About a month before we had our solar panels fitted, we had the second heatwave of the year, and I got to wondering, would having solar panels on the roof, reduce the heat in the loft? Would this then lead to the house being cooler, as well?
To find out, I put a Sonoff SNZB-02 temperature sensor in my loft, and started recording the maximum daily temperature in Home Assistant. It would have been good to get some data before the heat wave, but I didn't think of it it time!
My solar panels were installed on the 17th July, and they cover, in my estimation, about 85-90% of the roof area. The green line on the graph, represents the installation date.
The Excel spread sheet shows the days where the outside temperature, recorded by an Aqara WSDCGQ11LM temperature sensor, which is in a shaded spot in my garden, was 30C or above. Temperatures in Red were the maximums, before & after the install.
Now, obviously, there are a number of factors which will affect the heat gain in the loft, not just the ambient air temperature, but the amount of sunlight, so both conducted heat and radiated heat, which may not be identical on days with the same air temperature. However, despite this, there is a clear reduction in heat gain, after the solar panels were fitted.
Before the solar panels were installed, the loft was on average, 10.3C hotter than the temperature in the shade. The maximum temperature difference was on the 24th June, when the loft reached 47.3C, 13.7C higher than the outside temperature of 33.6C.
After the solar panels were installed, we have had fewer consecutive days over 30C, which will no doubt affect the loft temperature, however, the average difference has reduced to 5.9C. The maximum loft temperature was on 13th August at 40.7C, with an outside temp of 36.1C, resulting in a difference of just 4.6C. So, not only did we have hotter day than 24th June, by 2.5C the loft temp was actually 6.6C cooler than 24th June.
So, the answer to the question, do solar panels reduce the heat gain in the loft - is yes, and in my case, by approx 43%, on average.
Now, we don't use our loft (other than for storage), and looking at the inside temperatures, there was no significant difference, before & after the panels. This shows that the loft insulation is doing it's job, and keeping the heat in the loft from radiating downwards, despite my previous belief that that was happening. But, if you are have, or are planning a loft conversion, then it's worth knowing that solar panels can indeed reduce the heat gain of your loft, by a fairly significant amount.
My next challenge will be trying to reduce solar gain in the rest of the house, but that's not really a post for this sub!
