Derate Factor

A derate factor is a multiplier used to reduce a solar system’s nameplate capacity to a more realistic estimate of actual output. It accounts for real-world losses that reduce production below the ideal laboratory conditions used for panel ratings.

Common sources of loss included in the derate factor are: inverter conversion efficiency (96–99%), wiring and connection losses (1–3%), soiling from dirt and dust (2–5%), temperature effects (5–15% in hot climates), shading (varies), panel mismatch (1–2%), and system age/degradation. The overall derate factor for a typical residential system is usually 0.75–0.85, meaning the system produces 75–85% of its nameplate DC rating as usable AC electricity. For example, an 8 kW system with a derate factor of 0.80 effectively produces about 6.4 kW AC under real conditions. Tools like PVWatts (developed by NREL) use derate factors to estimate annual solar production for specific locations. Understanding the derate factor helps homeowners set realistic expectations and compare solar proposals that may use different assumptions.