Angle of solar panels on flat roof

We just had a solar panel install in the United Kingdom (latitude of 57 degrees). The contractor quoted us for panels installed at a standard 35 degree angle. However, the actual install was panels at a 10 degree angle (i.e. nearly flat).

The panels are installed on a flat roof, not on a pitched roof. The contractor claims that 35 degrees is standard for a pitched roof, but on a flat roof fitting at a 10 degree angle is standard.

I know that the panels will harvest less energy because of the smaller angle. My questions are:

• Is the contractor telling the truth?
• Is the difference in capacity substantive? Is this worth arguing about?

Thank you.

Update:

Many thanks to @juhist for the clear answer. Roof space is limited. The panels are set up in two rows (front row and rear row) --- so close together that there is some shading of the rear row even at a 10 degree angle (in the UK in winter; during other seasons I would expect no shading).

The contractor could raise the rear row by 40cm by adding a platform. This would eliminate shading and perhaps permit raising the panels to an angle of 20-25 degrees. (35 degrees is not possible.)

My other half says to leave it be. My mathematician head says to optimise. Comments welcome.

Oh yes it could be worth arguing about, but only if you have huge roof area and very few panels. With roof fully covered by panels, they should be flat.

Generally, if production is limited by panel count and unlimited by land/roof area, you'd want panels to be installed at an angle equal to your latitude and facing south. So at latitude of 57 degrees, you'd want 57 degrees. But if the panels start shading each other, then most of your production will be destroyed so you want to avoid that. Also quite few roofs are at a 57 degree angle.

If your goal is to even out power production so that you get less power during summer and more power during winter, even greater angles are justified. So 75 degrees could actually be very good if you have lots of panels, because then you have to sell less electricity during summer to the grid, and get more electricity during winter. However, the problem is that approximately no roof is at 75 degree angle -- usually roofs are at a smaller angle than 57 degrees too. Also at 75 degree angle shading during winter is really bad so that destroys the theory that 75 degree angle could be good, unless you have infinite amount of land. Furthermore, if there are trees nearby they'll anyway destroy winter production, so optimizing for winter production might not make sense given the site where the panels are installed.

So it's worth asking what is the expensive thing here -- is it the panels, or is it the roof and installation work? Usually roofs and installation work cost a lot, electricity is expensive but panels are cheap as dirt (because they are made out of dirt i.e. silicon), so you probably want to use your entire roof. Tilting the panels mean there are shaded areas, which you can't cover with panels, so actually it turns out to be the case that if your entire roof is fitted with panels, they could as well be installed flat on the roof at the natural angle of the roof.

So the big question is: is your entire roof covered with panels (if it is, then don't start arguing because you won't get single bit more power by tilting the panels but actually less due to shading), or do you have a partial installation? If the installation only covers partially the roof, and if the quote says 35 degrees, then I would demand them to re-do it at 35 degrees, if shading can be prevented. But generally it can be prevented only if you have a very big roof and only very little panels.

I have a program I can use to simulate production. At latitude of 57 degrees and panel rotation from zenith of 35 degrees (panels facing south), you get 979 full power hours worth of production. At angle of 10 degrees, you get 814 full power hours worth or production (17% less production, assuming no shading occurs in either case). That's assuming panel count is the limiting factor, and that no shading occurs. If you have so many panels that installing at 35 degree angle would mean some panels shade other panels, then don't start arguing. The theoretical production in that case wouldn't increase and the practical production would actually decrease, because shading even part of panels cause your production to be at few percent of what it could be. A partially shaded panels is actually about as bad as a fully shaded panel.

So, it depends on your roof size, your panel count and whether the panels would be shaded.

Actually during winter panel shading could be worse, so the 75 degree angle I suggested is worth it only if you have infinite amount of land but only small amount of panels. With a finite/small amount of land, you generally don't want to install panels at 75 degrees.

It might also make sense to think about future. So, if for example you covered only 10% of your roof with panels, if you now install them at an angle, later you might expand your system to cover 100% of your roof (or at least the areas of roof facing south), and if you could realistically do that then removing the 10% panels that are at an angle and installing them flat could cost a lot of money. Having them flat allows a cheaper pathway to covering 100% of your roof with panels.

My opinion is that the contractor isn't telling the truth. They might have done the right thing (if the entire roof is covered with panels then the panels should be flat to prevent shading), but they didn't say that and they didn't say anything about shading, but they said only about "standard practice". If that contractor can't explain why it's standard practice (shading), it could be a clueless contractor -- that still might have done the right thing, however.

Edit: I seemed to assume the roof is at 10 degrees. Now I read it's flat and the panels are at 10 degrees. That is probably still the right thing, however. At 10 degrees there's very little shading. If panels were completely free but roofs hugely expensive, then you would want panels at 0 degrees, but panels aren't completely free so given that, 10 degree angle sounds actually quite good -- if nearly the entire roof is covered by panels. If you have a huge roof but very few panels, you probably want even bigger angles because then you can add lots of spacing between panels to prevent shading.

Have a look at PVGIS which is a nice EU-funded tool (not all money is wasted !) that allows you to simulate production for any location in the world.

You can then compare your decrease in output between the 10° install and the 35° install.

A couple of points to note :

1. Wind : This is the main reason why installers go for lower profile installs on flat roofs. The higher the angle of the panel, the bigger the sail you are creating for the wind and my oh my wind will do damage. So on a flat roof, I would agree to go with low angle panels, unless you ballast the hell out of it which can bring it's own problems.

2. Impact on seasonal production : With a lower angle, you'll be producing a larger proportion of your annual usage in the summer half of the year. This means that your proportion of elec exported to the grid will increase if you compare with an installation that is optimised for winter or year-round production.

So I don't think the installer is lying to you, flat roofs really should have almost flat panels even if that has an impact on seasonal/overall production.