How this instrument works
A solar panel's power output starts from how much sunlight energy hits its surface — irradiance, measured in watts per square meter — multiplied by the panel's physical area, giving total incoming solar energy. Only a fraction of that energy actually becomes usable electricity, set by the panel's efficiency: a typical residential panel converts somewhere around 15-22% of incoming sunlight into DC electricity, with the rest lost as heat or reflected.
Even a panel's rated efficiency figure, tested under lab conditions, doesn't survive intact into a real installation, which is why this formula includes a separate performance ratio term. Wiring resistance, inverter conversion losses, panel soiling, imperfect sun angle, and temperature above the lab's test conditions all shave a further chunk off theoretical output — a system performance ratio of 0.75 to 0.80 is a common real-world figure for a reasonably well-installed system.
Multiplying area, irradiance, efficiency, and performance ratio together gives instantaneous power output at that specific sunlight intensity — useful for sizing a system against a known irradiance figure, like the standard-test-condition value of 1,000 W/m² used to rate panels, or a location's typical peak midday sun.
- Enter Panel area (m²) — the total collecting area of the array, not the roof space it sits on.
- Enter Solar irradiance (W/m²) — 1,000 W/m² is the standard test condition; use a lower figure for overcast or off-peak conditions.
- Enter Panel efficiency (0-1) — from the panel's datasheet, typically 0.15-0.22 for residential panels.
- Enter System performance ratio (0-1) — accounts for real-world losses from wiring, inverter, soiling and temperature; 0.75 is a common working estimate.
- Read Power output (W) — the array's instantaneous electrical output under these conditions.
Worked example — a 20 m² array at standard test conditions
A 20 m² solar array sits under standard test-condition irradiance of 1,000 W/m², with 20% panel efficiency and a 0.75 system performance ratio. Power = 20 × 1000 × 0.20 × 0.75 = 3,000 W.
That 3,000 W (3 kW) figure represents the array's real-world output under this specific sunlight intensity, already accounting for the roughly 25% of theoretical output lost to wiring, inverter conversion, and other system losses — the panels' raw theoretical output before those losses would have been 20 × 1000 × 0.20 = 4,000 W, a full 1,000 W higher than what the array actually delivers.
Questions
Why is real-world output always lower than the panel's rated wattage?
A panel's rated wattage is measured under standard test conditions — 1,000 W/m² irradiance, 25°C cell temperature, and a specific light spectrum — that real installations rarely match exactly. The performance ratio term in this calculator bundles up everything that separates lab conditions from an installed system: wiring losses, inverter conversion losses, dust and soiling on the panel surface, imperfect tilt or shading, and panels running hotter than the 25°C test temperature (which itself reduces output).
What's a realistic irradiance value to use for a full day, not just noon?
1,000 W/m² represents peak midday sun under clear skies, not an average across a full day — irradiance is much lower in early morning, late afternoon, and under cloud cover. For estimating daily energy rather than instantaneous power, solar installers typically use a location-specific 'peak sun hours' figure (equivalent full-sun hours per day) multiplied by the array's peak wattage, rather than running this formula continuously across 24 hours.
What efficiency should I use if I don't know my panel's exact rating?
Standard crystalline silicon residential panels, the most common type installed today, typically fall between 15% and 22% efficiency, with 19-21% now common for newer premium panels. Checking the panel's own datasheet gives the most accurate figure; when in doubt, 18-20% is a reasonable mid-range estimate for a modern panel.
Does panel area mean the whole panel or just the active cell area?
Use the panel's total physical area (length times width), since manufacturer efficiency ratings are already calculated against the full panel footprint including the small border and frame, not just the exposed cell surface. Using the datasheet's stated area alongside its stated efficiency keeps the two figures consistent with each other.
How does temperature affect solar panel output?
Panels lose efficiency as they heat up above their 25°C rating temperature — most silicon panels lose roughly 0.3-0.5% of output per degree Celsius above 25°C, which matters in practice since panel surfaces routinely run 20-30°C hotter than ambient air temperature on a sunny day. This effect is one of the real-world factors the system performance ratio in this calculator is meant to approximate, alongside wiring and inverter losses.