How many solar panels do you need for your RV or Camper?

This is part two of our series on sizing a solar PV system for an RV or camper.

In part 1 we covered how to size batteries for your system. So hopefully you now know how many batteries you need, or perhaps already have an RV with house batteries installed, and are ready to move on to sizing the solar panels.

Unfortunately, there is no simple rule that can be applied to size your solar relative to your batteries. For example, some people make general statements like, “You should install 200W of panels for every 100Ah of battery storage” The problem is, while these generalizations may work for some people, they will lead to an incorrectly sized system for others.

Instead, you have to consider how solar will perform in different regions, and different seasons so that you can calculate the system size that is going to be appropriate for your specific needs.

In this article we will start by working out how much sunlight your panels will be exposed to in the scenarios in which you plan to use them, we will then talk about losses in the system (something that is often overlooked), and finally, we will take this information and use it to calculate how many watts of solar panels you need to charge your batteries.

Calculating Peak Sun Hours

Since solar panels are rated at a specific number of watts, you may think you can simply take the watts in your batteries, and divide by the number of hours the sun is shining, to find the number of watts of solar you need to charge those batteries.

The problem is that solar panels only generate their rated power when they are being hit by 1000W/m2 of sunlight, and they are at 25°C, this is referred to as Standard Test Conditions (STC). The issue is further complicated because the intensity of the sun varies throughout the day, and at different times of the year.

To simplify this issue we need to think in terms of Peak Sun Hours per day.

One Peak Sun Hour is defined as one hour of sunlight at 1000W/m2.

As you probably noticed, the sunlight intensity in one Peak Sun Hour matches the sunlight intensity that solar panels are rated at. Therefore, if we can calculate the average number of peak sun hours per day, you can work out how much electricity your panels will generate in an average day (we will talk about temperature later).

So we need to find an appropriate number to use for our average peak sun hours. Many websites and books that talk about this topic just give one number for everyone to use. This doesn’t make a lot of sense though, since different locations and different seasons will have a completely different number of peak sun hours.

Instead, we have come up with the table below to help you find a number that is more specific to your needs. These are still only estimates, as they cover wide areas with large ranges in solar irradiance, but enable a much more accurate system size than using one number!

In the future, we are planning to write an article to teach you how to calculate this number for any location and season, so stay tuned, or let us know in the comments below if this is important to you and we will make it a priority.

Average peak sun hours in different regions and seasons

You will need to choose one of these numbers to use in the sizing calculation later in this article. Which one you use depends on where and when you will be using your RV.

For example, if you size your system based on the lowest number from the seasons and locations you are likely to travel in, then it should cover your needs. However the system could be very large, and either be too expensive or not fit on your roof. Designing for winter in northern Europe for example is usually not going to be practical.

Alternatively, you could size only for the warmer seasons and perhaps rely on alternator or generator charging to get through days with less sunshine. It entirely depends on your preferences.

Considering Losses

Now that you know the number of peak sun hours per day you are going to use in your calculation, we need to talk about losses.

As mentioned earlier, the rated power output of solar panels is affected by temperature. Solar panels can get quite hot when they are running, and as they get hotter, their efficiency goes down. Therefore you have to take this loss into account when sizing your system.

Solar panels also lose power due to other factors such as soiling and shading, so you also need to include these in your calculations.

To simplify things, we will combine all these losses into one number. We recommend using 20%.

Calculating how many watts of solar you need

The next step is to decide how quickly you want to charge your batteries off the solar.

Generally, you will want to size your system to charge the batteries in one day (especially if you are using AGM batteries as fully charging them daily will extend their lifespan).

Alternatively, some people with several days of autonomy may want to only size their solar for one day of energy usage, and rely on alternator charging for the rest.

Here we will show how to calculate how many watts of solar are required to charge your batteries in one day, and you can adjust the calculation to suit your situation.

The steps in the calculation are:

1. Multiply your usable battery Ah by the battery voltage to get the total usable watts of the battery system. (don’t forget that only 50% of AGM batteries should be considered “useable”)
2. Divide that by the number of peak sun hours, so you know how many watts you need to generate per hour.
3. Multiply by 1.2 to add allow for the 20% losses we just discussed.

The calculation is therefore:

Battery Ah x Battery voltage / Peak Sun Hours * 1.2 (20% losses) = Required solar in Watts

Let’s do an example:

Assume you have a 100Ah, 12V, Lithium battery. You tend to stay in SE USA and decide to size the system for spring and summer.

The equation becomes:

100Ah x 12V / 5.7 * 1.2 = 263W

Since you probably can’t find exactly 263W of solar panels you would probably round this up to 300W, and then buy three 100W panels, or two 150W panels, etc.

Final thoughts

You may find that after doing the calculation above, that you just can’t fit that many solar panels on your roof. This is still ok, as long as your system is also set up to be able to charge from an alternative source, such as your alternator, a portable generator, or shore power.

If you have the space for the system you want, but just can’t afford it right now, you could also consider installing fewer solar panels, but then buying a larger charge controller to allow you to add more panels later.