How efficient are solar panels?

The majority of solar panels are usually about 15% – 17% efficient on average. The best solar panels will have efficiency levels of around 22% or so. The efficiency of a solar panel is actually called its performance. The performance of a solar panel is tested under Standard Test Conditions (‘STC’). These conditions are a 25°C temperature and an irradiance of 1,000 W/m2. You can use the performance of a solar panel to determine how much watts each cell would generate under standard conditions.