When we think about the freedom and adventure of RVing, it is easy to forget that most campers will not function for long without being connected to some-sort of external power system. This article will walk you though all of the steps required to size, design and install a solar system in a RV. This article is written for the Newbie RV owner that wants to explore the world off-grid without needing to always be connected to a generator or shore power.
The goal of this article is to help you learn what is needed to build a RV solar system from the ground up and get you to the point of being able to confidently discuss off-grid solar systems with anyone. I have done my best to cover every topic in a simple and easy understand way. After reading this you should be armed with all of the info you need to install your own solar system or know what to ask for when hiring someone else to do the installation.
This is a long post that covers everything from definitions to sizing. You may want to save this page to your browser bookmarks so you can find this reference again as you design and build your system.
Table of Contents
Solar and Electrical Terms We Need to Know
If you found your way to this article you likely know what solar is, but let's run through a few key terms to ensure our terminology is the same.
Solar Array
When one sees a group of solar panels installed on a RV, house, building, etc. the term most often used to describe the installation is Solar Array. A Solar Array is made up of a group of two or more Solar Panels that are electrically connected together.
The term Solar Array is an informal reference to a group of connected panels that make up a system — it is not a scientific term.
Photovoltaic Array
When exploring solar, you will encounter the term “Photovoltaic Array.” Solar Array is a generic term that refers to the installation of solar panels. Photovoltaic Array is the scientific term used when describing power outputs and efficiencies of panels.
It is possible that you will see people interchanging the terms Solar Array and Photovoltaic Array, but Photovoltaic Array is the proper PRO term used to describe power outputs of interconnected cells inside of a single panel or the combined output/efficiency of a complete solar installation. Solar Array should be used when generically referring to a solar panel installation.
A Photovoltaic Array is defined as a grouping of solar cells that make up a single solar panel or group of panels.
Solar cells are the small square sections you see when looking at a solar panel.
The term photovoltaic (PV) originates from two root words: “photo” (light) and “voltaic” (voltage).
When you see the abbreviation PV, they are usually discussing something related to input or output specifications of one or more panels (e.g voltage or amps).
One must carefully read the docs referencing PV to determine what values are being discussed. Sometimes it is one panel and sometimes it is all panels combined.
Watt (W)
Watt (W) is a unit of power, and power is the rate at which energy is produced or consumed. Think of watts as a measure of electrical flow. Does an electrical device need a big flow or a small flow to work?
The easiest way I have found to describe Watts is to think about a garden hose. The amount of water that can flow through a hose is similar to Wattage. A small hose cannot move as much water as a larger one. This is the same concept in electricity. Lower wattage devices will not consume as much power as a higher wattage device.
A 100W light bulb, for example, uses energy at a higher rate than a 60W bulb. The 100W bulb is going to require a bigger flow of electrons to work than a lower output bulb.
Watts are calculated by multiplying Voltage times Amps of a device. In the 60W bulb example, we do not know the amperage, but we do know the operating voltage of 120V. We can determine the amps by dividing the rated wattage by 120V.
Watts (W) ÷ Volts (V) = Amps (A)
60 Watt Bulb ÷ 120 Volts = .5 Amps
In the solar context, Watts is the rate that the solar energy flows power from the panels to the RV Charge Controller.
Watt Hours (Wh) / Kilowatt Hours (KWh)
A common term that surfaces when we research solar is Watt Hours (Wh). This can be confusing at first. Watts and Watt Hours can sound similar, but they are different.
A Watt Hour (Wh) is a measurement of the amount of work performed or generated during a period of an hour. RV appliances and electrical devices perform “work” and that requires energy in the form of electricity that is supplied through batteries from a camper if it is not connected to grid (e.g. shore) power.
Watt Hour Example: All electrical components are rated in Watts (W). When seeing the term Watt (W) without Hours (Wh) next to it, the rating is referring to how much power the device will require to operate for one-hour. A 60W lightbulb, for example, will consume 60 watts of energy every hour, which equates to 60 Wh per hour.
When discussing Watt Hours, we are looking at how much energy is needed or has been consumed over time. A 60W bulb would consume 600 Wh's in 10 hours of use.
Watts (W) x Hours (H) = Watt Hours (Wh)
60 Watt Bulb x 10 Hrs of use = 600 Watt Hours
Kilowatt Hours (KWh) is Watt Hours that exceed 1,000. Utility company's use KWh to make the numbers easier to read. 1 KWh = 1,000 Wh. In the bulb example above, let's assume it was operating 24 hours a day for 7 days (168 hours). It would have consumed 10,080 Wh's. TO make the larger numbers more manageable, we divide Wh's by 1,000 to get KWh
Watt Hours ÷ 1000 = KWh
10,080 Watt Hours ÷ 1000 = 10 KWh
Megawatt: It is rare to see the term Megawatt used in a RV, but it is possible if calculating a year or more of use. Mega means million. The math works the same as converting to Killowatt. The only difference is you would divide by 1,000,000 instead of 1,000. In our 60W bulb example, it would have to be operating 24 hours a day for 6,849 days to get to 1 Megawatt!
Amps (A) and Amp Hours (Ah)
Amps is an abbreviation for Amperage. Amperage (Amps) is a rating that states how much power a device will require to operate at peak or in the case of a battery how much power it can deliver when fully charged.
Think of amps like water flowing through a garden hose. The amount of water that can flow through a given size hose is like Amps. For example, a big hose found on a fire truck is going to deliver a larger volume of water than a small garden hose at your house. A weather radio may consume 0.2 Amps whereas a space heater may consume 15 Amps.
Every electrical device will have an Amp rating. Most of the time when device manufacturers reference Amps, they will just put the letter “A” by the number. “A” stands for Amperage/Amps. The picture above is the back of the power transformer for a Ring Alarm system. In the Output section lists 2.5A. What this is saying is the Ring transformer can deliver power up to 2.5 amperage. This does not mean it consumes 2.5A 24×7, but it can deliver up to 2.5A without damaging the power supply or whatever may be connected to it.
