What Kind of System Is PVWatts Assuming If You Leave Every Setting at Its Default?
Summary
The web calculator starts at 14% total system losses, with line items such as 2% soiling, 1% shading, and 3% availability. The API defaults to a DC-to-AC ratio of 1.2 and 96% inverter efficiency, and V8 uses 2020 typical-year (TMY) weather. Raising losses from 14% to 20% cut New York's modeled output from 1,291 to 1,200 kWh per kW, about 7%, so that one field should match before any PVWatts figure is set beside an installer's estimate.
What sits inside the 14%
Defaults as listed on the PVWatts calculator page:
| Loss item | Default | Page description |
|---|---|---|
| Soiling | 2% | |
| Shading | 1% | an array with little or no shading |
| Snow | 0% | no snow on the array, or kept clear |
| Mismatch | 2% | |
| Wiring | 2% | resistance in DC and AC wires |
| Connections | 0.5% | resistance in electrical connectors |
| Light-induced degradation | 1.5% | |
| Nameplate rating | 1% | |
| Age | 0% | no weathering over time |
| Availability | 3% |
With age at zero, the output is a first-year figure. A 20-year-out estimate means changing that line or the total yourself. The listed items neither add nor multiply to exactly 14%, and the page does not explain the gap.
The other menus and the weather file
Module type offers Standard, Premium, and Thin film. Array type offers Fixed Open Rack, Fixed Roof Mounted, 1-Axis, 1-Axis Backtracking, and 2-Axis. V8 draws weather from 2020 TMY data in NLR's National Solar Radiation Database; V6 used data from around 2015.
The calculator carries a caution that its predictions do not reflect variations between PV technologies or site characteristics beyond the inputs, and that better modules are not differentiated from lesser ones. The expected range shown beside the result is based on 30 years of actual weather at the location and describes year-to-year variation for a fixed open-rack system.
Sources
- National Laboratory of the Rockies — PVWatts Calculator (Website Version 8.7.4, API 8.5)
- NLR — PVWatts V8 API runs (New York, 1 kW, tilt 20°, azimuth 180°, losses 14% and 20%; 2026-09-23)
- NLR — PVWatts V8 API runs (New York, 1 kW, tilt 20°, azimuth 180°, losses 14% and 20%; 2026-09-23)
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