How much power does a solar panel produce?
How much power does a solar panel produce? In a day, about its wattage × your peak sun hours × a loss factor of 0.75 to 0.85, in watt-hours. For example, a hypothetical 400 W panel on a roof that gets 4.5 peak sun hours, with a loss factor of 0.8, makes about 400 × 4.5 × 0.8 = 1,440 Wh, or roughly 1.4 kWh a day.
The rest of this guide explains each part of that formula, what a panel’s wattage rating actually means, what pushes real output up or down, and how big common panels are. Worked examples and answers to common questions come at the end.
The formula
Two lines cover one panel’s output:
- Daily output (Wh) = panel wattage (W) × peak sun hours × loss factor
- Yearly output (kWh) = daily output (Wh) × 365 ÷ 1,000
What each term means:
- Panel wattage is the rating printed on the panel’s label and datasheet, such as 100 W or 430 W. It’s the power the panel makes under standard lab conditions (see the next section).
- Peak sun hours is how much sunlight your location gets in a day, expressed as hours of full-strength sun. One peak sun hour is one hour at 1,000 watts per square meter. A place with 4.5 peak sun hours gets as much solar energy over the whole day as 4.5 hours of full midday sun, even though the sun is up much longer.
- The loss factor trims the ideal number for real-world losses: heat, dust, wiring, the inverter or charge controller, and panel aging. About 0.75 to 0.85 covers most setups. Use 0.8 if you’re unsure.
Divide by 1,000 to turn watt-hours into kilowatt-hours (kWh), the unit on your electric bill. For a monthly figure, multiply the daily output by 30.
What a panel’s wattage rating means
A panel’s wattage is measured under Standard Test Conditions (STC): 1,000 watts of sunlight per square meter, with the solar cells held at 25°C (77°F). Every manufacturer tests the same way, so STC ratings let you compare panels fairly. But a panel on a real roof seldom sees those conditions. Sunlight is usually weaker than 1,000 W/m², and cells in full sun run much hotter than 25°C.
Many datasheets also list a second rating, often labeled NMOT or NOCT, measured with weaker light and warmer cells. It’s typically around three-quarters of the STC rating, and it’s a better guide to what a panel delivers on an ordinary sunny day.
The temperature coefficient
Solar panels lose output as they heat up. The datasheet states how much as the temperature coefficient of Pmax, a negative percentage per °C. A panel rated at −0.30%/°C, for example, loses 0.30% of its rated power for every degree its cells run above 25°C.
Cells in direct sun often run 20°C to 30°C hotter than the surrounding air. Take a hypothetical panel at −0.30%/°C whose cells reach 55°C on a hot afternoon: that’s 30 degrees above STC, so it produces about 9% less than its rating at that moment. A smaller (closer to zero) coefficient means less loss in hot weather, which matters most in hot climates.
What changes real output
The formula’s three inputs move with your location and setup. These are the things that matter most:
- Sun hours by location. This is the biggest factor. For a south-facing roof, peak sun hours range from about 4.0 a day in Seattle to about 6.5 in Phoenix, according to PVWatts (version 8, checked October 2026). The free PVWatts calculator from the U.S. Department of Energy’s National Laboratory of the Rockies (formerly NREL) gives you the figure for your own address: its “solar radiation (kWh/m²/day)” result is your peak sun hours.
- Tilt and direction. In the U.S., panels facing south get the most sun. East- and west-facing panels produce noticeably less, and north-facing ones much less. A moderate tilt collects more sun over a year than lying flat or standing very steep. PVWatts lets you enter both.
- Shade. Even partial shade from a tree, chimney or vent pipe can cut a panel’s output sharply, because shading part of the cells limits current through the rest. Keep panels in full sun from mid-morning to mid-afternoon if you can.
- Heat. As the temperature coefficient section above shows, hot panels make less power. Panels with airflow behind them stay cooler than panels mounted flush against a surface.
- Dirt, dust and snow. A dusty or pollen-coated panel loses some output, and snow cover stops it entirely until the snow slides off or melts. Rain cleans most panels well enough. In dry, dusty places, an occasional rinse helps.