Amps and Watts can sound the same, but they are different. In our gardenhose example, Amps would be the volume of water flowing through the hose, whereas, Watts would be the power generated by the water running through the hose (i.e. think pressure washer or hydroelectric dam).
Amp Hours
Similar to Watt Hours, Amp Hours are the amount of Amps that are delivered in an hour.
Think about the process of filling a swimming pool with water. It could take days to fill a pool with a garden hose and a few hours to fill with a fire hose. Both methods will fill the pool to its max capacity. Larger hoses deliver more water and reduces the amount of time it will take.
In electricity, a 1 amp continuous output device would equal 1 amp hour. A 2.5 Amp device would equate to 2.5 Amp Hour. In the swimming pool example, a small hose may deliver 10 gallons an hour whereas a large fire hose could deliver 100 gallons an hour.
Let's take our 60W bulb example from above. A standard 60W bulb will consume approximately 0.5A in an hour. To find out how many amps a Bulb will consume in 10 hours we just multiply the amperage by time in hours.
Amps (A) x Hours (H) = Amp Hours (Wh)
.5A Bulb x 10 Hrs of use = 300 Amp Hours
Voltage (V)
Voltage is a term we use all of the time, but it can be a bit tricky to grasp the concept of what it means. Voltage is potential energy. It is like the amount of water behind a Dam. High voltages are like being behind a large dam. Low voltages are like being behind a small one. The more water there is behind a dam, the more potential energy the dam can deliver.
The term Potential energy can also be a bit tricky to understand as well. It is a bit easier to think about this as water. If you have ever used a siphon hose to move water from one bucket to another then you have experienced how potential energy works. The higher bucket has a higher potential than the lower one that is empty. A siphon hose creates the conduit that will move the water from the higher potential to the lower potential.
Next, think about what happens when the water levels equalize between the two buckets… The siphon hose stops flowing water — the potential energy between the two buckets has equalized. When the potential energy is the same, water — or in the case of electricity — electrons will not move.
The image above is showing what happens when siphoning water. When the two buckets have the same level of water, the siphon stops working. This is what happens in Electricity with Voltage. Higher voltage electrons want to move toward a lower voltage.
A battery, for example, may have a nominal voltage of 12.6 V. A battery charger must be rated for something above that (e.g. 14.6 Volts) to work. The battery charger has a higher potential voltage so the electrons want to move toward the battery until the battery voltage equalizes with the charger voltage. When the voltages equalize a battery will stop charging. This is similar to what happens when water stops flowing in a siphon.
A brief aside about battery chargers. If a battery charger does not reduce its output when the battery it is charging is full, it could damage the battery. Pushing more voltage into a device rated for lower voltage for extended periods of time may cause permanent damage. Always use a battery charger designed for the battery it is charging. Using a constant voltage lithium charger on a lead acid battery will destroy it.
Converter and Inverter
One of the most common terms that get interchanged by new RV owners is Converter and Inverter. They sound the same, but they are different.
A Converter gets its name because it “converts” standard 120 Volt Alternating Current (VAC) house-style electricity down to 12 Volt Direct Current (VDC) automotive voltages. The RV Industry calls these devices Converter/Chargers.
The difference between a Converter/Charger and a standard battery charger is the amperage output. Converter/Chargers are designed to power the 12V items inside of a coach when plugged in then the surplus power is used to charge the on-board batteries. Most standard auto battery chargers are designed to charge the batteries only. They have a lower output current and generally are not designed to power other DC powered devices like lights and a radio.
A Converter/Charger is a simple device. The electrical plug that powers it connects to a normal house-style 120 VAC socket. The other end has 12 VDC positive and negative terminals that connect to the DC Bus of a camper. The Converter/Charger pictured above is manufactured by Progressive Dynamics and rated for an 80 Amp output. Common stock RV converter/chargers generally range from 35-60A.
Inverters
Inverters are similar to Converters, but they work in the opposite direction. A converter changes AC voltage to DC. An Inverter changes DC voltage to AC. Inverters are used when a standard household-type electrical device needs to be powered from batteries.
It is common to see RV's ship with an inverter if the coach has a residential refrigerator. A residential refrigerator needs AC power to operate. When traveling, a RV needs AC power to keep the refrigerator running. The AC power is supplied by an Inverter.
High-end RV's with upgraded electrical systems may come with larger inverters, but generally larger inverters are installed after-market by owners who upgrade their electrical systems.
Solar System Components
Solar systems are actually fairly straightforward. Basic systems have two main components. The Solar Panels that convert light into electricity and a charge controller that transforms the voltage coming in from the solar panels down to something that can charge the batteries.
Solar Panels
Solar panels used in RV's typically come in one of two configurations — Monocrystalline or Polycrystalline.
Monocrystalline and Polycrystalline panels are both made from silicone. Monocrystalline panels are made from a single block of silicone that is cut to a uniform size and shape that are called wafers.
Monocrystalline panels are the most efficient because they are cut from pure-blemish free silicone stock. Mono panels cost more than Poly panels because the Mono manufacturing process is expensive and time consuming.
Polycrystalline panels are also made from silicone, but their construction comes from multiple silicone sources that are bought together to form the cells. The process is less expensive, but the imperfections in the Poly cell lowers its efficiency.
It is easy to identify the differences between Mono and Poly panels.
- Mono Panels are cut at a 45 degree angle at each corner because they are made from a round-stock of silicone. If manufacturers were to cut a perfect square they would have too much loss in material.
- Poly panels are perfect squares. They are manufactured out of multiple sources of silicone so the manufacturers can utilize every inch of space when forming the panel. The square shape provides more surface area that improves cell efficiency. This helps offset some of the loss due to how the panels are made.
Panel Construction (Rigid vs Flexible)
There are two types of construction methods of Solar panels used in the RV industry – Rigid and Flex. Rigid panels are generally what is thought of when someone thinks about a solar panel. They are the rectangular shaped aluminum framed panels that we see everywhere.
We see rigid solar panels everywhere because they are virtually indestructible. Rigid panels have been manufactured for decades. The manufacturing process is efficient and the panels are tested by power companies and large installations worldwide. Rigid panels are proven technology that will withstand years of highway driving, rain, hail, tree limbs bumping them, etc.
