Solar Panel Output Calculator
Enter a panel wattage, your site's peak sun hours, and a performance ratio to estimate how many kilowatt-hours your solar setup produces in a day.
Daily kWh at a glance
Enter the wattage, peak sun hours, and performance ratio and the calculator returns the daily energy output in kilowatt-hours (kWh).
It's an estimate
Real output swings with weather, season, tilt, temperature, and shading — treat the figure as a typical-day average, not a guarantee.
What does a solar panel output calculator do?
Turning watts and sunshine into kilowatt-hours
A solar panel output calculator estimates how much energy a panel or array produces in a day. It takes the rated wattage of your panels, the peak sun hours your location receives, and a performance ratio that accounts for real-world losses, and returns the daily output in kilowatt-hours — the same unit your electricity bill uses. Rated wattage alone tells you only the power under perfect lab conditions; what actually matters is how many full-strength sun hours fall on the panel each day and how much of that the system keeps after wiring, inverter, heat, and shading losses. This is the number behind sizing a rooftop array, estimating bill savings, and comparing one site or panel against another.
Enter your panel wattage, peak sun hours, and a performance ratio to get the daily energy output in kilowatt-hours instantly.
Daily energy output is the panel wattage multiplied by the peak sun hours and the performance ratio, then divided by 1000 to convert watt-hours into kilowatt-hours.
kWh/day = (W × peak sun hours × performance ratio) ÷ 1000Wattage and peak sun hours both scale the result directly, so doubling either roughly doubles the energy. The performance ratio trims the ideal figure down to what the system actually delivers. Divide by 1000 because wattage is in watts but energy bills and panel output are quoted in kilowatt-hours.
Suppose you have a single 400 W panel at a site that averages 5 peak sun hours a day, with a performance ratio of 0.75.
Multiply wattage by sun hours
400 × 5 = 2000 watt-hours — the ideal output before losses.
Apply the performance ratio
2000 × 0.75 = 1500 watt-hours — what the system actually keeps.
Convert to kilowatt-hours
1500 ÷ 1000 = 1.5 kWh per day — the daily energy output.
The two inputs that move the result the most are peak sun hours and the performance ratio. Peak sun hours are not daylight hours: they count only the equivalent hours of full-strength sun (1000 W/m²), so even a long summer day might deliver 5–6 peak sun hours while a short, overcast winter day delivers 1–2. This varies hugely by region — a site in southern Spain or Arizona may average over 5 peak sun hours year-round, while northern Germany or the UK often sits closer to 2.5–3. The performance ratio captures everything the textbook formula ignores: inverter and wiring losses, heat that pushes panels above their rated temperature, dust, and partial shading. A well-installed, unshaded system typically lands around 0.75, with 0.80+ for excellent installs and below 0.70 where shading or heat is severe. To estimate a whole array rather than one panel, enter the combined wattage — ten 400 W panels become 4000 W. Multiply the daily figure by 365 for a rough annual estimate, though real years average out higher-summer and lower-winter days.
The formula is a sound first approximation, but a few practical points are worth keeping in mind.
A typical-day estimate, not a forecast
This calculator gives a representative daily average, not the output of any specific day. Real production swings with weather, cloud cover, season, panel tilt and orientation, temperature, dust, and shading, and degrades slowly over a panel's lifetime. Peak sun hours and the performance ratio are themselves averages — for a precise, location-specific estimate use a modelling tool such as NREL's PVWatts, which factors in your exact coordinates, tilt, and local weather history.