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A solar panel produces roughly its rated wattage × your peak sun hours × 0.80 per day. A 400 W panel in a location with 4.5 peak sun hours produces about 400 × 4.5 × 0.80 = 1,440 Wh, or 1.44 kWh per day.
That single formula answers most “how much power” questions. Here is how it works and what changes the result.
Solar panel output calculator
Estimates use the formulas explained in this guide. Plan a complete system with the all-in-one calculator →
The daily output formula
Daily output (Wh) = Panel watts × Peak sun hours × 0.80
- Panel watts: the STC rating printed on the label (for example 400 W). STC means Standard Test Conditions: 1,000 W/m² of sunlight at a 25 °C cell temperature.
- Peak sun hours: how many hours of full-strength sun your location averages per day. It is not the same as daylight hours. See peak sun hours explained.
- 0.80: accounts for real-world losses from heat, dirt, wiring and inverter or charge controller efficiency.
Daily output by panel size
| Panel | 3 sun hours | 4 sun hours | 5 sun hours | 6 sun hours |
|---|---|---|---|---|
| 100 W | 0.24 kWh | 0.32 kWh | 0.40 kWh | 0.48 kWh |
| 200 W | 0.48 kWh | 0.64 kWh | 0.80 kWh | 0.96 kWh |
| 300 W | 0.72 kWh | 0.96 kWh | 1.20 kWh | 1.44 kWh |
| 400 W | 0.96 kWh | 1.28 kWh | 1.60 kWh | 1.92 kWh |
| 500 W | 1.20 kWh | 1.60 kWh | 2.00 kWh | 2.40 kWh |
Monthly and yearly output
Multiply the daily figure by 30 for a monthly estimate, or by 365 for a yearly one. A 400 W panel at 4.5 sun hours makes about 43 kWh per month and 526 kWh per year.
Keep in mind that peak sun hours change by season. In the northern US a panel can produce three times more in June than in December, so a yearly average hides big swings.
Why your panel produces less than its rating
A 400 W panel rarely produces 400 W, even at midday. The main reasons are:
- Heat. Output falls by about 0.3 to 0.4% for every degree Celsius the cells run above 25 °C. On a hot roof, cells often reach 60 °C or more, costing 10 to 15% of output. Our temperature coefficient guide explains this.
- Sun angle. Panels produce the most when sunlight hits them square on. A poor tilt angle reduces yearly output.
- Dust, pollen and bird droppings. Typically 2 to 5% loss, much more in dry, dusty regions without rain.
- Shade. Even partial shade on one cell can sharply cut a panel’s output. See shading and bypass diodes.
- Wiring and conversion losses. Charge controllers and inverters are typically 93 to 98% efficient.
Worked example: a small off-grid setup
You have four 200 W panels (800 W total) in a location with 5 peak sun hours.
- 800 × 5 × 0.80 = 3,200 Wh (3.2 kWh) per day
That is enough to run an efficient refrigerator (about 1.2 kWh/day), LED lights, a laptop, a Starlink dish and a few phone charges, with some left over to charge batteries for the night.
How to measure your real output
If you already have a system, your inverter app or charge controller shows a “yield today” figure in kWh. Compare it with the formula over a week of clear weather. If your real yield is more than 25% below the estimate, check for shading, dirty panels, loose connections or a mismatched string.
Worked example: estimating a month’s production
You have a 3 kW array in a location where PVGIS gives these figures for June: 186 kWh/m² for the month on your panel’s tilt.
- Peak sun hours per day = 186 ÷ 30 = 6.2
- Daily output = 3 kW × 6.2 × 0.80 = 14.9 kWh
- June output = 14.9 × 30 = 446 kWh
For December with 75 kWh/m²: 75 ÷ 31 = 2.4 sun hours, so 3 × 2.4 × 0.80 = 5.8 kWh per day, or about 180 kWh for the month.
Output per day for common system sizes
| System size | 3.5 sun hours | 4.5 sun hours | 5.5 sun hours |
|---|---|---|---|
| 1 kW | 2.8 kWh | 3.6 kWh | 4.4 kWh |
| 3 kW | 8.4 kWh | 10.8 kWh | 13.2 kWh |
| 5 kW | 14.0 kWh | 18.0 kWh | 22.0 kWh |
| 8 kW | 22.4 kWh | 28.8 kWh | 35.2 kWh |
| 10 kW | 28.0 kWh | 36.0 kWh | 44.0 kWh |
When the 0.80 factor is too optimistic or too pessimistic
- More losses (use 0.70 to 0.75): hot climates with roof-mounted panels and little airflow, dusty sites cleaned rarely, partial shading, PWM controllers, long low-voltage cable runs.
- Fewer losses (use 0.82 to 0.86): cool climates, well-ventilated ground mounts, new high-efficiency panels, modern string inverters on short cable runs.
Our solar calculator uses 0.80 as a sensible middle value.
Daily output vs instantaneous power
Daily output (kWh) tells you how much energy you collect. Instantaneous power (W) tells you what you can run at a given moment. A 3 kW array might produce 2.2 to 2.5 kW at noon on a clear summer day but only 300 W at 8 am. Loads like air conditioners need enough instantaneous power, not just enough daily energy. See solar panels for a 1.5 ton AC.
Common mistakes
- Expecting rated wattage all day. Output follows a curve that peaks around solar noon.
- Adding battery energy twice. Batteries store energy; they don’t add to daily production.
- Ignoring clipping. If a grid-tied inverter is smaller than the array, midday output is capped, which is normal and costs only a little energy.
FAQ
How much does a 1 kW solar system produce per day?
About 1 kW × peak sun hours × 0.80. At 4.5 sun hours that is roughly 3.6 kWh per day, or about 108 kWh per month.
Do solar panels produce power on cloudy days?
Yes, but much less, typically 10 to 25% of normal output under heavy overcast. Read more in do solar panels work on cloudy days.
How do I calculate how many panels I need from daily output?
Divide your daily energy use by the daily output of one panel. Our step-by-step guide on how many solar panels you need and the solar calculator do this for you.
How much does a 5 kW solar system produce per day?
About 18 kWh at 4.5 peak sun hours, ranging from roughly 14 kWh in cloudy climates to 22 kWh or more in very sunny ones.
How much does a 1 kW solar system produce per month?
About 108 kWh per month at 4.5 peak sun hours (1 × 4.5 × 0.80 × 30).
Related guides
- How Much Power Does a 400W Solar Panel Produce Per Day?
- What Can a 100W Solar Panel Run? (Real Daily Numbers)
- Bifacial Solar Panels: Are They Worth It?
- What Can a 1kW to 20kW Solar System Run? kWh per Day and Appliance List
Sources and further reading
- NREL PVWatts Calculator: solar output estimate for any address.
- European Commission JRC: PVGIS: free solar output and optimum-tilt tool for Europe and most of the world.
- Global Solar Atlas (World Bank): sun data and solar potential for any location.
- U.S. Department of Energy: PV Performance and Efficiency Basics: what limits panel efficiency, including heat.