The drawback of installing Rigid panels on the roof of an RV is they are bulky and need to be hard-drilled into the roof. Each panel will require 6 to 8 physical mounting points securing it to the RV roof. Every hole has the potential of creating a leak point. Accordingly, Airstream and rigs with metal roofs have the additional challenge of drilling through metal to mount the panels. If they are ever remove or change the panels, there would be unsightly holes in the roof that would not be easy to conceal.
Flexible Solar Panels
Flex panels are starting to show up more on RV's. The flexible nature of the panels make them easy to mount onto unusual shapes like an Airstream roof.
Another advantage to the Flex Solar Panels is they are adhered to the surface with a peal and stick adhesive. If they ever need to be removed, a heat gun and some elbow grease can pull them up without damaging the roof.
Flex panels are not as durable as rigid panels. It is very common to see nicks and cuts in a Flex panels that was caused by a tree limb or something nicking them.
Cuts will not necessarily prevent a Flexible Solar Panel from functioning, but they will reduce their overall efficiency.
Another issue with Flex panels is how the top surface is constructed. Flex panels have a soft plastic surface that covers the silicone wafers (Rigid panels are made with tempered glass). The soft plastic is susceptible to UV damage and will deteriorate over time. The top-surface clouding will cause the the entire panel to lose efficiency.
Flexible Solar panels have their advantages, but they are more expensive and will not last as long as a Rigid panel. Most quality flex panels come with a 10 year warranty, but the warranty does not cover labor to remove and install the replacement. Flex panel manufacturers may not cover panels damaged by nicks and scrapes either. Given the choice, rigid solar panels will work better for most RVs.
Charge Controllers
The second primary component of a RV Solar installation is the Charge Controller. What they do is normalize the electrical power coming in from the solar panel to something that can safely charge the on-board batteries.
There are two different types of Solar Charge Controllers — PWM and MPPT.
PWM Charge Controllers
PWM stands for Pulse Width Modulation. PWM controllers act like on/off switches. When they are on, the voltage coming from the panel feeds directly into the battery bank directly. The power output from the panel is not altered. When a battery is fully charged, a PWM controller will toggle the solar panels on and off to keep the batteries in absorption.
PWM systems required that the solar panels are precisely matched to the batteries being charged. If you have a 12 VDC normal battery bank, the panels will need to output a voltage slightly over 12 VDC ~ something around 18 VDC. If you remember our definition above about voltage potential, this is where it comes into play in solar. As soon as the panel output falls below 12 VDC, it stops charging the batteries. A PWM controller would switch the panel off so the battery doesn't try to feed the panel.
Installing multiple solar panels in a PWM system has to be carefully thought out. PWM controllers have a max input amp rating that cannot be exceeded and the solar panels need to output voltages over 12 VDC to work, but they cannot exceed 18 VDC or it can damage the batteries and components inside of the RV. Most of the time PWM systems have one or two panels.
MPPT Charge Controllers
MPPT stands for Maximum Power Point Tracking. MPPT controllers are basically DC to DC converters. They take input voltage form panels and step it down to a safe voltage for the system being charged. All MPPT controllers are designed to convert higher voltages coming form solar panels down to 12 VDC. The common input voltages for MPPT charge controllers are 50, 75, 100 and 150.
MPPT charge controllers are the most flexible. They let you find the best panel that fits the roof and then it does the conversion required to charge the batteries. Accordingly, by having higher input voltages solar panels can be connected in series that multiplies the voltage of the panels and does not increase amps. The advantage of this is lighter weight wires can be ran between the solar panels and charge controller.
Series and Parallel Connections
When discussing multiple solar panel configurations, it generally doesn't take long to get into the discussion on how they will be wired together (e.g. Series or Parallel).
The reason the wiring discussion is important is because how you connect the panels determines what charge controller you will need, what types of panels you can use and the gauge (thickness) of the wires running between the solar panels and charge controller.
Parallel Connections
Parallel connections are the most common wiring in the auto and RV industry. RV Batteries, for example, are almost always wired in parallel. What parallel means is all positives connect to each other and all negatives connect to each other.
When batteries or solar panels are wired in parallel, the voltage stays the same. Two 12 VDC batteries that are connected in parallel output 12 VDC to whatever they are connected to. This is the most common configuration in two battery automobiles and RV's.
What happens in a parallel connection is the output doubles. In the picture above, two 12 V 100 Ah batteries were connected. The result is it is now like having one big 12 V 200 Ah battery. The batteries are joined and double their capacity without changing the output voltage.
The challenge with parallel connections in solar arrays, however, is when two panels are connected in parallel their output amperage doubles. The higher amperage needs to be carried through wiring from the panels to the charge controller. In solar installations it is generally better to keep the amperage as low as possible.
Series Connection
Series wiring of batteries connects the positive terminal from one battery to the negative terminal to another battery. The cross connects would continue in the positive to negative configuration until the last battery. There would be one open positive and one open negative. Those would connect into the system.
The batteries in the photo above are 100 Ah 12 VDC. What happens in a Series connection is the voltage doubles to 24 V, but the amperage output remains the same.
Doubling of voltage is what happens to solar panels that are connected in series as well. Two 32 V / 9 A output solar panels would output 64 V at the charge controller when connected in series, but their amperage would remain the same at 9 A. The advantage of this configuration over parallel is that it would take a smaller gauge wire to carry 9 A in a series connection vs 18 A in a parallel connection.
Series-Parallel connections
In large complex solar installations, it is common to see panels wired in a series-parallel configuration. This configuration has to be thought out carefully. Everything needs to be identical in each section to work properly.
Series-parallel is a configuration where there could be groups of panels in series and then the groups combine together to form a parallel connection.
The diagram above shows 4 groups of solar panels (one per row). Each row is connected in series and the outside connections are in parallel. The panels are 19V 8A.
- The total output of the above series-parallel configuration is 38V 32A.
- If all 8 panels were connected in parallel, the output voltage would be 19V at 64A.
