What are Standard Test Conditions?

Standard Test Conditions (STC) define the reference environment used worldwide to measure and compare PV module performance. They are specified in IEC 61215.

The three STC parameters

• Irradiance: 1000 W/m² — bright midday value; India's annual mean is 500–650 W/m².
• Cell temperature: 25°C — the temperature inside the cell, not ambient air. In the field, cell temperature typically runs 20–35°C above ambient.
• Spectrum: AM 1.5G — standardised solar spectrum after passing through 1.5 atmospheres of air.

Why STC ratings overstate real-world output

Indian PV modules routinely operate at cell temperatures of 45–65°C in summer. Since all silicon panels lose roughly −0.35%/°C above 25°C, a 550 Wp module may produce only 460–480 Wp at 60°C cell temperature.

Real-world yield estimates should use NOCT conditions or temperature-corrected energy models rather than STC nameplate ratings alone.

STC on the datasheet

Every IEC 61215-certified datasheet lists: Pmax (Wp), Voc, Vmp, Isc, Imp, module efficiency (%), and temperature coefficients — all measured at STC.