- RV Slideout Mechanism Types - June 6, 2022
- Is a Garmin RV GPS Worth It? - May 14, 2022
- Portable 12V Air Compressor – Why Every RV Needs One - May 7, 2022
Last Updated on December 9, 2021 by Jessica Lauren Vine
When considering adding a solar power system to an RV one of the most common questions is “How many solar panels do I need for my RV?”. This sounds like a simple question, but unfortunately, it is not and nor is the answer.
Solar panels are only one component of an RV solar system and to determine how many panels are needed we must look at the system as a whole. Here we’ll go through how to properly size a solar system to provide the power that you need when you are off-grid. I know this sounds complicated but we’ll break it down into easily understandable parts starting with the components of a solar system.
What Parts Make Up an RV Solar System?
There are three required parts to an RV solar system.
- A solar panel to capture the sun’s energy and convert it into electricity.
- A solar charge controller to convert the energy from the solar panel into a form that is RV friendly and usable for charging the batteries.
- Batteries to store the energy from the charge controller/solar panels when the sun is shining, and to power your RV when it is not.
How Does an RV Solar System Work to Power My RV?
The easiest way to think of a solar system is that the battery is like a glass with a small hole in the bottom. If we take a pitcher of water (the energy from the solar panel) and pour it through a funnel (the solar charge controller) into the glass, water will start to flow out the hole. That stream is the energy your RV uses to power things like the refrigerator, lights, etc. If we add water faster then it drains through the hole in the bottom then the glass will fill to store energy to use later. We can stop adding water (nighttime or no sun), the water will continue to flow and power our RV until it runs out (dead battery). If we start pouring water into the glass (the sun comes back up) before the glass runs empty then we have a stable power system that can theoretically last indefinitely. This is the goal when sizing your RV solar system. Based on this example, we need to know three things:
- How much energy we will draw from the system (How big the hole in the glass is)
- How much total energy we can store (how big the glass is)
- How much energy we expect to take in from the sun.
As you can see, I ordered these opposite of how you were probably thinking. The solar panels come last. There is a very specific reason you should approach it this way. If you know how much energy you are going to use then you can use that to determine your battery size to provide enough power to cover your usage for a day. Once you know that, you can determine how many watts of solar panels you’ll need to keep the batteries charged. With that information, you have established a baseline for your solar system. From there you can look at all the other factors that can affect your solar system’s performance and make adjustments to your setup accordingly.
What Factors Affect a Solar Systems Performance?
There are many factors that will affect your RV’s solar system performance which will require you to change your design. These will fall into three categories. Variables in the amount of energy you use, variables in the amount of energy you take in, and variables in the efficiencies of your solar system. Let’s start with the efficiencies in the system first because these are one-time variables, or decay slowly over time so they are easy to account for.
Variables in System Efficiency:
With a basic solar system, there are two main points where efficiency can affect your system. The first is in the solar charge controller. This device converts the raw power from the solar panel into power that is RV-friendly.
That conversion process will result in some lost energy. There are two main types of solar charge controllers commonly used in RVs – PWM controllers and MPPT controllers. The acronyms stand for Pulse Width Modulation and Maximum Power Point Tracking. Without going into the gory details, these are simply two different methods of converting the voltages from your panels to the 12V power your RV needs. PWM is simple, cheap, and inefficient, with losses of up to 25%. MPPT is complex, expensive, and efficient, with losses as low as 2%. Don’t worry about the complexity of what they do, that is all hidden inside the box. Both types of controllers are equally easy for the end-user to operate.
The important difference for us is that 23% spread inefficiency. To put that into perspective, a 100-watt solar panel will give around 98 watts of power with an MPPT, but only about 75 watts with a PWM. That’s a huge difference and it is magnified the larger your system gets. While the MPPT controller is generally quite a bit more expensive than the same sized PWM, you will save that back by needing fewer panels in all but the smallest systems.
The next inefficiency in the system is in the batteries. As batteries age, they lose the ability to store power efficiently. There are several types of batteries commonly used in RV solar systems and each has its own properties, advantages, and disadvantages.
All the nuances of battery choice are beyond the scope of this article. Generally speaking, the following is true:
- Lead-acid batteries are the cheapest. They are heavy, require the most maintenance, require good ventilation, and perform well.
- AGM batteries are more expensive. They are also heavy, require little if any maintenance, can be used in non-ventilated areas, and also perform well.
- Lithium batteries are the most expensive. They are much lighter than the other options, require little if any maintenance, can be used in non-ventilated areas, and perform the best.
When designing your system you’ll want to take into account that older batteries may be able to store as little as 75% of their capacity when new. When they get to that point it is generally time to replace them. Regardless, those losses may be spread out over several years. However, 10 – 20% can still be a lot in a larger system so keep that in mind.
Finally, any components you add to expand your basic system will also introduce power losses to your system. An inverter to power AC outlets or appliances is a common example. These typically draw power even when not in use so make sure you include that in your base power consumption calculation. There will also be some power loss converting the 12 V DC battery power to 110V AC. Your inverter’s manual should tell you how much. While it may look insignificant it is best to not ignore it, even small numbers can add up quickly if you use it a lot.