- If all 8 panels were connected in series, the output voltage would be 152V at 8A
Series-parallel allows solar installers to get more wattage on the roof while keeping the input voltage and amperage into something that a charge controller can handle. Series-parallel also allows for installers to use smaller sized wires. The 32A output can be used on 10GA wires that are common in RV's that have preinstalled wires. The 64A output of the parallel connection would double the size of the wiring required.
Series wiring in this example would work fine with the standard 10 Gauge wiring in RVs with pre-wired solar connections. The output voltage, however, is too high. 152V output would push it outside of the specs of most charge controllers that cap at 150V input.
When designing a solar array, it is important to calculate the output voltage and amperage based on series, parallel, and parallel-series configurations. This will help you then pick the appropriately sized charge controller and wiring.
Sizing RV Solar Systems
The surface area on an RV ultimately defines the capacity of the system. The easiest way to start is to measure the roof and determine approximately where a solar panel could be installed.
Both images above are Grand Design RV's. The layout of OEM components on the roof such as air conditioners and vents determine where panels can be installed. The top photo is a 2020 Grand Design Momentum 376TH. The second is our 2020 Grand Design Momentum 397TH. They are both big rigs, but the roof layout is different.
Our rig was ordered with factory solar. The panel at the bottom of the picture shows the OEM panel. GDRV installed the panel in a location that is off center, but they did not leave room for a second panel. We added two more factory Jaboni panels in the front. The two panels in the front could not be even due to a LCI Cellular LTE antenna.
The image above is a mockup I created to add two panels to the OEM install. The measurements are not exactly to scale, but it helped get in the ballpark. We added two panels, ran larger wiring from the roof to the charge controller, relocated the wiring into a junction box and left the OEM panel in place. This configuration gives us 900W of solar capacity. To achieve more we would have to move the OEM panel and patch the holes in the roof. The panels are wired in series.
Calculating Solar Panel Output
| Dimensions | 77.16 X 38.97 X 1.57 inches |
| Maximum Power (pmax) | 300W |
| Maximum Power Current | 9.30 Imp |
| Open Circuit Voltage | 39.85 Voc |
After roughing in the initial lay-out I was able to calculate the output power needed to size the solar charge controller and wiring. The first step was to look at parallel connections. The panels output 9.3A max output. 3 panels in parallel would output 27.9A and maintain 39.85V.
The next step was to look at panel output when wired in series. The panels output 39.85 V maximum open current voltage. 3 panels in parallel would output 119.55V and maintain 9.3A.
Determining Charge Controller Size
The Grand Design Solar install came standard with a Jaboni 30A MPPT charge controller. The easiest solution would be to use the OEM controller; however, when looking at the Jaboni Charge Controller specs, it will not work with 3 300W Solar Panels.
| Max Solar Open Circuit Voltage | 45 Vdc |
| Max Input Current | 30 Adc |
| Max Wire Size | 10 AWG |
If we refer back to the above calculations, 3 panels in parallel would output 27.9 A. Although this is within the limits of the input amperage, the max size wiring supported is only 10 AWG. 10 AWG is max rated to 30A for solar installations. This configuration would push 10 AWG wiring to its limits, which increases heat and voltage drop.
Next we look at 3 panels in series, which output 119.55V. The max input voltage for the Jaboni Charge Controller is 45V. The Jaboni Charge controller can only support 1 300W solar panels in series.
The OEM charge controller can only support parallel connections due to the limitation of input voltage. Operating 3 panels in parallel is technically possible, it would push the amps on the wiring to the max supported by the Jaboni charge controller. Another limit of the Jaboni Charge controller is it can only output a max of 30A. 3 panels in series has the potential at charging at more amps with a different charge controller. Therefore, we eliminated the OEM Jaboni charge controller from our system.
Victron Charge Controller Options
Victron makes a wide range of solar charge controllers and are frequently used in RVs. It is important to find a charge controller that will fit the system without buying one that is too large or too small. A large system will not be as efficient as a smaller one. A small system can overload and shut down if more power than it can handle is pushed in or out of it.
Victron lists all of their charge controller's on there website. At first glance it may be confusing, but it isn't too hard to narrow down the product needed for our install.
The first item I looked at was what devices support Bluetooth. I wanted a controller that can be programmed and monitored directly from the Victron smartphone app. The devices that have integrated bluetooth are called SmartSolar and have a picture of a smartphone next to them (top row of image above plus the leftmost controller on the second row).
Now that we have narrowed to the Victron SmartSolar devices, we need to determine what controller we need. The output voltages are available in different configurations — 12V/24V/48V. Output voltage is the voltage that the system uses to charge the onboard battery bank. This is not the input voltage from the panels. I chose 12V output.
I like keeping everything at 12V in an RV because that is the standard voltage used in vehicles. Higher voltages are more efficient, but they require step-down DC to DC converters to work with inside RV systems and it increases the risk that an RV tech or someone not familiar with high-voltage systems could accidentally cross wires that could cause significant damage to components.
After determining the output of 12V in our case, the next step is deciding what controller we need based on the specs of the panels. Victron conveniently lists the input and output capability of their controllers in the name. The MPPT 150|70 can handle input voltages up to 150V and will charge at 70A. A MPPT 250|100 can support up to 250V input voltages and charge batteries up to 100A.
To pick the controller we need to refer back to our solar panel specs. The higher voltage input in general is more efficient because we can run smaller gauge cables from the roof. Looking at our 3 panel series connection, we would output a max of 119.55V, which takes us out of the 100V and below input controllers. We can use either the 150V input or the 250V input controllers.
Now we need to look at our charging potential to see what size controller we need. We need a bit of math to get us in the ballpark. In our 3-panel configuration we will be outputting 119.55V at 9.3A. The controller will step down the voltage to 12VDC. When stepping down voltages, the amperage will increase. We need to see how many amps we will be outputting to confirm the approximate controller size we need.
To calculate charging amperage, we need to reference the panel output wattage. Our 3 panel configuration is 900W (300W x 3). 900W output divided by the end-voltage of 12V works out to the maximum amperage output potential of the system (900W ÷ 12V = 75A).
