Sizing batteries and solar panels

One of the first steps in planning the electrical system is figuring out what size battery and solar panels we need.

That depends on a few key factors, which will vary for everyone. In our case, they are:

  • Price: we are price-sensitive!
  • Level of luxury / number of appliances: we don’t want a TV, microwave, Air-condioning, hot water, etc. So our electricity needs are quite modest
  • Dependence on electricity: at the moment, we’d like to mix our fuel usage, so gas will be a big part of the mix: mostly for cooking and heating
  • How close to civilisation: we want to go wild, not stay in campsites with hookup power. That means we want to go a long time on just the battery and solar power.

Sprinter Adventure Van have – as ever – a very useful post detailing how they broke down their power requirements and sized their battery accordingly: Calculating battery size. You should read that now, and then come back. Everything that follows builds on their approach, with a few adaptations.

Calculating Usage

Our power requirements fall into 2 categories. Appliances and fittings that run off the van’s 12V power, and battery-powered devices that need charging.

(An aside on AC power: All of these are DC, low voltage devices, so I don’t want to have any 240V AC power in the van. It doesn’t make any sense to invert 12V DC up to 240V AC, and then plug transformers into those sockets to bring it back to 5-20V DC. Instead, we’ll just have a load of 5V USB and 12V cigar-lighter sockets in the van. If we’re near civilisation, and want AC for something, we can always run an extension cable.)

Given these two different categories, it makes sense to calculate usage in two different ways. For 12V appliances, use the ‘power draw x daily usage’ method detailed by Sprinter Adventure Van. For charging batteries, use a calculation based on the size of the battery and the number of times it needs to be charged.

That gives us two tables for usage:

Usage: 12V Appliances

 Power draw on 12V (A)Daily usage (Hours)Daily power draw (Ah)
Total power draw for 12V appliances23.15
Fridge2.1816.8
LED strip0.821.6
3 LED task lights0.0830.75
Fan224

Usage: Battery charging

 Battery capacity (Ah)Charge efficiencyCharges per weekNumber of devicesDaily power draw (Ah)
Total power draw for battery charging14.36
12V Laptop charger680%212.14
USB iPhone charger280%725.00
USB iPad charger7.380%225.21
Other devices2.00

In total then, our daily power draw, given these assumptions, is 37.51Ah

Solar recharge

My method for calculating the amount of power we can get back into the battery differs slightly from the method they use on Sprinter Adventure Van.

We have to make assumptions about how much sunlight the panels get. I used this information from The Camping and Caravanning Club:

Solar panels sold in the UK will be marked with a power rating, which shows the power the panel will produce under prescribed standard test conditions (also known as STC).

On an average summer day in the UK, you could expect the equivalent of about 6-7 hours of STC illumination on a panel pointing towards the sun. Hence … a 100W panel will produce 600-700Wh. … In winter, however, you could be as low as 0.8 STC-equivalent hours a day, so that same 100W will only generate 80Wh.

The upshot is you get wildly different figures for summer and winter, so need to make different assumptions to fit.

In these calculations, I’ve doubled the size of the panel in winter. My premise is that we can fit one 100W panel now, and see how it goes. If, as expected, it doesn’t recharge the battery enough in winter, we can fit another 100W panel.

As the table below shows, in summer, we can expect a surplus of energy from the solar panel (42.5Ah, compared to our requirement of 37.5Ah), but in winter, even with an extra panel fitted, we’d still need a huge 300Ah battery, if we want to last 7 days without power from hookup or the alternator.

 SummerWinter
Battery size needed (Ah)n/a327
Hours of sunlight per day61
Panel size (Watts)100200
Efficiency of panel85%85%
Energy generated per day (Ah)42.514.17
Daily usage (Ah)37.5137.51
Daily shortfall (Ah)-4.9923.34
Battery size (Ah)100100
Available charge of battery50%50%
Number of days between hookups77

It’s worth noting these figures are pessimistic. Sunstore claim that, “in the UK summer”  their 100W panel will “charge your battery around 70 Ah per day”. A lot more than my summer figure 42.5Ah. And they might be right.