Solar Panel Output Calculator

How to use the solar panel output calculator

Enter your system size in kilowatts, the peak sun hours for your area, and a system-efficiency (losses) factor, plus your electricity rate, and you get an estimate of production — daily, monthly, and yearly — along with the bill offset that production is worth. The rate sets the dollar figure and varies by state: the US residential average is about 17.5¢/kWh as of 2026, so use the rate from your own bill.

Peak sun hours are not daylight hours — they’re the equivalent hours of full-strength (1,000 W/m²) sunlight a location receives in a day, bundling latitude, weather, and season into one number. Rough US planning ranges run from about 4 in the cloudy Northeast and Northwest to roughly 6 in the desert Southwest. Treat these as estimates and verify your actual address with a tool like NREL’s PVWatts before sizing anything for real.

The 80% efficiency factor is there because panels never deliver their nameplate kilowatts. Inverter conversion, wiring resistance, heat (panels lose output as they warm up), dust and soiling, and slow aging all subtract from the ideal — and 75 to 85% (a 0.80 default) is the standard real-world derate. It’s the reason a 6 kW array doesn’t make a full 6 kWh for every peak sun hour.

Be clear about what this estimates: production, the kilowatt-hours the array makes — not net savings. Your real savings depend on net-metering rules, time-of-use rates, the fixed charges that stay on your bill no matter what, and how much you use versus export. The bill-offset figure here values production at your flat rate as a ceiling, a useful ballpark rather than a guarantee.

To use the result, compare the yearly production against your yearly usage (add up a year of bills) to see what fraction an array would cover, then pair it with the Solar Panel Count Calculator to turn a target system size into an actual panel count. For a real quote, an installer will model your specific roof, shading, and utility — this gives you the back-of-envelope number to start from.

The formula

Production is the system’s size scaled by how much usable sun it sees and how much survives real-world losses. Monthly and yearly are just multiples of the daily figure:

daily kWh = system kW × peak sun hours × efficiency
monthly = daily × 30      yearly = daily × 365
bill offset = yearly kWh × rate

Worked example with the defaults — a 6 kW system, 4.5 peak sun hours, 80% efficiency, $0.175/kWh: 6 × 4.5 × 0.80 = 21.6 kWh a day, about 648 a month and 7,884 a year, worth roughly $1,379.70 a year at that rate.

The biggest swing factors are sun hours and losses. The same system at 6 sun hours makes a third more; bump the derate to 90% and 21.6 becomes 24.3 kWh a day. Because these are planning ranges, treat the output as a ballpark and confirm with PVWatts and an installer.

Frequently asked questions

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