During our time in Betws y Coed, sitting under tree cover and in rain for a couple of days, our solar panels failed to charge the Leisure battery. This is the first time it has happened as usually the panels are charging our battery even on overcast days.
When we were researching our solar panels we did a bit of investigation to determine how many/what size of panel we would need. This meant dredging up some of my GCSE physics, with that equation Watts = Volts x Amps
The aim is to keep our batteries topped up through various means, preferably our solar panels as – once paid for and installed – it’s ‘free’. Fully draining a battery is not good for it, and any other way of topping up the battery has a unit cost.
Our leisure battery is a 95 AH (Amp Hour) 12 volt supply. So. as an example, a 120W appliance would use 10 amps per hour and drain the battery in 9.5 hours. Assuming everything works efficiently – which, of course, it never does.
When looking at our solar panel requirements we had to ascertain which things use electricity. This includes obvious electrical stuff, and some things we had thought of as gas appliances. We’d be interested to see if anyone can spot anything we’ve missed:
Water Pump: Used for showering and washing and making drinks (unlike some people we do drink the water from our fresh water tank – we have had a look inside, seen the bits that float around in it, and decided it’s not going to hurt us. Because we full time it’s always being emptied and replenished so stays fresher), it’s on for less than half an hour per day.
Fridge: Although our leisure battery isn’t used for the actual cooling operation, it is used for the ignition and evaporator.
Lights: All our lights have been replaced with LEDs which typically are 3 watts or less. In the summer we don’t have the lights on much, but we’ll use them more with the longer nights of winter.
Phone chargers: We don’t tend to charge our phones directly from the leisure battery. This is because typically we want to charge our phones overnight while we are sleeping (the rest of the time we are glued to them of course, like any modern person). So instead we charge a power bank during the day and use it to charge our phones overnight.
Tablet, Laptop and Kindle chargers: These aren’t used every day. The laptop is particularly power hungry at 65 watts, but the tablet and kindle are both about 10 watts.
Boiler: Gas is used to heat the water, but electricity is required for ignition and the control for dumping water when the weather is cold (this stops the tank from splitting due to icing up).
Radio: We use a battery operated radio so that we can take it outside with us and listen without blasting everyone in a campsite with our music choices. Currently we don’t use rechargeable batteries, but I plan to change this.
Toilet: Strange as it may seem, our toilet does use a small electricity supply. This controls the indicator that tells us when the cassette is nearly full.
We think that, taking into account inefficiencies and other electrical use (for example the USB adaptors that fit into out 12v sockets all have LEDs which consume electricity), we will use approx. 200 watt hours of power per day. This should equate to about 16 Amp Hours from the battery (200 watts divided by 12 volts).
We have 200 Watts of solar panels on the roof. These will also have their inefficiencies. They will not be directly angled towards the sun as they are installed flat on the motorhome roof. And of course we never see perfectly blue sky. The maximum Amp hours we have seen from our solar panels so far is 7, but it has to be said that we would never be in the motorhome at midday on a lovely summers day. 13 Amp hours is reportedly the best we would get at peak efficiency (200 Watts, for a 12 volt battery, at 80% efficiency i.e. 200/12 * 0.8).
With our observations so far, a good summers day would give us maybe 60 Amp Hours of charge, far in excess of our usage. But a bad day may give us less than 10 Amp hours, which would see our batteries start to empty pretty quickly. An average day will probably be around 20 Amp Hours per day from our solar panels. So on average we should break even. Of course that would be fine if we had a battery of infinite capacity, but we don’t. So if we have a run of bad days our battery will be depleted and that is not good for it, or for our gadget usage.
Of course we have ways of mitigating the bad days. We could drive a long distance – charging our leisure battery through the alternator but using (and paying for) diesel in the process or we could pay for electric hook up, and charge our leisure battery through a mains supply – almost certainly we will use one or both of these options at times. We could get more solar panels, or we could buy a generator (and the diesel to power it) – but these options feel impractical or unreasonable, we don’t have the room for more solar panels on the roof and a generator is expensive and heavy. On top of these options we could also buy additional batteries so that we have more Amp hours use before we drain them – something we probably will do so that we can last longer before resorting to one of the other methods. We need to give all of these options some thought and do some cost benefit analysis. We’ll let you know what we decided. Until then we’re hoping that we have sunny days, and we’re making sure we don’t park under trees on the bad days.