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Solar Panels – Watts it all about?

If you’ve looked into solar panels for your caravan or motorhome you may well already be confused at the different units that are used, leisure batteries in Amp hours (Ah), devices rated in Watts (W) and solar panels rates in Watts peak (Wp). So how do you square this circle and work out what size solar panel you need?

First let’s sort out these units:

Power is measured in Watts (W) = Current in Amps (A) x Voltage in Volts(V)
Energy in Watt hours (Wh) = Power (W) x Time in hours(h)
Battery capacity in Amp hours (Ah) = Energy (Wh) / Voltage (V)

So let’s look at what happens if you run a 60W light bulb for 3 hours: That means is uses 60 x 3 = 180Wh of energy.  That is 180/12 = 15Ah. Compare that to a leisure battery of 100Ah and that’s well within its capacity.

Daily Power Requirements

To get an idea of what your daily energy use is when camping you need to repeat this for each electrical device you use – Watts times hours. This will give you a rough total to work with.

Item Power(W) Time(h) Energy(Wh) Amp hours(Ah)
Lights 30 3 90 7.5
TV 50 4 150 12.5
Water pump 40 0.25 10 0.8
Laptop 50 2 100 8.3
TOTAL     350Wh 29.1Ah

In the above example the daily energy consumption is 350Wh. Now let’s look at how solar panels are rated.

Solar Panel Ratings

To make comparing solar panels simpler they are rated by peak power generated under prescribed standard conditions known as ‘Standard Test Conditions’ (STC).  This rating is usually written as Wp – Watts peak. Of course this power will only be produced during times when they are fully lit by the sun, in the UK on an average summer day this is about 6-7 hours equivalent of STC.  Using these two figures we can calculate the energy produced per day, Watts x hours. So a 100Wp panel could be expected to generate 600-700Wh on an average summer’s day. In the winter, with low sun and shorter days we may get less than one hour of STC illumination, so the same panel would only produce about 100Wh.

Now let’s go back to our example above of 350Wh per day energy drawn from your leisure battery. To restore this power (in the summer months) you would need 350(Wh) / 6(h) = 58.3W rated panel. So in practive this would mean a minimum of a 60Wp panel. Of course if you are a 3 or 4 season camper you would need to increase the panel rating if you wanted to be self-sufficient year round.

We hope this helps you in selecting the right solar panel for your needs. We are happy to advise you on all aspects of solar panels for your caravan or motorhome, if you have any questions please email us at .