Our 900W system will need a charge controller that can output somewhere around 75A. Next, let's look at the specs of the Victron charge controller specs that are in the 75A range to pick the best one for our system.
| SmartSolar | 150/60 | 150/75 | 150/85 |
|---|---|---|---|
| Battery Voltage | 12 VDC | 12 VDC | 12 VDC |
| Max. PV Input Current (amps) | 50 A | 50 A | 70 A |
| Rated charge current | 60 A | 75 A | 85 A |
| Nominal PV power, 12V | 860W | 1000W | 1200W |
Victron controllers are built to compensate for panel and cable loss. When looking at the reference table, the SmartController 150|75 would work for our system. Our 900W panels are below the 1000W rated input specs. Our output voltage of 119.55V is below the 150A input voltage. And our input amperage of 9.3A from the panels is below 50A max limit.
In our 3 panel configuration the Victron 12VDC SmartSolar 150|75 will match our system perfectly. However, we may add another 300W panel to the system. What would happen if we did?
Victron Series Parallel Charge Controller Sizing
Based on our 397TH roof configuration, we could add one more Jaboni panel if we relocate the OEM panel. Adding another panel is somewhat easy, but the math changes if we do. We need to run though the numbers to ensure we have expansion capabilities.
4 300W panels in series would output 1200W at 159.4V. This output voltage would force us into the significantly more expensive 250V charge controller. However, if we configure the panels in series-parallel we can easily keep the output voltage below 150V.
Connecting 2 sets of panels in series and then combining those two panels in parallel changes the math.
To calculate the Series-parallel configuration we start by calculating the series legs. In the image above, each row consists of 2 300W Jaboni panels that output 39.85V at 9.3A each. The 2 pair series panels (row in pic above) works out to 79.7V (39.85 x 2) at 9.3A. The two series pairs are then combined into parallel, which works out to 79.7V at 18.6A.
Series connections add voltages together and leave amps the same. Parallel connections leave the voltage the same and add the amperage. When combining two series connections into a parallel leg the voltage will remain the same as the series pair and the amperage will double.
By changing to a series-parallel configuration, we are able to keep the voltage below 100V. This opens up the option to look at the 100V and up controllers; however, when referencing the 100V Victron Charge Controller specs, they only support panel configurations up to 700W.
Although our voltage output is lower than 100V, the series-parallel configuration will still require a 150V|X system that can handle the output load of the 1200W panels.
| SmartSolar | 150/75 | 150/85 | 150/100 |
|---|---|---|---|
| Battery Voltage | 12 VDC | 12 VDC | 12 VDC |
| Max. PV Input Current (amps) | 50 A | 70 A | 70 A |
| Rated charge current | 75 A | 85 A | 100 A |
| Nominal PV power, 12V | 1000W | 1200W | 1450W |
The SmartSolar table above shows the Victron Charge Controller specs that may fit. By adding the 4th panel we push outside of the 150|75 controller because our system will output more than 1000W. The 150|85 is now the best fit. It will support a panel configuration up to 1200W and charge at 85A. It would also possible to jump up to the 150|100, but that would additional cost that isn't necessary for the 1200W panel configuration.
I chose the 150|85A controller for our installation. Although we have 900W initially that will work with the 150|75, I will be adding another panel to our system. By stepping up to the next charge controller size I can use the same controller to work with our 900W system and support the expansion to 1200W.
System Wire Sizing
Solar to Charge Controller Wire Sizing
Now that we have landed on the number of panels and the charge controller, we need to calculate the size of wires we need. We will have two connections to cable — the feed from the Solar panels to the Charge Controller and the feed from the Charge Controller to the Batteries.
The system I have been documenting will start with 900W panels, but the plan is to expand it to 1200W in the future. When sizing, we need to look at both configurations and size to the largest wiring requirements.
We need to compare both 900W and 1200W system configurations because it is possible that a smaller system could put a heavier amp load on the wiring. For example, one could have a smaller system in parallel that multiples the amperage on the cables and then move to series in a larger system. In this situation the smaller parallel configuration would put a heavier amperage load on the wires.
We can use tables and reference data to determine wire size, but fortunately there are a number of online calculators that make the process easier. One of the calculators I like to use is hosted on the Renogy website. It is important to keep in mind that not all sizing calculators are designed to compensate for solar. Renogy sells quality solar components. Their calculator is safe to use for sizing solar wiring.
When reaching the Renogy calculator, select Solar Cable Gauge Calculator. Next we need to enter the specifications for the proposed configurations. We can start with the wire size from the panels to the charge controller. I will use the numbers generated for the 900W and the 1200W examples.
900W Configuration (3 300W Series)
For the 900W system, we need to round up to the next full number. I rounded the 119.55V output of the 3 series connected panels to 120V and rounded the 9.3A output to 10A. I entered 40 feet to ensure that I calculate the maximum length possible. Quality cables that are designed for solar have low loss. I used TemCo cables so I entered 2%. After entering the number click calculate button.
The wire gauge size needed for a 900W system would be 14 AWG minimum.
Next, we need to calculate the specs of the 1200W Series-Parallel panels.
In the 1200W Series-Parallel wire calculator, I rounded up the output voltage from 79.7V to 80V and the amperage from 18.6A to 19. I left the distance and the loss the same.
The wire gauge size needed for a 1200W system would be 10 AWG minimum.
Based on our calculations, the 1200W Series-Parallel system will require the largest 10 AWG cables. We need to use the 1200W configuration for sizing the wires.
I like bumping up to the next size cable. Larger cables will improve efficiency and provide for expansion down the road if needed. In our 1200W configuration, the 10 AWG wiring will support a max of 30A. If we jump to 8 AWG, we can safely carry 55A. The one size up adds significant headroom and only marginally changing price and weight.
I chose to run 8 AWG wiring from the roof to the charge controller.
Charge Controller to Battery Bank Wire Sizing
The next step of the process is determining the size of wires needed to charge the battery bank. The Renogy Wire Size Calculator is technically designed for solar panel to Charge Controller, but we can use the calculator to get into the ballpark of what we need to connect to our batteries as well.
