Installing solar is a significant investment. Whether you’re a homeowner looking to spend a few thousand dollars, a business looking at a system in the tens of thousands, or a solar farm investing millions, you want to get the most out of your investment.
Tilting your solar panels at different angles can have a big impact on energy output and therefore financial return.
How Big a Deal is Tilt Angle?
It’s pretty common knowledge that the output from a solar panel varies with tilt and direction. But just how big an effect is this? Does it drastically change the economics or is it only a minor benefit?
To answer this question we modelled a flat roof in Phoenix, Arizona, using Aurora Solar software. We started with flat panels and increased the angle of tilt to the south to see how much extra energy is gained through the year. A rule of thumb that seems to have spread around is that the optimal tilt angle is about equal to the degree of latitude of the location. Therefore we include a result at a tilt of 33.4 degrees, the latitude of Phoenix. Results are shown in the graph below.
Tilting the panels significantly increases energy output. The maximum output, at 30 degrees tilt, is 14% higher than the energy output of flat panels. Over the 25 year life of the panels, that’s a lot of energy. Therefore with fairly flat roofs tilting should be seriously considered.
However if you have a roof that already has a 20 degree pitch to the south, you’re only going to get an extra ~1.5% by using an extra 10 degrees of tilt to get to a total of 30 degrees. The small increase in energy output is probably not worth the extra expense: there is more hardware to buy to tilt panels, and more labour as well.
The graph shows that the optimal angle was pretty close to the latitude of Phoenix (33.4 degrees), but not exactly at this angle. Therefore using the latitude as the angle of tilt is only an approximate optimization.
How does this change if we move further away from the equator, to say Boston? Boston is about 42.4 degrees North.
The first thing to notice is that the energy output is a lot less than Phoenix. At the maximum point (35 degrees tilt), in Boston you can expect about 7,150 kWh/year, whereas in Phoenix the maximum was 9,600 kWh per year. This is due to Boston being further from the equator and also probably having more cloudy days.
Compared to flat panels, panels tilted at 35 degrees had 19% more energy output, so tilting had an even more positive effect than for Phoenix. Also the maximum point is further away from the value of latitude for Boston.
Again though, if you had a roof with a 20 degree pitch, the extra energy gained from tilting another 15 degrees might not be worth the extra cost.
What About East and West Facing Roofs?
The above results were for south-facing roofs which are the best for solar performance. However, what happens if your roof has East and/or West faces instead of south?
We’ve repeated the previous results for Phoenix, with east- and west-facing panels. The results are shown in the graph below.
Surprisingly, tilt actually makes things worst for both east and west-facing panels. Flat panels give the most energy output. However flat panels require more cleaning maintenance, as water doesn’t run off well and therefore the panels don’t ‘self-clean’. (Thankfully there are a range of inexpensive cleaning products for solar panels which have proved highly effective and easy to use.) About 10 degrees tilt is often recommended to give good self-cleaning. Looking at the graph again, the energy doesn’t drop off much at 10 degrees of tilt so 10 degrees of tilt seems a good option for east- and west-facing panels.
A common belief is that north-facing roofs aren’t worth putting solar panels on. Let’s see how tilt can help north facing roofs.
We set up a model roof with a 20 degree pitch to the north, again in Phoenix, and varied the angle relative to the roof as shown in the diagram below.
The graph below shows that tilting can increase the output from panels on north-facing roofs a lot. For panels flat on the roof, the output was 6,552 kWh per year. Adding 40 degrees of tilt relative to the roof increased this to 9,289 kWh per year.
By comparison the south-facing panels for Phoenix gave 9,600 kWh per year. Not that much difference! Getting the right amount of tilt can definitely make north-facing panels worthwhile.
What if Space is Limited?
Optimal tilt angle is a little more complicated if there is limited space available for panels. This is because tilted panels cast a shadow and therefore rows of panels need to be spaced apart to avoid panels being shaded. In a restricted space, is it better to have more flat panels or less tilted panels that produce more energy per panel?
To answer this question we’ve modelled a 36 feet x 36 feet flat roof in Phoenix. When the panels were tilted, the rows of panels were spaced sufficiently apart to minimize shading. The amounts of energy per year for the different tilts are shown in the table.
|Tilt Angle||Number of Panels||Energy per Year (kWh)|
This might seem confusing. Didn’t we just say that tilting was better? Well yes, it is, when you’re looking at energy output per panel, and if space isn’t an issue. If your space is limited, then you need to look at energy output per area of roof. In this case it’s better to have the panels flat so you fit more panels into the space, even if the output per panel is less than for tilted panels.
However once again, maintenance is more of an issue with flat panels. It might be worth losing a bit of energy and having 10 degrees of tilt to make maintenance less time-consuming.
What About Solar Farms?
Solar farms are obviously on a much larger scale compared to residential and commercial solar arrays. They are also almost always placed on land, which gives more flexibility as to how the panels are mounted, compared to a mounting on a rigid structure.
Many solar farms use a technique called ‘tracking’. Tracking basically means the angle of the panels changes during the day to maintain the optimal angle to the moving sun.
Tracking can be ‘single-axis’ which means the panels rotate around one axis, as shown in the figure below. To get the maximum energy output, tracking can be ‘dual-axis’ which places the panels at the exact optimal angle to the sun.
Single-axis tracking can give 25-35% more power compared to no tracking, while dual-axis tracking can give 35-35% more power. Experience has shown that the best ‘bang for buck’ is single-axis tracking: the extra energy from dual-axis tracking isn’t normally worth the extra expense.
Tilting panels can make a big difference to the energy output from a panel, however each specific situation must be looked at carefully to see if tilting is the best option.
If you have a flat roof and plenty of space, then tilting will increase the energy output per panel and can give great value for money. If your roof faces north, tilting can increase the energy output a lot.
However if you have a flat roof and space is limited, you may be better off having flat panels or panels with a small ~10 degree tilt to help with self-cleaning.
Is tilting important?
Tilting can significantly increase the power output of a solar panel and therefore needs to be considered at the design stage. If your roof already has a good angle on it, tilting may not be worth the extra expense.
Can tilting help output from a north-facing roof?
Most definitely. Tilting up to 40 degrees relative to the roof can greatly increase the output energy, to nearly the same you can get from a south-facing roof.
I have a flat roof. What is the best tilt angle?
It depends. If space is not an issue, then about 30 degrees of tilt to the south (depending on location) will give the most energy output. However is space is limited, more panels can be fitted in if the panels are flat and can produce more total power
Are flat panels ok?
If space is limited then flat panels will give the most output as you can fit the most panels in the space. However since water does not run off well, ‘self-cleaning’ is not very good and the panels will require more checking and cleaning. 10 degrees of tilt is enough to give a good amount of self-cleaning.
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Geoff Edwards has worked in the renewable energy sector for more than 15 years, initially at the forefront of lithium ion battery technology, and more recently in solar power combined with energy storage. He has over 15 patent applications in various fields. Geoff has a degree and PhD in engineering from the University of Queensland in Brisbane, Australia.
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