Watt-Peak (Wp)

Watt-peak (Wp) is the standard unit for rating the maximum power output of a solar panel under Standard Test Conditions (STC). When a manufacturer labels a panel as "400W," they mean it produces 400 watt-peak under the specific laboratory conditions of 1,000 watts per square meter of irradiance, 25°C cell temperature, and AM1.5 spectrum.

This standardized rating allows apples-to-apples comparison between panels from different manufacturers. Without a common test standard, comparing panels would be meaningless because output varies dramatically with light intensity and temperature.

In real-world conditions, a panel rarely operates exactly at its watt-peak rating. On a hot summer afternoon, cell temperatures may exceed 60°C, reducing output well below rated Wp. On a cool, clear spring day with direct sun, a panel might actually produce close to or slightly above its Wp rating.

Watt-peak is used to calculate system sizing. If you need 4,000 watts (4 kWp) of solar capacity and are using 400 Wp panels, you need 10 panels. System designers then apply derating factors for temperature, wiring losses, inverter efficiency, and shading to estimate actual annual energy production in kilowatt-hours (kWh).

Related terms include kilowatt-peak (kWp, equal to 1,000 Wp) used for residential systems, and megawatt-peak (MWp) used for commercial and utility-scale installations.