Using the Renogy solar calculator for the battery interconnect cable is the easiest since we are on this page already. All that is needed is to change the Solar Array Voltage and Operating Current to the output specs of the Charge Controller. In our case, we are using the Victron 150|85 that has an operating voltage of 12VDC and a max charging amperage of 85A. The distance from the charge controller to the batteries should be as short as possible. I used 6 feet in this calculation, which is about double the actual length of approximately 3 feet.
The Renogy calculator indicates that we need to use a 2 AWG size wire to connect to the battery bank. This will support the max charge amperage and voltage. However, before locking in 2 AWG, we need to reference the Victron Charge Controller documentation. We need to verify that the Charge Controller has terminals large enough to fit 2 AWG wiring.
Based on the Renogy calculator and the Victron Charge Controller documentation, 2 AWG wire size will fit the terminals and carry the 85A max charging load.
Charge Controller Distance to the Battery Bank is Important!
The Charge Controller should be mounted as close as possible to the battery bank. To illustrate why this is important, I changed the battery calculation slightly. I increased the distance from the controller to the battery bank from 6 feet to 10 feet.
Adding 4 more feet to the distance the minimum wire size moved form 2 AWG that will fit the charge controller to 0 AWG that will not fit the output terminals of the charge controller. Mount the Charge Controller as close as possible to the Batteries.
Choosing House Batteries
The heart of a RV solar system isn't the solar panels it is the battery bank. The solar panels do not actually operate anything. Their sole purpose is to charge the batteries that store the power required to operate the equipment in the RV.
Battery Bank Voltage
When selecting batteries, the first decision to make is the voltage configuration that will be used. Higher voltage battery banks such as 24V and 48V configurations improve overall efficiency of a solar system, but changing battery voltage from 12VDC adds risk. Most RV's and vehicles in general are built around 12VDC.
Standard 12V configuration enables us to jump start from one vehicle to another without damaging the electrical system. All DC powered components installed in an RV including systems such as the furnace, slide controllers, generator starter, lights, radio, water heater, etc. are designed to solely work off of a 12V system.
When deciding to change from 12VDC to something else, the battery bank must be isolated from all other RV components. A device called a DC to DC converter must be installed between the battery bank and the RV components. DC to DC converters will step down the battery operating voltage to 12V that is safe to use internally.
DC to DC converters function similarly to solar charge controllers. They are reliable, but they do add another point of complexity and failure point in the system.
The further we move from standard, we move further away from what RV techs, mechanics and most RV owners understand. Solar connected systems are complex and have very expensive components. A simple mistake like trying to charge a 24V battery bank with a standard 12V charger can cause permanent damage to parts, blow fuses, and could start a fire!
In a home or off-grid situation, higher voltages are preferred, but in an RV I highly recommend keeping everything 12VDC. Keeping the electrical system as close to standard as possible is safer.
- We may need to leave our rigs at dealers for maintenance,
- We may sell our rigs to someone that doesn't understand the system,
- We may have a breakdown and need to jump start the generator from the tow vehicle,
- We could brake down on the road and a tow truck driver is not familiar with the system.
Battery connections
Similar to solar panels, batteries can be configured in parallel, series and a combination series-parallel. For this discussion I will generally target parallel configurations since we like sticking with 12V in a RV.
It is important to note that there are some RVs that have installed 6V Golf Cart Batteries in their rigs. 6V batteries need to be wired in a Series-Parallel configuration so they output 12V into the RV. Series-parallel is similar to the discussion above regarding solar panels. Two 6V batteries are connected in Series. The output of those two batteries are connected to the the other two-group of batteries in parallel.
The 6V configuration is not as common now that there are more powerful 12V options, but it is possible that you may encounter a 6V parallel-series connected system if you are upgrading an older rig. It is always a good idea to take pictures of existing wiring before changing anything and then test voltages with a multimeter. It can cause significant damage to systems if 6V and 12V systems are inadvertently connected to each other.
Battery Chemistry
There are 3 types of chemistries used in most RV Batteries – Lead Acid/Wet Cell, AGM/Gel Cell and Lithium-Ion. Outside of those chemistries, there are a myriad of marketing differences from one battery manufacturer to another. We may hear about cold craning amps, deep cycle, marine, maintenance free, etc. Regardless of what the batteries are called, they all end up being one of the 3 chemistries.
Lead Acid / Wet Cell
The Lead Acid Battery is by far the most common battery found installed in RVs. The technology has been around for over 100 years. Lead Acid batteries are also routinely called Wet Cell batteries. The reason they are called wet cell is because water needs to be added to the batteries regularly.
Lead Acid batteries require a lot of maintenance from owners. When Lead Acid batteries are charged the internal cells generate heat. The hot plates inside of the battery heats the surrounding water that is intended to keep the cells cool. The water will heat up above its boiling point and create steam that evacuates outside of the battery. When the battery cools, the escaped steam is lost and the water level decreases. This process happens during every charge cycle.
The lower water level exposes the internal lead acid plates to air, which exposes them to high-heat during charging. Heat will destroy the charging plates reducing their overall power output. This is why wet cell lead acid batteries must be regularly serviced with water. One charge cycle without water covering the internal plates will cause permanent damage to the battery.
Another disadvantage of lead acid batteries is they are not efficient at storing electrical loads over time. A fully charged battery that is disconnected form everything will lose 5% of its capacity every week. A battery sitting 1-2 months without a trickle charger will be completely dead.
Fully discharged lead acid batteries will destroy them. A fully charged battery pushes the Acid electrolyte that holds the charge into the surrounding water. As the battery discharges, the acid attaches to the lead plates. When removing the Acid from the water the freeze point increases. Frozen water expands and will destroy the lead acid plates and can crack the housing. This is the main reason why car batteries die in winter.
Most of us have been taught to trick charge our batteries when in storage, which is great advice. The problem of trickle charging lead acid batteries; however, is the plates always stay warm. The warm plates evaporate the water in the cells. Batteries being trickle charged still need to be inspected regularly to ensure water level in the cells is sufficient. If water goes unchecked, the cells will be exposed to air and overheat during trickle charging. This is why we find lead acid batteries that will not hold a charge after winter storage even though they were stored in a warm location and trickle charged.
