Pump Up the Power: Page 2 of 5

tilt can make a difference in an array’s energy production. For best year-round performance in most locations, fixed arrays should be oriented to true south—as opposed to magnetic south—which means taking into consideration the site’s magnetic declination.

Array tilt also plays an important role in energy production. For optimal production, arrays generally should be tilted at an angle equal to your latitude. However, most PV arrays are mounted parallel to the roof plane, and have the same tilt as the roof, which is typically pitched at an angle less than the latitude. An array mounted parallel to the roof surface at a tilt less than latitude will produce more energy in summer, when some utilities have higher per-KWH rates.

If your site does not allow for true south orientation or tilt equal to latitude, you can simply factor the production losses into your system design and compensate by using a slightly larger array. See the table for design factors that can be used for less than optimal orientations and tilts. To determine the KWH impact of various tilts and other factors for any PV system at any site, use the National Renewable Energy Laboratory’s PVWatts online calculator (see Access).

Components

PV Performance Parameters. PV module power ratings (nameplate ratings) are determined at “standard test conditions” (STC)—1,000 watts per square meter of solar irradiance at a PV cell temperature of 25°C (77°F). A system’s size is nominally stated by multiplying the STC rating by the number of modules—but you shouldn’t count on this being an accurate reflection of the system’s actual output.

STC testing is performed in a laboratory setting where modules are flashed with a light source and power output is measured. This measurement doesn’t account for temperature or wind variations, which can drastically affect performance. Like any material exposed to sunlight, PV modules heat up as they absorb solar infrared radiation, becoming less efficient at converting light to electrical energy. For cell temperature to be 77°F, the same as STC, the ambient air temperature has to be much lower (about 23°F to 32°F)—unusually low temperatures in most circumstances.

Another standard, PTC (PVUSA test conditions) was developed to better simulate real-world installations. PTC is conducted at the same irradiance, but at a somewhat more realistic ambient temperature of 68°F (with cell temperature about 113°F), and at a wind speed of 1 meter per second (2.24 mph). Because temperature-related power loss averages -1/2% per °C rise for crystalline PV modules, their PTC ratings typically range from 85% to 90% of the STC rating.

The underlying lesson is to provide for sufficient airflow around mounted PV modules to minimize production losses due to heat. In general, allow a 3- to 5-inch unrestricted air gap between the roof and flat-mounted modules. Modules tilted up from the roof plane fare even better—but to some, tilting can be aesthetically undesirable.

Warranted PV Minimum Power Ratings. To most accurately project long-term performance,