An array derate factor is a multiplier between 0 and 1 used to calculate the real-world power output of a solar photovoltaic (PV) array. It reduces the ideal nameplate rating (STC) to match actual expected performance after accounting for system losses like wiring, dust, heat, and inverter limits. A typical combined residential derate factor is around 0.80 to 0.85 (80% to 85%).

Common Losses Included #

• Soiling: Dust, dirt, or leaves blocking light (1% to 5% loss).
• Wiring resistance: Power lost as current travels through DC and AC cables (2% to 3% loss).
• Module mismatch: Minor performance variations between individual panels (2% loss).
• Inverter efficiency: Conversion loss turning DC power into usable AC power (2% to 5% loss).
• Temperature effects: Power reduction as solar panels heat up under direct sun.

How It Is Calculated #

• Find the manufacturer’s peak DC wattage rating for the solar array.
• Convert each individual loss percentage into a decimal factor (e.g., a 3% loss becomes 0.97).
• Multiply all individual component factors together to get the total net array derate factor.
• Multiply the ideal nameplate DC capacity by this final factor to estimate real-world AC output.