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» Learn about Photovoltaics
» Grid-tied
» System Sizing
» Factoring in Equipment Efficiency in Array Size
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Factoring in Equipment Efficiency in Array Size
No electrical device is perfectly efficient. Because of this, it is important that we calculate the expected
efficiency of our system in order to determine an accurate idea of output. All modules are rated under
Standard Test Conditions (STC). STC is equivalent to an irradiance of 1000 W/m2 and a cell temperature of
77°F. Most PV modules will operate under much warmer conditions in the real-world. For this reason, a more
realistic rating system called PTC (PV USA Test Conditions) was developed. This test uses an ambient
temperature(rather than 77°F cell temperature with STC) of 68°F. The cell temperatures are much higher
under these conditions providing a much more accurate calculation of expected output. The average PTC rated
wattage is approximately 88% of the STC rating. Therefore, a 175 watt module will realistically produce 154
watts. A list of the PTC ratings for all available modules can be found at the following link:
http://www.gosolarcalifornia.org/equipment/pvmodule.php
Also, we must determine the efficiency of our grid-tie inverter. The California Energy Commission has
independently tested all major inverter manufactures for efficiency. Most inverters fall in the 92-96% range.
A list of the results can be found at:
http://www.gosolarcalifornia.org/equipment/inverter.php
Continuing our example, if we factor in our inverter and module efficiencies we will need additional modules.
4,700 watts ÷ .96 inverter efficiency = 4,985 PTC watts
4,985 PTC watts ÷ .88 module efficiency = 5,664 STC watts
Therefore, we need an array size of 5,664 STC rated watts. If we use 32 of the 175 watt modules, our array
size will be just about perfect at 5,600 STC watts.
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