How to size the batteries for your RV/Camper Solar system
This is part one of our series on sizing a solar system for your RV or camper. It will cover how to size your batteries when installing a new system.
If you already have a battery setup in your camper you can consider skipping this article and moving on to the next article in this series, which covers how to size the solar panels.
This article is broken down into the following three steps:
- Estimate your daily electricity usage
- Decide how many days of autonomy you want in the batteries
- Size the batteries
Estimate your daily electricity usage
The first step you need to take is to calculate how much electricity you expect to use in your camper/RV on an average day.
A few important notes before we begin:
When completing this step don’t forget to also include any future electrical loads you may be planning to add.
If you don’t already have a good understanding of DC and AC electricity, Amps (A), Watts (W), and Volts (V), then it would be a good idea to take a look at our Basics of Electricity for Solar article to brush up on these topics.
We will calculate our load based on Watt-hours (Wh). You may see other methods that calculate the loads based on Amp-hours (Ah), and you can do this if you prefer. Each method is essentially the same, but using Wh is slightly easier, as you don’t have to worry about the different voltages of DC and AC equipment.
To start, you need to list out all of your electrical loads and calculate how many watt-hours per day each one is likely to use.
Watt-hours per day, as the name suggests, is calculated simply by finding how many watts a device uses, and multiplying that by the number of hours per day on average that you expect to use that device.
For example, assume you have six 2W downlights in your camper, and you use them on average for 4 hours per day. Then your daily usage for these lights would be:
6 x 2W x 4h = 48Wh
Watts can usually be found either on the device itself or on a tag attached to the cable.
Some devices don’t show the number of watts they use and instead give voltage and amps. In this case, watts is easily found by multiplying voltage x amps.
For example, if you have a phone charger that uses 2A at 5V then the watts would be:
2A x 5V = 10W
For some devices, it can be harder to estimate the hours used per day. A DC fridge for example may say that it uses 3 amps, however, this is only when it is actively cooling. In reality, it may only actively cool for 30% of the time, so to get your daily usage hours you would have to multiply 24 hours by 0.3 to get 6 hours.
Since fridge usage depends on the model, the temperature it is set at, the ambient temperature, and other factors, it is best if you are able to test the fridge yourself or check online for typical numbers for your fridge. If this isn’t possible, then using 40% running time is a reasonable first estimate.
Once you have a list of all your electrical appliances, along with their average daily Wh used, you simply add them all up to get an estimate of the total Wh you typically use in one day.
Depending on how you feel about the accuracy of your estimates you may like to add a buffer of 10-20% to give you the flexibility to use more electricity than you have estimated.
Also, after you go through the next steps, think about how you came up with your estimates. If they were quite generous, and you end up sizing a large expensive system that you can’t afford, consider playing with the numbers and see if you might be able to get away with a smaller system if you are more conservative with your energy use.
Decide how many days of autonomy you want
Now that you know how much energy you use in a typical day, the next step is to decide how many days you would like your batteries to last when they are not being recharged by the solar panels due to bad weather. This is called days of autonomy.
There is no set value to use here, and you will have to determine what is most appropriate for your needs and budget (batteries are expensive).
A fairly common rule of thumb is 2 days of autonomy, this would be sufficient for the average person’s needs and is often a reasonable balance between cost and comfort.
Many cost-conscious people size their batteries only for one day of autonomy, perhaps with a 20% buffer, and rely on alternator charging or a generator to get them through bad weather. Others like to be able to go off-grid for extended periods in all weather and have large systems of 3 or 4 days of autonomy.
To help you decide what is right for your situation, here are some examples of when you may want more or fewer days of autonomy:
When you might want fewer days of autonomy:
- You rarely go off-grid, typically camp at RV parks, and have a system where you can plug into the mains to charge.
- You are doing a long overland trip, which involves lots of daily driving, and you will be charging the battery from the alternator.
- You only plan to use your camper in perpetually sunny areas, for example in some southern states in the US.
- You plan to take a portable generator to get you through cloudy days.
- You are on a tight budget
When you might want more days of autonomy:
- You like to get out in the wilderness for days on end in remote locations.
- You expect to be in areas where long periods of bad weather are not unusual.
- You have a large budget.
Keep in mind that you can also change your energy usage to match the weather conditions. If you design your system for 2 days of autonomy, you may be able to stretch that to 3 or more, if you see several days of bad weather in the forecast, and then reduce non-essential usage of your electrical appliances.
Sizing the batteries
Once you have decided how many days of autonomy you need, you are ready to size the batteries.
First, since batteries are rated in Ah, you need to convert the Wh value you calculated in the previous step. To do this you simply divide it by your system voltage (typically 12V unless you have a very large system).
Next, multiply this Ah number by your days of autonomy, to find the total battery storage capacity you need.
For example, if you calculated that you use 1200Wh in a typical day, and you want to have 2 days of autonomy, the equation would be:
Battery storage needed = 1200Wh / 12V x 2 = 200Ah
Now, before you go out and buy a battery that matches your Ah number. Be aware that Lead Acid batteries should only be discharged to around 50% of their maximum capacity. Regularly discharging them lower than this will reduce their lifespan. This includes both Absorbed Glass Matt (AGM) and Flooded Lead Acid (FLA) (not recommended) batteries.
Lithium batteries, on the other hand, can be discharged almost fully without reducing their lifespan.
Therefore, if you have calculated that you need 100Ah of battery storage, you should purchase either:
- 100Ah of Lithium batteries
- Or 200Ah of AGM batteries
Usually, your required Ah will not perfectly match the available battery sizes, in this case, it is generally better to round up to be on the safe side. Although if you want to save money and think you can get by with slightly less you can also consider rounding down.