Kilowatt-Peak (kWp)
The rated maximum power output of a solar system under Standard Test Conditions — the standard unit used to size and compare solar installations.
Kilowatt-peak (kWp) is the nameplate power rating of a solar panel or system under Standard Test Conditions (STC): 1,000 W/m² solar irradiance, AM 1.5 spectrum, and 25°C cell temperature. It represents the theoretical peak output under ideal laboratory conditions.
Real-world output is always lower than kWp due to heat (panels lose efficiency above 25°C), suboptimal sun angles, system losses (inverter efficiency, wiring resistance, soiling), and non-perpendicular irradiance. A performance ratio of 75–85% is typical — meaning a 10 kWp system realistically delivers 7.5–8.5 kW at peak.
Annual energy production (kWh) is more useful than kWp for estimating savings. Production equals: kWp × peak sun hours per day × 365 × performance ratio. A 10 kWp system in Phoenix (5.5 peak sun hours) with 80% performance ratio produces roughly 10 × 5.5 × 365 × 0.80 = 16,060 kWh/year. The same system in Seattle (3.5 peak sun hours) produces only ~10,220 kWh/year.
Real-World Example
Two quotes both offered "10 kW systems" — but one used twenty 500W panels while the other used twenty-five 400W panels; same kWp, different roof space requirements and potentially different inverter architectures.