Quick answer
Solar Capacity Factor is the ratio of the electricity a solar system actually produces over a period to what it would produce running at full rated power every hour of that period.
US utility-scale solar averaged 24.4% in 2025, according to the EIA. A fixed rooftop system is usually lower because it does not track the sun.
Quick facts
The key facts about solar capacity factor, with sources:
Key takeaways
- Capacity factor is real output divided by the most a system could make if it ran at full power all day, every day.
- It is not efficiency. A 16% capacity factor does not mean the panels waste 84% of the sunlight.
- US solar farms averaged 24.4% in 2025. A fixed south-facing roof in Charleston models at 16.6%.
- Roofs score lower because they cannot follow the sun and are rated on panel (DC) watts.
- Use it to compare places, designs and quotes on the same basis. Use dollars, not capacity factor, to judge value.
What capacity factor means in plain terms
Imagine a car rated for 100 miles an hour. If you drove it at full speed for 24 hours, you would cover 2,400 miles. If you actually covered 400 miles that day, your "capacity factor" was 400 ÷ 2,400, or about 17%.
A solar system works the same way. The EIA defines capacity factor as the energy a generator produced in a period, divided by the energy it could have produced at continuous full power over that same period.
Solar can never come close to 100%. There is no sun at night, the sun is low in winter, and clouds pass. Even at noon, panels rarely reach their rated power. So a solar capacity factor in the teens or twenties is normal, not a sign of a fault.
Sources: [2]
How to calculate it, step by step
For a full year, use this formula: capacity factor = annual kWh ÷ (system kW × 8,760 hours). There are 8,760 hours in a non-leap year.
Take a 7 kW array that PVWatts models at 10,179 kWh a year in Charleston. The most it could make is 7 × 8,760 = 61,320 kWh. So its capacity factor is 10,179 ÷ 61,320 = 16.6%. The same 7 kW in Atlanta, at 9,812 kWh, is 16.0%.
One detail matters a lot. PVWatts divides by the DC rating of the panels 5. Utility statistics usually divide by the plant's AC rating, which is the inverter size. The AC rating is smaller, so the same sunshine gives a higher percentage. Always check which basis a figure uses before you compare.
Types of capacity factor you will see
| Type | Divides by | Where you see it |
|---|---|---|
| DC capacity factor | Panel rating in kW DC | PVWatts results and most home solar quotes 5 |
| AC capacity factor | Inverter or plant rating in kW AC | Utility and government statistics |
| Net capacity factor | Rated capacity, after the plant's own use is subtracted from output | Power plant reporting 2 |
| Monthly capacity factor | Rated capacity × hours in that month | EIA monthly tables; shows the seasons 1 |
US utility-scale solar capacity factor by year
| Year | Solar PV capacity factor |
|---|---|
| 2016 | 25.0% |
| 2017 | 25.6% |
| 2018 | 25.1% |
| 2019 | 24.3% |
| 2020 | 24.2% |
| 2021 | 24.4% |
| 2022 | 24.4% |
| 2023 | 23.2% |
| 2024 | 23.2% |
| 2025 | 24.4% |
Example: how roof direction and angle change capacity factor
We ran NLR's PVWatts version 8 on 7 October 2026 for 7 kW DC in Charleston, SC: standard module, fixed roof mount, 14% system losses. Only the direction or tilt changed. PVWatts reports capacity factor on a DC basis 4.
| Layout | Yearly kWh | Capacity factor |
|---|---|---|
| South, 20° tilt | 10,179 | 16.6% |
| Southeast or southwest, 20° tilt | about 9,800 | 16.0% |
| Flat (0° tilt) | 9,148 | 14.9% |
| East or west, 20° tilt | about 8,870–8,880 | 14.5% |
| North, 20° tilt | 7,325 | 11.9% |
Each figure is the PVWatts result for that layout. You can check any of them: kWh ÷ (7 × 8,760).
Where capacity factor shows up
Home quotes rarely print capacity factor. You can work it out from two numbers that they do print: system size in kW DC and year-one production in kWh.
PVWatts shows it in its results, next to annual kWh 5. Government reports use it to compare power sources. The EIA publishes it for solar, wind, nuclear and other sources every month 1.
You may also see it in news stories that compare solar with other plants. In those, the figure is almost always on an AC basis for large plants, not for home roofs.