Sealed Lead Acid/Maintenance Free Batteries
Sealed Lead Acid Maintenance free batteries are very common. Battery manufacturers love marketing sealed lead acid batteries as being Maintenance Free. Unfortunately Maintenance free batteries and lead acid batteries are one in the same. They are made up of the exact same chemistry.
The only difference between a sealed lead acid battery vs a standard lead acid battery is accessing the cells. Standard serviceable lead acid batteries have caps that can be removed to enable inspection and water servicing. Sealed battery caps are designed to capture and cool the steam released from the water when the batteries are charged. When the hot steam cools it creates condensation that is directed back into the cell.
Although sealed lead acid batteries will last longer without maintenance, the process of converting water to steam and back does not capture 100% of the water. Over time the water level will decrease and expose the lead plates to air. Maintenance Free batteries that are never serviced will suffer the same fait as standard lead acid batteries that are not serviced properly.
Interestingly, standard lead acid batteries could outlast maintenance free batteries if they are properly maintained.
It is possible to add water to some maintenance free batteries, but this process generally requires removing labels and prying off the covers. When servicing them, however, the water recapture system can be damaged turning them into standard lead acid batteries that need to be regularly serviced.
AGM/Gel Cell Batteries
AGM/Gel Cell batteries and lead acid batteries function basically the same way. The difference is the material that holds the charge (e.g. electrolyte). Lead acid Batteries use water. AGM (Absorbent Glass Mat) batteries use a Fiberglass material that is situated between the lead plates. Gel Cell batteries use a gelatin material that holds the electrolyte. The electrolyte is exchanged between AGM and Gel cell batteries in exactly the same way that Lead Acid and Water works. The main difference is stability. AGM and Gel batteries do not suffer from evaporation like lead acid batteries and are truly maintenance free. They cannot be serviced.
Gel cell batteries are not commonly used in RVs or solar. They do not work well when high charge or discharge rates. Starting generators, moving slides, etc. can overdrive a gel battery and cause it to internally short circuit. Accordingly, fast charging form large solar systems will also damage a gel cell battery. The main reason we see gel cell batteries is they work very well in slow discharge conditions and they can operate in environments that have high heat better than AGM or Lead Acid.
AGM batteries are commonly found in RVs. AGM batteries are sealed and can be mounted in any configuration — sideways or upside down. AGM does not off-gas as much as Lead Acid batteries, but they still create toxic fumes when charging. They cannot be installed inside of a living space and must be vented externally.
AGM batteries will not last as long as a properly maintained Lead Acid Battery and they are more expensive; however, the maintenance free design of an AGM battery and the ability to mount them in any configuration make AGM the most common aftermarket battery used in RVs.
Lithium Ion (LI) Batteries
The new kid on the block is the Lithium Ion battery, which is often abbreviated to LI. Lithium batteries hit the market initially in laptop computers, smartphones, and personal devices. The consumer electronics market drove the initial LI use case because the batteries can be built in virtually any shape or size. LI technology was pushed into high-demand applications by Tesla the electric car manufacturer. Tesla vehicles use a powerful LI battery configuration that carries their vehicles 300 miles or more on a single charge.
Automobile and consumer electronic manufacturers have invested billions of dollars into research and production of stable and safe Lithium Ion battery technology, which has enabled aftermarket battery manufacturers to create drop-in sized batteries that have the exact same shape and voltage output of a normal Lead Acid, AGM or Gel Cell battery.
The drop-in form factor of Lithium batteries has rapidly moved LI into the mainstream market making LI the most common battery chemistry used in solar and high-power applications.
The main advantage of Lithium technology is they can deliver to their full rated power. A 100AH LI battery can deliver a full 100AH at a stable voltage from fully charged to fully discharged. Lead Acid, AGM and Gel Cel batteries all suffer from the same technology limitations. A 100AH traditional battery chemistry, for example, can only safely deliver 50% of its rated capacity (e.g. 50AH). Discharging a traditional battery beyond 50% will decrease its output. Several deep discharges of a traditional battery will destroy them. LI batteries can be fully discharged and recharged thousands of times without damaging the battery. Most quality LI battery manufacturers cover their batteries for 10 years or more. Lion Energy batteries come with a lifetime warranty. They are guaranteed to deliver no less than 80% of their initial rated capacity for 3,500 full charge/discharge cycles.
LI battery chemistry holds voltage consistent as they discharge even when under a significant load. A traditional battery will have a voltage drop when delivering full power and the voltage will drop over time as the battery discharges. Voltage fluctuations of traditional batteries can damage equipment, which is another reason why it is not safe to discharge traditional batteries beyond 50% of their capacity.
LI batteries are also completely sealed and maintenance free. Like AGM batteries, they can be mounted in any configuration — on their sides or even upside down. Where LI batteries further diverge from traditional battery chemistry is they do not off-gas. They can be safely installed inside the cabin of an RV. Mounting inside is critically important for small RVs like an Airstream that does not have large external storage compartments. Many Airstream owners install LI batteries in the storage compartment under the bed. LI chemistry is the only battery option that can be safely used inside a living space.
The final key differentiator between traditional batteries and Lithium ion is the weight. A type 27 lead acid battery will weigh somewhere between 54 to 70 lb. A Lion Energy UT1300 Lithium battery weights 23 lb!
The primary disadvantage of LI batteries is the upfront cost. LI batteries can be 5 to 10 times more expensive than traditional battery technology. It is not uncommon to pay more for LI batteries than all other components of a solar system. The advantages of weight, discharge capacity, safety and stability make them a very good choice for RVers.
I recommend looking close at LI batteries for any solar system build. If you are on a budget, starting off with one or two batteries and expand the battery bank over time might be a better choice than building around traditional battery technology. Adding capacity later is relatively easy if the system is installed in an area that has room to add more batteries.
LI batteries have a better return on investment over time than traditional batteries because traditional batteries need to be replaced every 2-3 years. It also takes 2 traditional batteries to deliver the same output capacity of 1 LI battery.
