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Solar Panel Output Calculator
Find out how much electricity your solar panels will produce. Enter the panel wattage, how many panels you have, your local peak sun hours and system losses to see kWh per day, month and year, what that power is worth and how output declines over 25 years.
Last updated:
Your panels
W
Nameplate (STC) watts on the panel label.
h
Yearly average for your location, roughly 3 (cloudy north) to 6+ (desert southwest).
%
Inverter, wiring, heat, dirt, shading. 14% is the PVWatts default.
Value and comparison (optional)
$/kWh
kWh
From your power bill, to see how much the panels cover.
%/yr
Typical warranties assume about 0.25–0.8% per year.
%/yr
Enter your values to see the result.
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How to use the Solar Panel Output Calculator
- Enter the panel rating in watts and how many panels you have or plan.
- Enter your peak sun hours, the yearly average for your location.
- Adjust system losses if your panels are shaded or hot (14% is typical).
- Add your electricity rate and monthly use to see the value and coverage.
- Review the 25-year table and the sun-hour comparison, and download them if needed.
Solar panel output formula
- System size (kW) = number of panels × panel watts ÷ 1,000
- Daily kWh = system kW × peak sun hours × (1 − losses)
- Monthly kWh = daily kWh × 365 ÷ 12; yearly kWh = daily kWh × 365
- Capacity factor = yearly kWh ÷ (system kW × 8,760 hours)
- Year y output = first-year output × (1 − degradation)y−1
Output of one panel by size and sun hours
kWh per day with 14% losses.
| Panel | 3 h | 4 h | 5 h | 6 h | Per year at 4.5 h |
|---|---|---|---|---|---|
| 100 W | 0.26 | 0.34 | 0.43 | 0.52 | 141 kWh |
| 300 W | 0.77 | 1.03 | 1.29 | 1.55 | 424 kWh |
| 400 W | 1.03 | 1.38 | 1.72 | 2.06 | 565 kWh |
| 450 W | 1.16 | 1.55 | 1.94 | 2.32 | 636 kWh |
Tips for getting more from your panels
- Avoid shade: on a string inverter, one shaded panel can pull down the whole string. Microinverters or optimizers limit that loss.
- In the Northern Hemisphere, south-facing panels tilted close to your latitude give the most yearly energy.
- Rain cleans most roof panels; in dusty or dry areas occasional cleaning can recover a few percent.
- Check your monitoring app against this estimate; a big gap often means a fault or shading.
Frequently asked questions
How much electricity does a solar panel produce?
Daily kWh = panel watts ÷ 1,000 × peak sun hours × (1 − losses). A 400 W panel with 4.5 peak sun hours and 14% losses makes 0.4 × 4.5 × 0.86 = 1.55 kWh per day, about 47 kWh per month or 565 kWh per year.
How much does a 20-panel solar system produce?
With 400 W panels (8 kW), 4.5 sun hours and 14% losses it produces about 31 kWh per day, 940 kWh per month and 11,300 kWh per year. The same system produces about 25% less with 3.4 sun hours and about a third more with 6.
What are peak sun hours?
Peak sun hours are the number of hours per day the sun would need to shine at 1,000 W/m², the panel test condition, to deliver the day's total solar energy. A location that receives 5 kWh/m² of sunlight per day has 5 peak sun hours. Typical yearly averages range from about 3 to 6.
Why do panels produce less than their rated watts?
The rating is measured at 25 °C cell temperature and 1,000 W/m² of light. In real use panels run hotter, the inverter and wiring lose a few percent, dirt and shading block light and the sun is rarely at full strength. Together these losses are commonly modelled at around 14%.
How much does solar output decline over time?
Most crystalline silicon panels lose roughly 0.25–0.8% of output per year. At 0.5% per year, a panel still makes about 89% of its first-year output in year 25.
Does this calculator account for seasons and tilt?
It uses a yearly average of peak sun hours, so monthly figures are averages: summer months will produce more and winter less. Tilt and orientation are reflected in the sun hours and losses you choose; for a site-specific monthly model use a tool such as NREL PVWatts.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.
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