- Age. Panels lose a little output every year. NREL’s research puts the median for crystalline silicon panels at about 0.5% a year (NREL, Photovoltaic Lifetime Project, checked October 2026), so a panel might make around 10% less after 20 years than when new. Most manufacturers warranty power output for 25 years.
Output by panel size
Higher-wattage panels are physically bigger, more efficient, or both. This table shows typical sizes for common panel types, so you can see how much space a given output takes up. These are typical figures from the sources listed, not a full survey of the market. Dimensions vary by model, so check the datasheet of the panel you’re considering.
| Panel type | Typical wattage | Typical size | Area | Source | As of |
|---|---|---|---|---|---|
| Folding portable panel (camping, power stations) | 100 W | About 42 × 21 in unfolded | About 6 sq ft | Westinghouse WSolar100p specs | Checked October 2026 |
| Rigid 12 V panel (RVs, sheds, off-grid) | 100 W | About 41 × 21 in | About 6 sq ft | Renogy Eclipse RNG-100MB specs | Checked October 2026 |
| Residential rooftop panel | 400–460 W for about 97% of panels quoted; 430–440 W most common | About 5.8 × 3.5 ft (60-cell) to 5.7 × 3.7 ft (current high-output models) | About 20–21 sq ft | EnergySage, solar panel output | First half of 2026 (data); checked October 2026 |
| Large-format panel (72-cell, mostly commercial and ground-mount) | 555–575 W | About 7.5 × 3.7 ft (2,278 × 1,134 mm) | About 28 sq ft | Jinko Tiger Neo 72HL4-(V) datasheet | Checked October 2026 |
Notice that the watts per square foot stay in a fairly narrow band: roughly 16 to 23 W per square foot across these types. A panel’s size tells you its output within a fairly small range, which is why roof space is often what limits a home system.
Worked examples
These examples use made-up combinations of panel wattage and sun hours, with a loss factor of 0.8, to show the method. They aren’t predictions for any real location. Swap in your own panel’s wattage and your peak sun hours from PVWatts.
| Panel | Peak sun hours | Daily output | Monthly output (× 30) | Yearly output (× 365) |
|---|---|---|---|---|
| 100 W | 5.0 | 100 × 5.0 × 0.8 = 400 Wh (0.4 kWh) | 12 kWh | About 146 kWh |
| 400 W | 4.5 | 400 × 4.5 × 0.8 = 1,440 Wh (1.44 kWh) | About 43 kWh | About 526 kWh |
| 430 W | 5.5 | 430 × 5.5 × 0.8 = 1,892 Wh (1.89 kWh) | About 57 kWh | About 691 kWh |
Changing the loss factor moves every result in proportion. At 0.75, the 400 W panel makes about 1.35 kWh a day; at 0.85, about 1.53 kWh.
These figures are for one panel. To turn them into a full home system, see our guide to how many solar panels you need.
Frequently asked questions
How much power does a 400 W solar panel produce per day?
Multiply 400 by your peak sun hours and a loss factor of about 0.8. With 4 to 6 peak sun hours, that works out to roughly 1.3 to 1.9 kWh a day. At 4.5 peak sun hours, it’s about 1.44 kWh.
How many kWh does a solar panel produce per month?
Work out its daily output and multiply by 30. A 400 W panel at 4.5 peak sun hours and a 0.8 loss factor makes about 1.44 kWh a day, or about 43 kWh a month. Output is higher in summer and lower in winter, so a single month can be well above or below the yearly average.
Do solar panels produce power on cloudy days?
Yes, but much less. Panels respond to the total light reaching them, and thick cloud can cut output to a small fraction of a clear day’s. Thin or broken cloud costs less. Peak sun hours figures from PVWatts already average in a typical year’s cloudy days. Panels make no power at night.
How many solar panels does it take to power a house?
That depends on your home’s yearly electricity use as well as each panel’s output, so it’s a separate calculation. Our guide to how many solar panels you need walks through it step by step.