In one line
The laboratory reference conditions — 1,000 W/m², 25°C cell temperature, AM1.5 spectrum — at which module nameplate power is rated.
What it means
Every module datasheet headline ("580 W") is measured at Standard Test Conditions: 1,000 W/m² irradiance, 25°C cell temperature and an AM1.5 solar spectrum. These conditions are reproducible in a flash tester but rarely coincide in the field, because 1,000 W/m² of sunlight heats a cell far above 25°C. NOCT (Nominal Operating Cell Temperature) and its successor NMOT describe a more realistic condition — typically 800 W/m², 20°C ambient, 1 m/s wind — and the NOCT power figure on a datasheet is usually 25–27% below the STC rating. Designers size strings using STC values corrected for site temperature extremes, and estimate energy using NOCT-like behaviour.
Worth knowing
- STC is a rating benchmark, not an operating condition.
- Real cell temperatures at full sun commonly run 25–35°C above ambient.
- NOCT/NMOT figures on the same datasheet describe realistic output far better than the headline watts.
- Two modules with identical STC ratings can differ meaningfully in field energy.
At a glance
The same module, two rating conditions
| Parameter | STC | NOCT / NMOT |
|---|---|---|
| Irradiance | 1,000 W/m² | 800 W/m² |
| Cell temperature | 25°C fixed | Derived from 20°C ambient |
| Wind speed | Not specified | 1 m/s |
| Purpose | Nameplate rating, comparison | Realistic operating estimate |
| Typical 580 W module | 580 W | ≈ 430–440 W |
Field notes
- 1Size string voltage using the coldest expected temperature and STC Voc — that is where the maximum system voltage limit is breached.
- 2Size string current and inverter loading using hot-day behaviour, not STC.
- 3When comparing two modules, compare temperature coefficients and NOCT figures, not just STC watts.
Common mistakes
- Assuming a 580 W module produces 580 W on a hot clear afternoon — 430–470 W is more typical.
- Comparing bifacial modules on STC front-side power alone, ignoring rear-side gain.
Frequently asked
What are Standard Test Conditions for solar panels?
1,000 W/m² irradiance, 25°C cell temperature and an AM1.5G spectrum. All nameplate module power ratings are measured at these conditions so that products can be compared on a common basis.
Why do solar panels never produce their rated power?
Because STC specifies a 25°C cell temperature at full sun, which almost never occurs — cells at 1,000 W/m² typically run 45–65°C. Combined with soiling, wiring and inverter losses, real output at full sun is commonly 75–85% of nameplate.
Sources
Related terms
Browse every term in the renewable energy glossary, or read the latest analysis on Solar PV.