Derating Factor

kWp capacity is measured at Standard Test Conditions: irradiance of 1,000 W/m² and a cell temperature of 25°C [IEC 61215]. In Indonesia, real operating conditions are far different — module temperature can reach 50-65°C during the day, soiling from tropical dust accumulates quickly, and system losses compound from the panel to the export meter.

Typical derating components often used as calculator defaults: tropical temperature derating ~5-8% (the Pmax coefficient of crystalline silicon is around -0.35%/°C), soiling 2-5% in urban environments with routine cleaning, inter-module mismatch 1-2%, DC and AC cable losses 2-3%, and inverter efficiency 96-98%. The accumulation of all factors yields a combined derating of 80-85%, used as a conservative multiplier against STC output.

Beyond this "instantaneous" derating, solar panels also undergo annual degradation of 0.5-0.8% per year [IRENA 2023] — a permanent decline in capacity due to cell-material ageing. After 25 years, installed capacity is generally down to 80-85% of the original nominal rating, and this must be factored into long-term production projections.

Indonesian PLTS Application Example

A 5 kWp system in Surabaya with a PSH of 4.9 kWh/m²/day [NASA POWER 1984-2023] and a combined derating of 0.82 yields an estimate of: 5 × 4.9 × 0.82 = 20.1 kWh per day, or ~7,330 kWh per year. By year 10, assuming 0.7% degradation per year [IRENA 2023], net output falls to ~6,840 kWh per year.