Solar Panel Output explained
Solar panel output refers to how much electricity a panel generates. It's expressed in two ways:
- Peak power (Wp or W): the maximum output under Standard Test Conditions (STC) — 1,000 W/m² of irradiance, 25°C cell temperature. This is what's printed on the panel spec sheet.
- Annual energy yield (kWh): the total electricity produced over a year in real-world conditions at your location.
Real-world output is almost always less than the STC rating because:
- Panels operate at higher temperatures in summer, reducing efficiency
- The sun rarely delivers a perfect 1,000 W/m² in the UK
- Shading, dirt, and inverter losses reduce output further
A reasonable rule of thumb for the UK: expect 850–950 kWh of annual output per kWp installed. So a 4 kWp system will typically generate 3,400–3,800 kWh/year. In sunnier regions, this figure rises: Southern France and Spain average 1,200–1,400 kWh/kWp, and Arizona reaches 1,500–1,800 kWh/kWp.
Factors that reduce output include poor azimuth (facing away from south), steep or shallow roof pitch, shading from trees or chimneys, and high ambient temperatures in summer.