ROOF PITCH

Solar PV systems operate most efficiently when the sun is perpendicular to the surface of the panels. Because the height of the sun varies over the course of the year, the best year-round angle for an array is approximately equal to the latitude where the array is located. In this region, that angle is 40° .

However, decreasing the tilt of the array does not cause a dramatic decrease in energy output until the tilt drops below about 10° to 15° . A south-facing array tilted at 15° from horizontal will produce about 93.5% of optimum, and one tilted at 10° will perform at 90.3% of optimum.

A tilt-up array is recommended for flat roofs. In this case, there are trade-offs between tilting the panels at a steeper, more optimal angle and tilting it at the minimum angle of 10° to 15° . The factors that come in to play in this decision include aesthetics, the wind loading in your area, and whether the system is tied in to the roof structure or is "ballasted." A ballasted system uses concrete weights to hold down the array against wind loading, thereby avoiding the need to penetrate the roof and tie in to the joists. On the other hand, attaching the array to the roof involves more design and installation time, and not all roof designs are suitable for this installation method.

Because of the prevalence of high winds in this region, these roof-mount tilt-up systems must often be restricted to a more shallow angle than ground-mount systems. Also, the roof must be designed such that it can handle the extra weight of the modules and mounting system.

Here is a 10.56 kW array with a ballasted system. The azimuth is 180° and the tilt angle is 10° . The output of this system is 90.3% of optimum:

Here is a system that is tied to the roof structure. This is a 10.32 kW array tilted at 15° with an azimuth of 135° . The tilt could not exceed 15° due to the high wind loading on the roof of this 8-story building. The output will be 91.4% of optimum: