Hansen Pole Buildings’ Designer Rachel asked me today what the best solar panel roof slope?

There are three main factors which go in to the calculation of how much sun a roof receives. Roof angle or pitch, roof orientation (how south-facing a roof is) and location. Since Rachel asked about roof slope, I will focus on this one aspect.

There is an optimum angle for solar panels on a roof. This is the angle where, over the course of the whole year, the panels get the most amount of direct sunlight which produces the greatest amount of electricity.

The question is: what is the perfect angle, and if the roof is not at exactly this angle how much energy will be missed out on? I’ve arbitrarily picked a location of 49 degrees north (generally the United States – Canadian border in the western U.S.) and a south-facing roof for this example.

So for this roof the optimum angle is 35° (a bit over an 8/12 roof slope) and at this angle the roof generates 1,265kWh/m2/day. The energy reduces as we move away from this optimum angle. At 30° the energy drops to 1,260kWh/m2/year – a reduction of just 0.4%. At 20° (15° away from the optimum angle) the energy drops to 1,229kWh/m2/day – a reduction of 2.6%. It’s only with very steep roofs – say over 60° – when the solar energy really drops off.

If you want to see how this works for your roof you can create a free report at:  https://www.solstats.com/ – take a look at the bottom of the report and you’ll see a version of the chart above for your location. WARNING: Entering your zip code will NOT take you to United States locations, it takes scrolling across the world map on the website.

It is obviously preferable to have a roof at the perfect angle. While I am not a fan of government subsidies, they are very generous when it comes to solar energy. Including them in the calculation, it absolutely makes sense to install on roofs which are not at the perfect angle. You’ll lose a few percent of solar energy but it should still make financial sense.