Battery Bank Sizing
As I discussed above, the number of solar panels are defined by the size of the roof. RVs do not have an unlimited roof space. There is a limit to the maximum amount of power than can be generated per day. Battery banks are similar. Batteries take up space in the RV and they are expensive. The more power on board, the longer a RV can be powered by the rig, but again there is still a charging limit. We can't fill our coaches with batteries and not have sufficient solar to charge them. We need to size a battery bank with our solar array.
The challenge with battery bank sizing is that if the batteries will only be charged by solar panels, the time to charge plus the consumption rate need to be look at. A small 100W panel charging a 12V battery on paper will charge at 1200Wh in full sun. One Lion UT1300 battery holds 1250 Wh's (105Ah x 12V). In a perfect world it would take a bit over an hour for a 100W panel to charge a UT1300.
Of course, we do not live in a perfect world. We cannot charge batteries at the full panel rate. There are losses in the system from the wiring used to the energy consumed by the charge controller to covert panel voltage to 12VDC. In my 1200W system, I landed on an 85A Victron Charge controller. The max rate I can charge at is 1020W (85A x 12V) per hour.
Another consideration we need to take into account is the hours of sun available. The standard usable solar calculation in the US is 5 hours per day. There will be days where we receive more than 5 hours of sun and there will be overcast and rainy days where there will be no charging whatsoever. We need something to calculate from, so 5 hours is a good baseline to use as an estimate. In my 1200W system, I will generate approximately 5,100Wh's per day (1020W x 5h).
Now that we know the approximate amount of power we can generate per day, we can look at optimal battery bank sizing. A Lion UT1300 holds 105Ah of power, which equates to 1,260Wh (105Ah x 12V). I can effectively charge 4 UT1300's from dead in 5 hours (5100Wh charging ÷ 1260Wh/UT1300).
It isn't a good idea to cycle batteries fully dead and up frequently. The larger we make our battery bank, the less likely we will need to fully discharge the batteries. For planning purposes, we should try to get to 2x more battery bank storage than what a solar array can deliver to keep the battery discharge rate down to approximately 50% of capacity.
Battery life is extended if do not discharge fully regularly. A full discharge would equate to 4 UT1300's in our 1200W solar array (e.g. 5,100Wh used and recharged). If we size the bank somewhere between 6 and 8 LION UT 1300's we will increase the lifespan of the batteries by not fully discharging them and have more reserve power on hand to cover cloudy days.
Calculating Energy Consumption
One needs to balance the size of the battery bank with the amount of energy that will be consumed while taking into account the length of time it will take to charge the batteries.
It may be easier to take the batteries out of the equation for a minute to bring the point home. A 1200W solar array can deliver 5,100Wh of power per day on average. 5,100Wh is our daily consumption limit.
If one consumes more power than a solar array can deliver in a day, then the there will not be sufficient power to charge the battery bank. Every watt consumed form a battery bank over what the panels can push back into energy storage will slowly deplete the battery bank to the point where they will eventually be fully discharged and disconnect.
In a perfect world, one would just add solar panels and batteries to cover everything, but as we have been discussing RVs have limited space. It isn't possible to make our roofs bigger. What we need to do is to limit our energy usage based on the amount of charging power the panels can provide.
RV solar sizing is a different approach than what would be done for a home install where we build a system to fit the consumption. We do not have the space in a RV to satisfy max consumption, so we have to fit our consumption into what our system can deliver.
Getting back to 5,100Wh's of max daily consumption, we need to estimate what it will take to get there. It can get a bit tricky to do the calculations since some components operate on 12 VDC and some use an inverter to convert to 12 VAC that has an overhead loss. We don't need to worry too much about all of that. We just need to get into a rough estimate of what we can consume per day and make sure we are looking at the same numbers.
We can back into the numbers by looking at amps per hour or by watts per hour. I find it easier to convert to watts per hour first so we are looking at solar panel output through to the batteries using the same numbers.
The quick way to convert into Watt Hours (Wh) is to reference amp rating of the appliance on its label and multiply by the operating voltage. For example, a computer is rated for .2A at 120 works out to 24 Watts per Hour (.2A x 120V = 24). That same computer would consume 576 Wh per day (24Wh x 24 Hours).
| Appliance | Voltage | Amperage | Watts Per hour |
|---|---|---|---|
| Computer | 120 | .2 | 24 |
| Microwave | 120 | 10.4 | 1248 |
| Satelite receiver | 120 | .2 | 24 |
| Blow Dryer | 120 | 7.5 | 900 |
| Refrigerator | 120 | 4.0 | 480 |
| Fireplace | 120 | 11.1 | 1332 |
| A/C 1500 BTU | 120 | 12.9 | 1548 |
| Furnace | 12 | 12.5 | 150 |
| Water Pump | 12 | 6.8 | 81.6 |
| Kitchen Fan | 12 | 1.6 | 19.2 |
Some always on appliances like a refrigerator are not technically running 24 hours per day. The inside temp stabilizes and power consumption is reduced. For calculations, let's assume a fridge runs for 12 of the 24 hours consuming 480 watts per hour. That works out to 5,760 Wh's consumed by the refrigerator per day!
A Refrigerator operating for 12 hours per day consuming 480 Watts per Hour will exceed our solar power generating capacity in a single day! For those of you who are planning on operating off-grid, a RV fridge running on propane is a more efficient use of power.
We all have dreams of living off-grid with no external power running our Air Conditioners all day, but the reality is that it isn't possible in a RV. We need to strike a balance of what we absolutely need to power off of electricity and use alternative power sources such as propane whenever possible. It is also important to unplug any component that isn't being used. Every plugged in electrical device will consume small amounts of power even when turned off.
Final Thoughts
On the surface solar looks to be complicated, but the reality is there are only 4 main parts that make up the system — solar panels, wires, charge controller and battery bank.
For those powering AC appliances like an Air Conditioner or Microwave, there is one more component called an inverter that turns battery power to standard household appliance power. I will write more about inverters in a future post.
We can spend a lot of time doing an energy audit trying to calculate the exact power needed, but the reality is the max solar power is limited to the space on the roof. Solar panels are cheap. Find a panel that will fit the roof the best and get as many of them as you can fit on the roof. Then take the specs of the panel and move forward though the system calculating everything needed from the wire size to the battery bank.