Is Your Roof Angle Suitable For Solar Panels?
How does the slope of your roof affect the performance of your solar panels?
The angle that your solar panels are mounted at will affect how much power they provide you.
Generally speaking, (unless your roof is flat) the pitch of the roof on your home is going to be the angle that your solar panels are mounted at. In Australia the standard roof pitches are 18° or 22.5°so your panels will most likely be mounted at one of those angles:
If your roof is totally flat, most installers will offer you a racking system (at extra cost!) to mount your panels at an angle:
Although before you go ahead and order the extra racking, I highly recommend that you people with flat roofs read the blog post I wrote on the cost benefits of extra racking.
Does the amount of tilt matter?
Yes – the tilt of your solar panels will affect how much power they produce because the tilt will affect how much sunlight you capture.
Consider a solar panel flat on the ground that is 1m wide.
When the sun is high in the sky (e.g. at midday in summer), then a 1m wide shaft of sunlight will be completely captured by that solar panel:
Now, if the sun is at an angle of 30° from horizontal, that same 1m wide shaft of light actually is spread out over 2m when it hits the ground:
The flat solar panel, in this example, will only get half the sunlight, and therefore produce half the energy compared to the sun being directly overhead.
The solution in this example is, of course, to tilt the panel by 60° so that it captures all the sunlight:
So what is the best tilt for a solar panel?
The angle of the sun in the sky depends on both the season and your location on the earth:
In winter the sun is lower in the sky. In summer the sun is higher in the sky.
And the closer you live to the equator (i.e. the lower your latitude), the higher the sun is in the sky all year round.
So you may think that calculating the optimum tilt for the panels on your roof is going to be really complicated…
Luckily it is actually dead simple.
The ideal angle for your solar panels (to maximize the power produced over the whole year) is simply the latitude of your location:
So for my house in Adelaide, the ideal solar panel angle is 35° from horizontal.
Why is the perfect solar panel angle simply your latitude?
Because your latitude is the same as the angle of the sun in the sky halfway between midwinter and midsummer. The sun will be about 15 degrees higher in the sky in summer and 15 degrees lower in the sky in winter. So tilting your panels to the midway point will maximize the sun captured throughout the year.
How much power do you lose if your roof is at a different angle to your latitude?
In a perfect world (well my perfect world anyway) all builders would be forced to build roofs with pitches the same as the latitude of the house. However as mentioned before, most roofs in Australia are at 15° or 22.5°.
So how much solar power are you losing if your panels are not at the perfect tilt angle?
Well, it depends on the direction of your roof, the location of your roof and the pitch of your roof. If you want to calculate it yourself then the easiest way is use these tables from the Clean Energy Council. Just pick the table for the town nearest to you and find the power loss:
Let’s use my home in Adelaide as an example. The roof pitch is 15°, which is 20° lower than the ideal angle of 35°.
When you crunch the numbers, the efficiency hit is only 0.7% - almost nothing.
However if my roof was flat the power hit would be 9% lower compared to having the panels at 35°.
So the moral of the story is: if your roof is pitched, your panels might not be at the theoretically perfect tilt angle, but they will be close enough.
If your roof is flat, then it may be worth considering mounting your solar panels on frames. Just make sure that the extra cost of the frames and installation isn’t more that the extra power generated over the life of the system (i.e. 30+ years).