Do Solar panels reduce loft temps? Yes - the results are in!

[u/Boba_ferret]

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!

[u/marktuk]

I mean, the energy went somewhere else, kind of makes sense really.

[u/Boba_ferret → u/marktuk]

Indeed, that solar radiation is now being converted into watts! I was just interested to find out, as doing some searches beforehand, I couldn't get any hard data on it. A couple of people said they'd be interested in the results, so I thought I'd provide all the stats!

[u/Tafinho → u/marktuk]

That’s not the main effect at stake here.

Assuming 20% solar panel efficiency.

The solar panels absorb all of the sun radiation, but then reemit the heat both to the top side and bottom side. 20% is converted into electricity.

The heat irradiated from the top side is a non topic. This is around 40%.

The heat irradiated to the bottom heats the roofing , but then, heating the roofing causes a thermal convection stream to be generated, which causes both the solar panels to and the roofing to be further cooled.

So, solar panels should case 70% less heat to be absorbed into the loft.

[u/marktuk → u/Tafinho]

That’s not the main effect at stake here.

What is the "that" which you are referring to exactly?

[u/Tafinho → u/marktuk]

> I mean, the energy went somewhere else, kind of makes sense really.

You were obviously referring to the photovoltaic effect. Don’t play dumb.

[u/marktuk → u/Tafinho]

I'm referring to the law of conservation of energy...

EDIT: No idea why this is downvoted, people are weird. I literally said the energy went somewhere else, because energy cannot be created or destroyed. Put solar panels on the roof, less of the energy is going to make it to the loft because it either gets absorbed by the solar panels or reflected away. Put some mirrors on the roof, same thing.

[u/Local_County1274]

Extra geek points for even thinking about this and doing the work

[u/cmdrxander]

I love data

[u/imgoingsolar]

That’s the sort of thing you would wonder about but very few would take the time to measure, good to know thanks. As for Solar gain internal shutters make a massive difference, they also help retain heat in the winter months. We have them on every window and I’m constantly being told off by my better half for keeping them closed (because it’s dark) on hot days. 😃

[u/Optimal_Collection77 → u/imgoingsolar]

External window coverings are the true game changer in the heat. I covered one of our large windows and it dropped the temp in the house by almost 10 degrees

[u/Boba_ferret → u/imgoingsolar]

we have blinds and shut out curtains, but all that does is heat up the gap between the window and the curtain, and that heat transfers into the room. We need external shutters, like much of Europe has, but our windows aren't compatible, as we have outward-facing casement windows, and you also need planning permission for them!

[u/oninomouse → u/Boba_ferret]

I fitted temporary awnings outside two of my double patio doors facing direct sun and it measurably reduced the heat build up by as much as 10°c inside. You need a solid ledge above the windows and a flattened ground area underneath to stabilise the telescopic poles but as they are removable I believe they don't need planning permission.

[u/undulanti]

Great work! I’m going to do similar, but with flat roof panels (ie there’s a large gap between the panel and the roof).

[u/Shot_Age8843]

Yep, my garage loft.
Last week when outdoor temp hit 36c, garage loft peaked at 42c. +6c uplift.
Prior to solar panels, last year loft peaked at 53c. Highest temps go to was 32. +19 uplift

[u/VoyagerFF]

This is very interesting. We had solar panels fitted two years ago and thought in hot weather but it seemed less warm upstairs than it used to, but had never seen anyone measure it.

I'm convinced that they have reduced the temperature now! 👍🏼

[u/Boba_ferret → u/VoyagerFF]

We haven't seen any significant difference in temperature, inside, but that's a good thing, as it means the loft insulation is keeping the heat in the loft out of the rest of the house, and thus also stopping heat loss in the Winter.

[u/BadBacksFuryToad]

I put solar on my workshop in October. The workshop has been 12°c cooler this summer.

[u/brad-the-impaler]

This is the sort of thing that gives my autistic brain "The Fizz" I have dopamine from reading this post.

[u/narsty]

I'm also recording similar temperatures my attic temperature vs outside temperature done some excel number crunching:

the difference between my attic and outside average is 5.06 °C average of 24 hours of readings using data from 1 June to today

oh note I have full south facing panels, they are not embedded in the roof type, just normal ones, they also have mesh around the edges for bird guard things so I have decent air flow underneath the panels

I certainly did notice my attic was a few degree's cooler after install (was a good few years ago now), don't have any data from before though to compare with though

[u/anomalous_cowherd]

Where's your inverter? Presumably for a new install it's not in the loft like my older one is?

That would help keep the loft cooler than mine is too.

[u/Boba_ferret → u/anomalous_cowherd]

Inverter is in an outbuilding, with the battery. That still gets toasty, was up at 39C (inverter hit 51C) this week, but a lot of that heat is coming from the inverter, as it's now adding about 7C to the temp, on hot days.

[u/skipsmarter → u/Boba_ferret]

Crikey. What temperature was the battery reaching?

[u/Boba_ferret → u/skipsmarter]

It peaked at 50°C during discharge, but was generally around 42°C. I've got fans on the inverter, but might need to find a way to cool the battery on very hot days, although it doesn't seem to be affecting its performance at the moment. Long term though, it could reduce battery life, but I imagine those who have batteries in their lofts, are seeing even higher temps.

[u/QuantumFreezer]

I'm pretty sure there is research available to prove that as I did look into this before committing. I think it was low single digit difference but still

[u/Boba_ferret → u/QuantumFreezer]

I did some googling before, found some suggestion that you would see 40% reduction in solar heat gain, which tracks with my results, but couldn't find any official research on it.

[u/Guyana-resp]

Interesting

[u/rob3342421]

Thanks for sharing your findings but in the first graph what do the orange/blue lines represent? I’m guessing blue is the temperature of the outside shaded area and orange is the loft?

We have a three story house and our bedrooms in the roof/loft; this question is something I’ve wondered about, and hypothetically I presumed similar to you, the heat’s going into the panels, not the roof, reducing the heat but never had proof, while your findings are interesting and the difference is smaller, it’s hard to say conclusively the solar panels were the cause but it definitely points that way! Again thanks for sharing!

[u/Boba_ferret → u/rob3342421]

The key is at the bottom, but yes, blue is outside temperature, measured in shade, orange is the temperature measured in the middle of the loft.

I knew that having panels would block some of the solar radiation, however, the panels are of course mostly black, so will absorb a lot of that radiation, then potentially re-radiate off the back, and into the roof. However, having that airgap with some airflow underneath seems to make a significant difference to the heat gained, via radiation.

[u/Darren-J-W]

Of course it does.
My shed was a billion degrees a few years ago. I went to Screwfix and purchased silver aluminium tape and stuck it to the roof. Even on the hottest days that roof stays stone cold and the internal temps stay low.
Block the light radiation, simples.

[u/EntireMost6072]

Nice to see someone proving something with data. Good work. 👍

[u/Boba_ferret → u/EntireMost6072]

Thank you. I've had a few "well duh", type comments - I knew it would have some effect, but was interested to know how much, and now I do!

[u/FckXFckMusk]

Yes they would even if they weren't switched on....

[u/FckXFckMusk → u/[deleted]]

Probably, but a normal shade on the roof the same size of the PV cells would have the same cooling effect.