Capacity factor by power source, 2025
| Source | Capacity factor |
|---|---|
| Nuclear | 91.0% |
| Geothermal | 65.9% |
| Hydroelectric | 35.3% |
| Wind | 34.2% |
| Solar photovoltaic | 24.4% |
| Solar thermal | 23.6% |
Why your roof scores lower than a solar farm
| Factor | Utility-scale plant | Home rooftop |
|---|---|---|
| Mounting | Often trackers that follow the sun 6 | Fixed at the roof's angle and direction |
| Rating used | Usually AC (inverter) capacity | Usually DC (panel) capacity on quotes and in PVWatts |
| Site choice | Chosen for sun and open land | Whatever roof the house has, with its shade |
What capacity factor is good for, and its limits
Useful for
- Comparing two locations or two designs on the same basis.
- Checking a production claim on a quote.
- Comparing your real year with the estimate.
- Seeing the effect of seasons, using monthly figures.
Not useful for
- Judging panel quality. It is not an efficiency figure; see panel efficiency.
- Judging value. A kWh at a time-of-use peak can be worth more than one at noon; see time-of-use rates and solar.
- Comparing DC and AC figures with each other.
- Deciding if solar pays. That depends on cost and your rate.
Limits of the number
A higher capacity factor is not always the better design. Adding more panels than the inverter can handle, a high DC-to-AC ratio, raises the AC capacity factor. But some power is lost on the sunniest hours, which is called clipping.
It also ignores cost. A tracker lifts capacity factor, but the DOE notes trackers add upfront cost and maintenance 6. On a house, a fixed mount is almost always the practical choice.
How capacity factor links to cost and lifespan
We do not quote dollar prices here. Capacity factor helps you turn a price into a cost per kWh. See cost per watt for current figures.
- Same price, higher capacity factor, cheaper energy. More kWh from each kW spreads the cost over more energy.
- Levelized cost. Analysts fold capacity factor into the levelized cost of energy, a lifetime cost per kWh.
- Slow decline. Panels lose a little output each year, so capacity factor drifts down. One data sheet allows no more than 0.33% loss a year after year one 8.
- Lifespan. DOE guidance says a system should produce for at least 30 years, with major parts typically under 25-year warranties 7.
How to work out your own capacity factor
- Find your system size in kW DC on your contract or permit drawings.
- Export 12 months of production in kWh from your monitoring app.
- Multiply system kW by 8,760 to get the most the system could make.
- Divide your yearly kWh by that number.
- Compare the result with the installer's estimate, on the same DC basis.
If you do not have a full year yet, run your address in PVWatts. It reports capacity factor with each result.
Keeping your capacity factor where it should be
You do not maintain a capacity factor directly. You keep the system healthy, and the number follows. DOE guidance says a roof system has no moving parts and should need no routine maintenance. Rain usually keeps panels clean enough.
Check your yearly figure once a year. A small drop each year is normal ageing. A large drop from one year to the next points to a fault, new shade or a failed inverter.
Sources: [7]
Warning signs: when to call a pro
Call your installer if any of these show up. See why a system underproduces.
- Your yearly capacity factor falls by more than a few points from one year to the next.
- A quote implies a fixed rooftop capacity factor well above the high teens in the Southeast, on a DC basis.
- Monthly figures fall far below the same month last year.
- The inverter shows a fault code or the app shows zero on a sunny day.
Rules and reporting behind the figures
No building code or utility rule sets a minimum capacity factor for home solar. The national figures come from federal reporting. The EIA collects data from utility-scale plants, those of at least 1 megawatt, and publishes capacity factors monthly 1 3.
Small rooftop systems are reported differently. The EIA estimates their total output rather than collecting plant-by-plant data. It counts systems below 1,000 kW as small-scale 3.
What rules do shape is system size. Your utility's size cap limits the kW you can connect. That changes total kWh, not capacity factor.
Capacity factor in SC, GA and VA
Our PVWatts runs, with the same inputs, gave a DC capacity factor of 16.6% in Charleston, SC and 16.0% in Atlanta, GA 4. These are models, not measured state averages.
Size limits differ by state: Duke Energy Carolinas caps residential systems in SC at 20 kW AC 9, Georgia Power at 10 kW 10, and Virginia law allows residential net metering up to 25 kW on investor-owned utilities 11.
Common misconceptions
- Myth A 25% capacity factor means the plant runs a quarter of the time.
- Reality It runs whenever there is daylight, mostly below full power. The percentage is an average across all 8,760 hours, nights included.
- Myth Low capacity factor means solar is a bad investment.
- Reality Capacity factor ignores cost. Payback depends on what each kWh saves you; try the payback calculator.
