Solar hot water systems usually have an electric back-up heater on a thermostat, for times of prolonged overcast weather. How crazy if this is set to default, heating your water every night so you hardly need to use solar!
This is a bit of a bug-bear I have. When we were deciding which system to buy, we walked around our local area asking neighbours about their solar hot water and whether they were happy with their choice. We also got quotes from all the distributors and heard their ‘spiel’. It was a bit of a shock to realise that many people are underutilizing their solar collectors and quite unaware.
Some systems don’t even have a manual switch, so the booster is always ready to kick in automatically. In my opinion, these are the brands to avoid.
Where there is a switch, I gather many people follow the seller’s advice. The sales reps recommend to leave the booster heater on and forget about it. Of course, then they have happy customers who report, “We never run out of hot water!”
However, if your peak demand for hot water is in the evening, for washing up and showers, the thermostat in the hot water system will register a drop in temperature and trigger the booster heater to come on and heat the water overnight. When the sun comes out next morning the water is still hot so the solar collectors are hardly used!
Instead it is more efficient to leave the booster off and switch it on manually only on grey days if required. When it has been raining all day and you run the tap and find the water isn’t very hot, then turn on the switch. Ours takes about half an hour to heat enough.