There are many environmental factors that can affect how much power a system can produce. These factors are combined for an estimated efficiency/derate value used in array sizing. (For more detail on these individual factors, see “Pump Up the Power—Getting More from your Grid-Tied PV System” in *HP127*.)

An overall average system efficiency of 66% is used in the example calculations to account for the following:

**88% derate for energy lost due to module heating (12% loss)**

**95% for inverter efficiency (5% loss)**

**97% for DC and AC wiring inefficiencies (3% loss) **

**95% for module production tolerance and mismatch (5% loss) **

**95% for module power loss due to dust and dirt (5% loss)**

**90% shade factor to account for array shading before 8 a.m. and after 4 p.m. (10% loss)**

To arrive at 0.66 (66%), multiply all the efficiency factors together:

**0.88 × 0.95 × 0.97 × 0.95 × 0.95 × 0.90 = 0.66**

While this 0.66 is a general value used for estimating a batteryless PV grid-tied array’s size, a derate factor can be adjusted to match each system and site specifics. For the next example, let’s say the array will consist of microinverters, which will eliminate losses due to module mismatch, and will use modules that have a positive-only production tolerance. In this case, the module production tolerance and mismatch loss will be zero (or will have an efficiency of 100% for a factor of 1.0), increasing the overall efficiency factor to 69%.

**0.88 × 0.95 × 0.97 × 1.00 × 0.95 × 0.90 = 0.69**

Conversely, let’s go back to the original string inverter and after performing a shade analysis on the roof, we find that the solar window is really from 8:30 a.m. to 3:30 p.m. and the shading factor is 0.85. This will decrease the efficiency factor to 62%.

**0.88 × 0.95 × 0.97 × 0.95 × 0.95 × 0.85 = 0.62**