- Myth Capacity factor and efficiency are the same.
- Reality Efficiency is the share of sunlight a panel turns into power. Capacity factor is output over time against full power.
- Myth A roof figure can be compared directly with a solar farm figure.
- Reality Roofs are usually rated in DC and farms in AC, and farms often track the sun.
When capacity factor matters to you
Use these rules:
- If a quote's production looks high, work out its capacity factor and compare with a PVWatts run for your address.
- If you are choosing between roof faces, compare their modelled capacity factors.
- If you read a news figure, check whether it is AC or DC before comparing.
- If your own figure falls year on year by more than normal ageing, call the installer.
- Next step: estimate your output with the system size calculator and read about solar production.
Questions about solar capacity factor
What is a good capacity factor for solar?
For US utility-scale solar, the national average has stayed between 23.2% and 25.6% since 2016, according to the EIA. Fixed home systems in the Southeast model lower: about 16% to 17% on a DC basis in our Charleston and Atlanta runs. Compare your figure with a PVWatts run for your own address.
Why is solar capacity factor lower than nuclear or gas?
Because there is no sunlight at night and less in winter and on cloudy days. A plant that can run around the clock can stay near full output for most hours of the year. US nuclear plants averaged 91.0% in 2025, against 24.4% for utility-scale solar.
How do I calculate capacity factor for my system?
Take a full year of production from your monitoring in kWh. Divide it by your system size in kW, then divide by 8,760, the hours in a year. Use the same DC or AC size your installer quoted, and say which one you used when you compare.
Is capacity factor the same as efficiency?
No. Panel efficiency is the share of sunlight hitting a panel that becomes electricity, measured in a lab. Capacity factor is how much energy a system makes over time, compared with running at full power nonstop. A panel can be 22% efficient and have a 16% capacity factor on a roof.
What is the capacity factor of rooftop solar?
It depends on location, direction and tilt. Our PVWatts model of a 7 kW south-facing roof at 20° gave 16.6% in Charleston and 16.0% in Atlanta, on a DC basis. Facing east or west dropped Charleston to 14.5%, and a north-facing roof to 11.9%.
Why do solar trackers raise capacity factor?
A tracker turns the panels to follow the sun, so they face the light more directly for more hours. The DOE says this gives more energy, but it adds upfront cost and maintenance. That is why trackers are common on large ground systems and rare on homes.
Does capacity factor change by season?
Yes. The EIA publishes monthly capacity factors, and solar's are much lower in winter than in spring and summer. In January 2024, US utility-scale solar ran at 13.4%, well below its yearly figure. Your own system will show the same pattern in its monthly output.
Does a higher DC-to-AC ratio raise capacity factor?
On an AC basis, yes. Adding more panels behind the same inverter raises the energy made per kW of inverter. On the sunniest hours, some power may be lost to clipping. On a DC basis, the effect is small or slightly negative, which is why the basis matters.
Sources
- U.S. EIA, Electric Power Monthly, Table 6.07.B Capacity Factors for Utility Scale Generators Primarily Using Non-Fossil Fuels (released 24 Sept 2026), retrieved .
- U.S. EIA, Glossary: Capacity factor, retrieved .
- US EIA, Solar explained: Photovoltaics and electricity, retrieved .
- PVWatts V8 API runs by HyreSolar, 7 October 2026 (7 kW, standard module, roof mount, losses 14%, tilt and azimuth as stated, NSRDB PSM V3 TMY), retrieved .
- NLR (formerly NREL), PVWatts V8 API documentation, retrieved .
- US DOE Solar Energy Technologies Office, Solar Photovoltaic System Design Basics, retrieved .
- US DOE Weatherization Assistance Program, Solar Frequently Asked Questions, retrieved .
- Qcells, Q.TRON BLACK (Q.TRON BLK S-G3R.12+/BFG 435–450) data sheet, 2025-08 Rev04, retrieved .
- Duke Energy Carolinas (SC), Rider RSC Residential Solar Choice, retrieved .
- Georgia Power, Behind-the-Meter Interconnection Summary for Residential Customers (rev. 15 Aug 2025), retrieved .
- Code of Virginia §56-594 (net energy metering), retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Annual and monthly figures are copied from EIA Table 6.07.B as released 24 September 2026. Rooftop figures are PVWatts V8 outputs from 7 October 2026, with inputs listed, recomputed by hand with the formula shown. Tracker and maintenance statements follow the DOE pages cited.
Suggest a correction. We fix errors and say what changed.