Please note: The Inverter's DC-to-AC Conversion Efficiency has a default value of 96% not included in the system loss percentage (separate input area) Also, PVWatts calculates temperature-related losses as a function of the cell temperature, so you should not include temperature loss in the system loss percentage.  

Soiling

Losses due to dirt and other foreign matter on the surface of the PV module that prevent solar radiation from reaching the cells. Soiling is location- and weather-dependent. There are greater soiling losses in high-traffic, high-pollution areas with infrequent rain. For northern locations, snow reduces the energy produced, depending on the amount of snow and how long it remains on the PV modules. NREL continues to work on improving the modeling of soiling and snow and is working to include historical datasets as available.

Snow

Reduction in the system's annual output due to snow covering the array. The default value is zero, assuming either that there is never snow on the array, or that the array is kept clear of snow.

Mismatch

Electrical losses due to slight differences caused by manufacturing imperfections between modules in the array that cause the modules to have slightly different current-voltage characteristics. The default value of is 2%.

Wiring

Resistive losses in the DC and AC wires connecting modules, inverters, and other parts of the system. The default value is 2%.

Connections

Resistive losses in electrical connectors in the system. The default value is 0.5%.

Light-Induced Degradation

Effect of the reduction in the array's power during the first few months of its operation caused by light-induced degradation of photovoltaic cells. The default value is 1.5%.

Nameplate Rating

The nameplate rating loss accounts for the accuracy of the manufacturer's nameplate rating. Field measurements of the electrical characteristics of photovoltaic modules in the array may show that they differ from their nameplate rating. A nameplate rating loss of 5% would indicate that testing yielded power measurements at Standard Test Conditions (STC) that were 5% less than the manufacturer's nameplate rating. The default value is 1%.

Age

Effect of weathering of the photovoltaic modules on the array's performance over time. The default value is zero.

Availability

Reduction in the system's output caused by scheduled and unscheduled system shutdown for maintenance, grid outages, and other operational factors. The default value is 3%.