How Much Energy Does a Solar Panel Produce Per Day?

Key Takeaways

• Rule-of-thumb: a 400 W panel ≈ 1.3–1.6 kWh per day, annualized, across most of the US.
• The formula is three terms: wattage × peak sun hours × derate (≈0.75–0.85) — everything else is refinement.
• Panel ratings are lab numbers; real roofs deliver 15–25% less than nameplate, which honest math bakes in up front.
• Your exact figure is free: PVWatts turns an address into an annual estimate that embarrasses every rule of thumb, including this one.

The formula behind every estimate

Solar production math reduces to one line:

Daily energy = panel wattage × peak sun hours × derate factor

A peak sun hour is the industry's way of standardizing sunshine: one hour of light at the laboratory intensity panels are rated under. A raw day of sun — morning slants, noon blaze, afternoon haze — compresses into a single equivalent number, and most US locations average roughly 3.5 to 5.5 peak sun hours daily across the year: the desert Southwest at the top, the cloudy Northwest and Northeast at the bottom.

The derate factor is honesty about the gap between lab and roof: heat (silicon loses efficiency as it warms), inverter conversion, wiring, dust, and imperfect orientation collectively shave 15–25% off nameplate. Use 0.75–0.85 and you're modeling reality rather than the brochure.

Worked example, assumptions labeled: 400 W × 4.5 peak sun hours × 0.8 = 1.44 kWh per day — the middle of the range every credible estimate lands in.

The range across the country

Assumptions: 400 W panel, 0.8 derate; annualized averages that smooth summer peaks and winter troughs. Your address's real number: PVWatts.

Two readings of that table matter. First, geography moves output by roughly ±25% around the average — real money over 25 years, but not the difference between sense and nonsense; solar works in Maine, just with more panels. Second, these are annual averages: the same panel might deliver 2+ kWh on a long June day and under half its average in December. Systems are sized against the year, not the solstices.

From one panel to your roof

Multiply by your panel count and the arithmetic scales linearly: twenty 400 W panels at the US-average 1.4 kWh each ≈ 28 kWh a day — almost exactly the EIA-average American home's consumption, which is why our companion guide lands on ~20 panels for the average house. The two rules of thumb are the same math wearing different units.

The refinements that move your number beyond geography: orientation (south-facing captures the most; east/west costs a modest slice), tilt (roof pitch near your latitude is ideal; flat and steep both trim), shading (the disproportionate killer — see our shade guide), and panel degradation (output declines gently with age; roughly half a percent a year is the industry's planning figure).

Getting your real number in five minutes

Skip the thumb-rules entirely: NREL's PVWatts calculator ingests your address, roof tilt, azimuth, and system size, applies decades of local weather data, and returns monthly and annual production estimates — the same class of tool installers use. Run it before requesting quotes and you hold the referee's number: any proposal promising meaningfully more production than PVWatts for the same roof is either counting panels you don't see or sunshine that isn't coming.

One panel's daily kilowatt-hour-and-a-half sounds humble. Multiply by twenty panels, 365 days, and 25 years — roughly 260,000 kWh from an average roof — and the humility inverts: that's a quarter-million kilowatt-hours of fuel-free electricity from hardware with no moving parts, which is the entire quiet argument for putting glass on roofs.

Keep reading: panels to power a house · panel direction · sizing for 1,000 kWh.