It has long been taught that a PV array tilted to an angle “equal to latitude” will yield close to the maximum annual output for a fixed array. While this holds true in some locations and climates (like sunny Phoenix, Arizona, and Tampa, Florida), it’s not true for every location. For example, in cloudy Portland, Oregon (latitude 45.6°), an array tilted to about 32° optimizes annual system output. This is due to the extremely cloudy winters in this location. An array with a lower tilt angle can better take advantage the high summer sun, maximizing output. Although not as extreme, another example would be Chicago, Illinois (latitude 41.8°), where a tilt angle of about 33° will optimize system output.
If you use PVWatts (bit.ly/PVWattsV1) to compare the output for various tilts in cities across the United States, you will find that a “slightly shallower than latitude” tilt often yields the highest output. We randomly chose several locations and found that, for a south-facing array (azimuth = 180°), a tilt of latitude minus 3° to 5° is predicted to provide the most output from a PV array. (Only one of our example locations—Portland, Oregon—suffered a greater than 1% decrease in output by tilting modules at latitude.)
We also need to consider the array’s orientation—the direction it faces. In “PV Array Siting & Mounting Considerations” in this issue, we compare various PV array mounting methods. Flush-mounting is a common technique used for rooftop PV arrays, which dictates the tilt and orientation. A roof’s angles are rarely perfect for maximizing system output. It is helpful to see what the penalty is, since we may choose to offset this loss by increasing our array capacity. PVWatts can also be used for this—choose the location, enter the system size, and then enter various tilt and orientation angles to pinpoint which combination maximizes output. Using this maximum kWh per year as your “optimal” value, you can compare the output at other angles and calculate the percentage of optimal for each.
For our example, let’s use Salt Lake City, Utah (latitude = 40.8°). A flush-mounted PV array on a southeast-facing roof (azimuth = 135°) with a tilt of 25.8° (latitude minus 15°) will generate 95% of the site’s maximum production. In this case, we could simply accept that small reduction in output or increase the array capacity by 5% to make up for the roof’s nonoptimal orientation and tilt. This could be achieved either by installing more modules or by using higher-efficiency modules.
Note: This article assumes that there is no shading on the array; if there is some shading, a more complete analysis would be required to figure out the comparative loss at each tilt and orientation.