Jim Paull, ERS, for Zondits
Why in the world would anyone mount a photovoltaic array facing north? Didn’t anyone tell them that in this hemisphere the sun is to the south? What were they thinking?
The best orientation for a solar installation – whether it’s thermal or photovoltaic – is facing due south at an elevation from the horizontal equal to the latitude. For this Massachusetts location, the latitude is 42.7˚ north. That represents a roof with about 11-in-12 pitch – which is fairly steep. In the real world, very few existing roofs have the exact angle and orientation to maximize solar harvesting. Add those pesky trees, and most existing homes are far from ideal for the installation of rooftop solar.
So how much do you lose with less than ideal orientation? The good news is that for many combinations of azimuth (compass direction) and elevation angle, the loss is not as much as you might think.
Elevation angles, or tilt, is most forgiving when it is lower than ideal. Flat roofs where module angle is restricted due to wind loads typically generate more than 90% of what they would at the ideal elevation angle (at this latitude). A perfectly flat installation (usually not advised) would be 87% of ideal. And when the installation angle is low, it is not very sensitive to azimuth.
Performance is not as kind to installations at steeper angles. A vertical installation facing east, west, or south only gets from 63% to 79% of the ideal. For building-integrated photovoltaic (BIPV) facades or windows where this is the only orientation possible, 78% is better than nothing.
And what about trees? Massachusetts is heavily forested, and a majority of residential solar sites have some shading from trees. Shade not only reduces the amount of electrical current from shaded modules, but in systems with string inverters it reduces the current even from unshaded modules in the string. But as with BIPV, often something is better than nothing, and there may be non-economic motivations for installing solar, as evidenced by the author’s peculiar siting of his own photovoltaic system facing a large pine tree.
So, what about our friends with the north-facing photovoltaic installation? While I would never recommend a north-facing photovoltaic system installation, because it’s a fairly flat installation, it’s not too bad. Without the trees, it would generate 77% of ideal – about the same as a vertical installation facing due south.