How do you size a photovoltaic system?

We thought we'd do a series of videos on the most important elements of an offgrid camper system, from how we size the system, to what batteries, panels, controller and adjacent items are worth buying. Let's look at the electrical system as a whole and learn how to correctly size the elements, in a simple version with panel, battery, controller, consumers.

The most frequently asked question on camper or caravan groups goes something like this: "If I put in an x watt panel and a y amp battery, will that be enough for a weekend?"

And I come and ask... enough to what? Do you want to keep two phones charged and a light or do you want to charge electric bikes, watch TV and sweat? Any offgrid or offcamping system must be thought from the consumers and time of use, never from the resources.

The calculation is not difficult, but there are a few things to consider: how much does each appliance consume? Basically it says on them, most of them are 12V, but you will have the consumption expressed in amps or watt hours. So, if the fridge says 3 amps, multiply 3 amps by 12 volts to find out how many watts it consumes, i.e. 36wh. That is, for 24-hour operation, you get 24 hours times 36wh, 864wh per day. So to keep a fridge going for 2 weekend days, you need 1.7kwh of energy or just over 140 amp hours.

Clearly, not all equipment works consistently. If you have an LED strip that is only used for 4 hours at night and consumes 1 amp hour, or 12 wh, that's 4 hours times 1 amp times 2 days times 12 volts, 96wh or 8 amp hours, depending on how you want to put it. But, as a rule, you have to take them all and make a realistic calculation and in no case an optimistic one, in the "let me not consume so much" way. You usually consume, and sometimes even more, and there's nothing worse than running out of power on holiday.

Now, you've approximated for fridge, lights, blender, inverter, TV, phones, water pump and everything else around the camper and you'll use and let's assume for ease that we got 2500w consumed in 2 days, hypothetically. Moving on to the next step, let's see where we get the power from and how we put it back. I'll do another episode on the different types of batteries and the differences between them, but for this case, let's assume we're working with a cheap gel battery variant, the cheapest possible. Gel batteries can be discharged up to 50% of their capacity, so if you have a 100Ah or 1280wh battery, you can actually discharge half of it so you have 50Ah usable or 640wh. If you go over once it's not really a drama, but repeated discharges below this percentage will lead to premature battery damage. So, if I want to have power for the whole weekend, I would need about 400Ah or 5120wh from which to draw constant power. But there is also the aspect of reloading.

And this is where the panel comes in. There's a lot to talk about panels and I'll discuss that part separately but, to keep things simple, we'll take a standard 300w farm panel as a reference. With an average of 3-6 hours of actual production, in my experience this panel charges somewhere between 1000 and 1500wh in a day. Sure, it can vary if there are clouds, if it's too hot, if the panel is clear or at a favorable angle. But if we were to take an annual average, I'd say 1280wh, for easy math. So, on a good day, a 300w panel can produce enough to compensate for what you consume. In the cheapest version then, for a super basic weekend setup, you would need a 300w panel, 2 batteries of 100Ah or 1280wh each, so you get close to 50% consumption and a MPPT solar controller.

The example is rudimentary to present the calculation method. In principle, in a better system you will also have bluetooth control for example, to see what you consume, how much you charge and approximately how many hours your system can run in its current mode. As we've been talking about, in our Sprinter 4x4 we have 320Ah lithium from Victron, a Smart Solar 100/30 with bluetooth and a single 300w panel. In general, if we keep it clean, it will provide us with power without much drama throughout the summer. Of course, this is where the lithium battery bank and the rest of the components come in, but that's a story for another episode.

So let's recap how we think about a system for a caravan or motorhome: Step 1, we see what consumers we have. Step 2, we estimate how long these consumers stay on. Step 3, we calculate about how much a battery bank should be to hold our system without charging. Step 4, we see how many solar panels and what kind we can put to compensate for consumption. Step 5, we put everything on paper and see what results. Finally, let me give you the most common option that works for a good part of the cases: a 300w panel or larger if you can find it, a correctly sized MPPT such as a Victron Smart Solar 100/30 and a 220Ah gel battery, also from Victron Energy. This was my first sprinter setup and I did very ok with it 2 summers.

In future articles we will talk about types of panels and types of batteries. There is a lot to tell so each one will have its own dedicated article. So if you want to be as clear as possible how to build a PV camper system and what is worth investing in when it comes to components, follow the blog and sign up for the newsletter.