Is Your Roof Angle Suitable For Solar Panels?
The angle that your solar panels are mounted at will affect how much power they give 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 common roof pitches are 15° or 22.5° so your panels will most likely be mounted at one of those angles:
If your roof is totally flat, then I strongly recommend mounting the panels at an angle of at least 10°. This is really important because it will allow any rain to run off the panels. If the rainwater pools on the surface of the solar panel, it is more likely to eventually get through the panels' seals and into the solar cells themselves. If this happens it is game over for the panel. You will get an earth fault and you will need to replace the panel. Many panels have a warranty condition that they must be mounted at at least 10° from horizontal.
Don't listen to anyone that tells you to have the panels horizontal and just add a panel or two to make up for lost efficiency. Horizontal panels are asking for trouble down the line. They will also get a lot dirtier because they will have very little ability to self-clean in the rain, so you'll need to manually clean them more often.
For these reasons, if you have a flat roof, most good installers will add the cost of a tilt frame to their quote.
As I've said, tilt frames on a flat roof are a good idea. But sometimes an installer will propose tilt frames for a roof that already has a small pitch to increase the angle of the panels and get a little more power (for the reasons described in the next section). If your installer is proposing tilt-frames for a non-flat roof, I highly recommend that you read the blog post I wrote on the cost benefits of tilt frames before proceeding.
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 angle 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 2% - 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 at more than 10°, your panels are probably not at their theoretically perfect tilt angle, but they will be close enough.
If your roof is totally flat, then I advise that you mount your solar panels on tilt-frames to avoid water pooling problems and to allow for self cleaning in the rain. And if you are getting tilt-frames, you might as well mount the panels at the perfect tilt angle for your latitude to maximise the power you generate over the course of a year.