kWpKilowatt-peak — nominal power of a system

The STC conditions (Standard Test Conditions) are laboratory conditions: irradiance of 1 000 W/m² (midday sun equivalent in summer), cell at exactly 25°C, and standardized solar spectrum AM 1.5. In real conditions, your panels almost never produce exactly their peak power — the temperature is often higher than 25°C (which reduces production), the irradiance is often lower (cloudy weather, morning, evening), and the angle of incidence is not optimal. A performance ratio (PR) of 80-85% is realistic in French-speaking Switzerland. For an 8 kWp installation with PR=80% in the canton of Vaud (irradiation ≈ 1 380 kWh/m²/year): production ≈ 8 × 1 380 × 0.80 ≈ 8 832 kWh/year.

In French-speaking Switzerland, the production ratio varies from 1 000 kWh/kWp/year (Jura, foggy regions) to 1 400 kWh/kWp/year (Valais due south). An 8 kWp installation therefore produces 8 000 to 11 200 kWh/year depending on the location — enough to cover 60 to 100% of the needs of a family of 4 people (average Swiss consumption: 4 500-6 000 kWh/year excluding electric heating).