How much heat are you getting from your solar panel?

If you have solar panels heating your hot water, do you know how much heat you are getting from them? The reason I ask is that I recently read a report on the performance of hot water panels in the field [1]. The researchers found that most people were very happy with their panels regardless of how much, or how little, solar energy they were getting from them. The proportion of hot water actually supplied by the panels varied from 9% to 98%. I suspect most people don't know what their panels are doing and quite possibly don't have an easy way to find out. I speak from experience here as our hot water system failed back in 2012 and we didn't realise anything was wrong for a couple of months.


I have written before about Optimising your solar water heating. We have programmed the gas boiler to top up the hot water in the tank in the late afternoon when we have got as much heat from the panels as we are going to and before we need it in the evening, for washing up. In the winter time the tank always needs topping up but the days are getting longer now and you should be getting a good half of your hot water from the panels. Unless you have a heat meter, which is very unusual, the only way to tell is to check the temperature of the tank in the late afternoon before the boiler has topped it up. After a sunny day recently ours was 56C, so it needed very little extra to get it to the 60C recommended to kill Legionella bacteria.

You must have some kind of temperature measurement in your tank because it controls the pump from the panels. It will only run the pump if the tank water is cooler than the panels. However, this temperature may not be displayed anywhere for you to see. In our system the central heating thermostat programmer also displays the temperature at the top of the tank and at the level of the solar heating coil. It still isn't easy to see though as this function is buried deep in the menus and you have to enter a password to find it.

Easy or not we do check it from time to time, because of our previous bad experience. This was caused by air in the pipes between the panel and the tank - eventually a large air bubble formed around the pump which stopped it working altogether. The panel was hot but none of this was getting to the tank.

If you haven't got any way to see the water tank temperature then it is worth installing something so you can. If you have a solar water system you most likely have a dual coil hot water tank with integrated insulation which means you can't just feel the tank with your hand. However these tanks come with holes in the insulation where you can insert sensors for the heating controls. If there is a spare hole you can insert a thermometer probe such as  this one. Failing that, you can check the temperature of the pipe between the panel and the tank. You can put a temperature sensor like this one onto the return pipe that goes back to the panel - this is a very good guide to the temperature in the tank when the pump is running.  You need to catch it on a sunny day but before the tank is so hot that the pump automatically shuts off to stop over heating.


[1] Here comes the sun:  a field trial of solar water heating systems (EST 2011)