Variables in the Amount of Energy Used:
When you do your initial energy use assessment you will get a good number for that day. Keep in mind that variables like location and time of the year will affect your energy usage. Given that your RV is mobile, these factors may significantly influence your average daily energy use.
Variances in temperature (desert vs mountains, north vs south, summer vs winter) will change how much you use your fans or furnace. Changes in latitude and season will affect day length (days are longer in the summer and in the north than in the winter or in the south) which can change your lighting needs. When you calculate your power needs make sure you include some cushion for these factors depending on your camping habits.
Variables in the Amount of Energy Captured:
When you are sizing your solar panel they will be rated in watts. That rating is the peak energy you can expect to get from them on a perfectly clear day with the sun directly overhead and the suns rays hitting perpendicular to the unobstructed panel surface. There’s a lot to digest and all of those things will affect the amount of power the panel actually produces.
To simplify it, let’s just call it solar intensity. Solar intensity will rarely be the 100% the panel is rated at. The solar intensity on a clear day in Alaska in January will be much less than a clear day in Key West in July. Other things that lower the solar intensity are shade from trees, clouds, haze, dust, dirt, and those pesky Mud Daubers that decided to build a nest on your panel.
The NREL generates maps that show solar intensity across the US. These are a great resource for determining how much solar power your panels are likely to generate at any given time of the year at any specific location. For the purposes of sizing your system, you’ll want to find the low point in the location where you camp and size based on that.
Putting It All Together – A Real World Example:
Step 1: Determining your usage. I know you probably didn’t need an excuse to go camping, but this would be a good time to do that. When you do, keep something handy to mark down when you use things that require power. Keep a log of the item used and for how long every time you use it. Also, note if it is a DC or AC device. Each day, start a new log so you can determine a daily average. When you return from your trip, go to each of those devices and look up how much power they use. It may be shown on the device’s label. A lot of things may not have such a label. Information on those can usually be found in their owner’s manuals or on the manufacturer’s website.
Power usage is usually given in watts, amps or milliamps. The conversions for these are:
amps = mA/1000 (eg, 500 mA = 500/1000 = 0.5A)
amps = Watts/Volts (eg. 4 Watts running at 12V = 4/12 = .33A)
|4” round LED ceiling lights||4W||4||0.33||1.32||10||10.32|
|RV CO/Propane alarm||24||0.11||2.59||1||2.59|
|Norcold NA8LXF refrigerator running on DC/Propane||24||1.36||32.64||1||32.64|
|Water Heater on Propane||2||2||4||1||4|
|12 V Hair Drier||216W||0.17||18||3.1||1||3.1|
Step 2: Sizing your batteries. Using these numbers, we use 69 amp hours over 24 hour period. Batteries are rated in total available amp hours. You should not drain your batteries more than 50% of their rated capacity, doing so will significantly shorten the life of the battery. Given that, we should have a battery bank no smaller than 138 amp hours. A typical two 6V golf cart battery set-up in series to give you 12V will give you around 225 amp hours of storage. This is the set-up I would choose for this situation since it gives extra capacity to cover the variables discussed previously.
Step 3: Sizing your solar panels. To determine the solar panels we need, we’ll work the 69 amp hours backward. Those 69 amp hours are consumed in 24 hours, while we are replenishing them in 12. To do that we’ll need to generate 5.75 amps (69/12) in capacity to keep things in balance. That works out to be a 69-watt solar panel (5.75amps*12V). With an MPPT controller, I would go with a 75 watt or larger panel depending on where you are in the country and the time of year. With a PWM controller, a 100 watt would be the minimum.
Step 4: Adjust for variables. So far we have a system that will run perpetually in a 24-hour cycle assuming 12 hours of perfect sun. To ensure you don’t run out of power due to one or more cloudy days you can add additional panels and additional battery storage. A typical minimal off-grid system is usually one with at least 200 amp hours of battery storage and between 200 and 400 watts of solar panels. In our example above, 200 watts of panels will recharge the batteries every day with 50% sunshine over 12 hours, 400 watts will do it with 25% sunshine. The key is to have enough storage to last at least 24 hours with no recharge at all. You can add more battery capacity (days without a recharge) to give you more of a safety factor.
So, How Many Solar Panels Do You Need on Your RV?
Solar panels are sold in different sizes. The most common is the 100 watt, although 10, 20, 50, 75, 150, 200, 225, 250, 275, and 300-watt models can all be found. They are generally priced around a $1/watt regardless of size so there’s no discount or penalty for getting bigger panels.
The bigger panels are heavier and their physical size can make it harder to find a location for them on your roof. For these reasons, most people just buy 100-watt panels. So in the example above you would need at least 1. Personally, I would put on at least 2 and if I could afford it I would do 4. We travel a lot all over the country so having the extra capacity never hurts.