How Much Electricity Does a Solar Panel Produce?

The honest answer fits in one line of arithmetic: panel watts, times your local peak sun hours, times a real-world derate of about 0.8. For a typical 400 W panel that works out to roughly 1.2 to 1.8 kWh per day. Here is where every number in that formula comes from, and how to run it for your own roof.

The core formula, with nothing hidden

Most solar sites bury this in marketing copy, so here it is up front. The daily energy from one panel is:

Take a common 2026 residential panel rated at 400 W, in a location averaging 4.5 peak sun hours per day: 400 × 4.5 × 0.8 ÷ 1000 = 1.44 kWh per day, or roughly 500 to 525 kWh per year. Modern panels span about 400 to 575 W, so scale accordingly — a 550 W panel in the same spot makes just under 2 kWh per day. The 0.8 is a combined derate factor covering inverter losses, wiring, temperature, dust and mismatch; we unpack it piece by piece below.

Peak sun hours are not daylight hours

This is the number people get wrong most often. A peak sun hour is one hour of sunshine at the standard test intensity of 1,000 W per square metre. Your location might get 14 hours of daylight in June, but the morning and evening sun is weak, clouds interrupt, and the sun sits low for much of the day. Add up all the actual solar energy landing on your roof and express it as equivalent full-intensity hours, and most inhabited places average between 2.8 and 5.8 peak sun hours per day across the year.

Those are annual averages: the same panel might do double the table figure on a clear June day and a third of it in December. See our guide on winter and cloudy-weather performance for the seasonal detail.

Capacity factor: the honest annual yardstick

Utilities describe generators by capacity factor — actual annual output divided by what the nameplate rating would produce running flat-out 24/7. A 400 W panel running continuously would make 3,504 kWh a year; in reality it makes 330 to 640 kWh. That is a capacity factor of roughly 15% in cloudy climates to 22% in deserts. It sounds low, but it is simply physics: the sun sets, and it is rarely directly overhead. Any quote implying dramatically better than this deserves scepticism.

What moves the number, and by how much

• Orientation: facing the equator is the baseline. Due east or west typically costs 10 to 20% annually; north-facing (in the northern hemisphere) can cost 30% or more.
• Tilt: within 10° of the optimum, losses are small (2 to 5%). Flat mounting in mid-latitudes loses around 10% and soils faster. Our tilt angle calculator finds the best angle for your latitude.
• Shading: the wildcard. A chimney shadow crossing panels for two hours daily can cut a string 20 to 40%; heavy tree shade can cost 50 to 80% without microinverters or optimizers.
• Temperature: panels lose roughly 0.3 to 0.4% of output per °C above 25°C cell temperature. On a hot roof, cells run 20 to 30°C above ambient, so expect 5 to 12% loss in warm climates during summer afternoons.
• Dust and soiling: 2 to 5% typically, up to 7%+ in dry, dusty or high-pollen areas between rains.
• Inverter and wiring: modern inverters run 96 to 98% efficient; add cabling and mismatch and you lose 4 to 8% here.

Multiply the survivor fractions together and you land near 0.75 to 0.85 for a decent unshaded install — which is why 0.8 is the honest default derate.

The per-kW shortcut

Because the formula is linear, professionals think per kilowatt of installed capacity: 1 kW of panels produces about 3 to 5 kWh per day (1,100 to 1,800 kWh per year) depending on location. Multiply by your system size and you have a sanity check for any quote in under ten seconds. It also makes panel counts interchangeable: 2.5 kW is 2.5 kW whether built from five 500 W panels or six 415 W panels — the roof layout changes, the annual energy barely does. When an installer quotes annual kWh, divide it by system kW: if the answer falls outside 1,100 to 1,800 kWh per kW per year, ask which assumption is doing the heavy lifting.

The shape of a year

Output follows a smooth seasonal curve. In mid-latitudes, the best summer month typically produces 1.5 to 2 times the annual monthly average, and the worst winter month 40 to 60% of it. At high latitudes (Scotland, Scandinavia, Canada) the spread widens: June can deliver four or five times December. Design for the annual total if you have net metering; design nearer the winter floor if you are off-grid.

Why nameplate never equals real output

The wattage on the label is measured at Standard Test Conditions (STC): 1,000 W/m² irradiance, 25°C cell temperature, a defined light spectrum. Real roofs are hotter, hazier and dustier than a flash-test lab, so a 400 W panel peaking at 340 to 370 W on a good day is normal, not defective.

Reading a datasheet in 60 seconds

Worked example: powering a 20 kWh/day home

1. Rearrange the formula: kW needed = daily kWh ÷ (sun hours × 0.8).
2. At 4.5 sun hours: 20 ÷ (4.5 × 0.8) = 5.6 kW → fourteen 400 W panels (or eleven 500 W panels).
3. At 3.5 sun hours: 20 ÷ (3.5 × 0.8) = 7.1 kW → eighteen 400 W panels.

For a fuller treatment including usage patterns and roof constraints, see how many solar panels do I need or run the system size calculator.

Common mistakes

• Multiplying panel watts by daylight hours instead of peak sun hours — this overestimates output by 2 to 3 times.
• Forgetting the derate entirely and treating nameplate watts as deliverable watts.
• Using a summer month to size a system that must also work in December.
• Ignoring the temperature coefficient when comparing panels for a hot climate.
• Averaging away shading: a shadow that hits at your peak production hour hurts far more than the same shadow at 8 a.m.

Frequently asked questions

How much electricity does one solar panel produce per day?

A typical 400 W panel produces about 0.9 to 1.8 kWh per day depending on climate, using the formula watts times peak sun hours times 0.8 divided by 1000. In an average sunny location (4.5 sun hours) that is roughly 1.4 kWh per day, or about 500 kWh per year.

What are peak sun hours and how are they different from daylight hours?

A peak sun hour is one hour of sunlight at the standard intensity of 1,000 W per square metre. Weak morning, evening and cloudy light is converted into equivalent full-intensity hours, so a location with 14 daylight hours may only average 4 to 5 peak sun hours. Most inhabited areas fall between 2.8 and 5.8.

Why does my 400 W panel never actually output 400 watts?

The rating is measured at Standard Test Conditions: 1,000 W/m2 and a cool 25 C cell temperature. Real rooftops run 20 to 30 C hotter and rarely see full lab irradiance, so peaks of 340 to 370 W on a clear day are completely normal for a healthy 400 W panel.

How many solar panels do I need for 20 kWh per day?

Divide 20 by your peak sun hours times 0.8. At 4.5 sun hours you need about 5.6 kW, which is fourteen 400 W panels or eleven 500 W panels. At 3.5 sun hours you need about 7.1 kW, or eighteen 400 W panels.