In the first articles of our Solar 101 series, (“Is my roof ready for solar?“) and (“Attaching your solar system to your roof”) we discussed how the age and physical characteristics of a roof affect the design and payback of a prospective solar system. We considered weather concerns and panel setbacks to better estimate the number of solar panels that will fit on a roof.
Now, we will take a look at the relationship between a panel’s latitude, pitch, and azimuth to its solar electricity output.
In a perfect world, solar panels always face the sun at a perpendicular angle. But most prospective solar buyers are stuck with pre-existing realities: a home they already own, and a roof with preexisting conditions. Partial shade, a less-than-ideal azimuth, or a less-than-ideal roof pitch are often the norm for a residential installation.
But that begs the question: To what extent will these factors affect my system’s return on investment?
Most of us are looking to maximize electricity generation on an annual basis. Thus, the best azimuth is due south and the ideal pitch is roughly equal to your home or business’s latitude. We can test this hypothesis using the U.S. Department of Energy’s PV Watts tool to estimate solar electricity generation.
Here is what it looks like:
PV Watts is not a perfect tool; it is a rough guide, but it is close enough for our purposes. If you live in a region with heavy snow, dust, tree cover, or other sunshine-blocking variables your electricity production will be lower.
Let’s start this analysis with a completely average city – Normal, Illinois – as our test site. For this analysis, we adopt the default variables in PV Watts, changing two variables: the Tilt (deg) of the roof and the array type to Fixed (roof mount).
A rule of thumb for optimizing the angle of your solar panels is to mount them at an angle equivalent to the site’s latitude, facing due south. The latitude of Normal, Illinois, is 40.5°.
As you can see in the chart below, the peak generation from this roof is not exactly 40.5°, but is somewhere between 35-36°. According to this “Normal” data, a panel’s pitch can range anywhere from 20° to 50° and still generate more than 97% of its peak potential.
One interesting observation from this chart is that if your solar panels are horizontal (corresponding to the far left of the chart) or 90° vertical (at the far right), your system will still produce a decent amount of electricity. Compared to the maximum output value of 5,623 kWh/year, a flat roof produces 15% less and a vertical wall of solar panels produces 38% less. Not optimal, but not useless.
Keep in mind that horizontal panels on a flat roof will collect dirt. That’s why at least a few degrees of pitch are needed to allow rain to rinse off dust and pollen. A slight angle will also prevent rainwater from settling on the solar panels and leaving a residue as the water evaporates. One study has shown that panels in very dusty environments with infrequent rain resist dust deposition best when angled at around 26%.
Now let’s consider the direction that the roof surface faces. This factor is called the “azimuth” and refers to the direction the roof surface faces relative to the North Pole, which is located at 0°. This makes due south 180°, west 270° and east 90°.
Returning to our completely average city of Normal, Illinois, we’ll keep the system’s roof angle at 40.5° for our calculations.
As you can see in the table, the Normal house facing due South can generate 5,608 kWh. When facing due east, that value falls to 4,350 kWh, a 22% loss. When facing due west, we get a similar result: 4,303 kWh, a 23% loss.
But that’s Normal, Illinois. Generation losses due to azimuth become less pronounced as we approach the equator. For instance, in San Diego, California, (latitude 32.7º) a default system modeled using PV Watts fell from 6,314 kWh to 5,511 kWh. That drop was just under 13%.
In the case of San Diego, elevated land to the east blocks some sun, at least compared to the beautiful open ocean to the west. This means a due south solar system actually, slightly, underperforms in terms of electricity generation. We found that the peak value was somewhere closer to an azimuth of 197-198º.
Other factors will affect your roof’s solar generation, including shadows from trees, neighboring houses, and even your own house’s features. Make sure that your solar contractor accounts for all these factors when giving you a quote and a financial model.
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Love PV Watts and use it regularly, but usually have had to explain it to lay people; you did an excellent job of explaining it and illustrating its use. Great job, thank you.