☀️ Solar
Solar Panel Efficiency
Calculate a solar panel's efficiency and energy output from its size, rated power and peak sun hours. Press Calculate.
About the Solar Panel Efficiency
Solar panel efficiency is the share of incoming sunlight that becomes electricity. At the standard test condition of 1,000 W/m², a 400 W panel with an area of 2.0 m² converts 400 ÷ 2,000 = 20% of the incident energy. Commercial panels are typically 18–23%.
Rated power is measured at 25 °C cell temperature and 1,000 W/m². Real output falls when the panel heats up, and inverter, wiring and soiling losses take another 10–20%. The annual estimate here uses peak sun hours, which already fold the sun's angle and season into one number.
How the calculation works
- Area (m²) = length × width.
- Efficiency = rated power ÷ (1,000 W/m² × area) × 100.
- Power per area = rated power ÷ area.
- Daily kWh = rated W × peak sun hours ÷ 1,000.
Reference values
| Panel | Area | Efficiency | Watts/m² |
|---|---|---|---|
| 250 W | 1.6 m² | 15.6% | 156 |
| 300 W | 1.7 m² | 17.6% | 176 |
| 350 W | 1.8 m² | 19.4% | 194 |
| 400 W | 1.9 m² | 21.1% | 211 |
| 450 W | 2.0 m² | 22.5% | 225 |
Worked example: a 400 W panel
A 400 W panel measuring 1.72 m × 1.13 m has an area of 1.944 m². Efficiency is 400 ÷ (1,000 × 1.944) = 20.6%. At 4.5 peak sun hours, daily generation is 400 × 4.5 ÷ 1,000 = 1.8 kWh before system losses; at 85% efficiency, about 1.53 kWh.
Peak sun hours already fold the sun's angle, season and weather into one number, so they are more useful than a simple “hours of daylight” figure. Real output also falls when the panel heats up, and inverter, wiring and soiling losses take another 10–20%. Use the estimate for comparison, not for a guaranteed bill saving.
Temperature, shading and real-world losses
Panel efficiency is measured at 25 °C cell temperature, a condition that rarely occurs in strong sun. A roof panel can reach 60 °C, and the output falls by roughly 0.3–0.4% per degree above 25 °C, so a hot afternoon can cost 10% or more. Winter sun is weaker but colder, which partly offsets the shorter day.
Shading is worse than its area suggests. Because cells are wired in series, shade on one cell can reduce the output of a whole string, and a chimney or antenna that shades the array in the morning can dominate the annual yield. Micro-inverters or optimisers reduce that penalty, but siting the array away from shade is still the cheaper fix.