About the Solar Panel Wattage Calculator
This calculator tells you how many watts of solar panels you need to cover a given amount of daily electricity use, and how many individual panels that translates to. It is the core sizing calculation used by solar installers and DIY off-grid planners before they ever price out equipment.
The formula
Required array wattage (W) = (Daily energy use in kWh ÷ (Peak sun hours × System efficiency)) × 1000
Once you know the required wattage, the number of panels is simply that wattage divided by the wattage rating of a single panel, rounded up to a whole panel. The calculator also reports the estimated daily energy output of the sized array, computed the same way in reverse: Daily output (kWh) = Array wattage (W) × Peak sun hours × System efficiency ÷ 1000.
Why "peak sun hours" instead of daylight hours
A peak sun hour is the equivalent of one full hour of sunlight at 1,000 watts per square meter — the standard test irradiance used to rate solar panels. Real sunlight varies throughout the day from near-zero at sunrise to a peak around solar noon, so meteorologists and solar engineers convert a day's total irradiance into an equivalent number of "peak" hours. A location might get 13–14 hours of daylight but only 4–6 peak sun hours. Typical values: Phoenix, AZ ≈ 6.5, Los Angeles ≈ 5.5, New York City ≈ 4.2, Seattle ≈ 3.4, and most of the continental US falls between 3 and 6.5 depending on latitude and cloud cover. Always use a peak-sun-hour figure for your specific location (from a solar irradiance map or your utility) rather than guessing.
Why a system efficiency (derate) factor is needed
Panels rarely deliver their full rated wattage in real installations. Losses come from inverter conversion (DC to AC), wiring resistance, panel soiling and dust, temperature above the 25°C test standard, shading, and inverter clipping. The U.S. National Renewable Energy Laboratory's PVWatts tool uses a default overall derate factor of about 0.86, but real-world residential systems commonly land between 0.75 and 0.85 once age and local conditions are included. This calculator defaults to 0.80 as a conservative, widely used planning figure — adjust it upward if you have a newer high-efficiency inverter and clean panels, or downward for older equipment or hot climates.
Worked example
A household uses 30 kWh per day and lives somewhere with 5 peak sun hours and an assumed 0.80 system efficiency. Required wattage = (30 ÷ (5 × 0.80)) × 1000 = (30 ÷ 4) × 1000 = 7,500 W, or 7.5 kW DC. With 400 W panels, that's 7500 ÷ 400 = 18.75, rounded up to 19 panels.
Typical reference values
- US average household electricity use: roughly 29 kWh/day (about 886 kWh/month, per the U.S. Energy Information Administration).
- Residential solar panel wattage today: most panels sold for home installs are rated 350–450 W, with 400–420 W being the most common in 2025–2026.
- Typical home system size: most US residential solar arrays fall between 5 kW and 10 kW DC.