Bar scale runs from 0 to 20%. UK benchmarks are annual figures from DESNZ; compare a single month or quarter with the seasonal table below the tool instead.
Formula and breakdown
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This solar capacity factor calculator works out how hard a solar PV system worked over a year, month or any period, as a percentage of what it would have made at full rated power all the time, and compares the result with measured UK load factors published by the Department for Energy Security and Net Zero (DESNZ). It is for UK and European homeowners, installers and anyone checking a generation meter or an installer's estimate.
Quick answer: A solar capacity factor calculator divides the energy generated by the rated power times the hours in the period. A 4 kWp array making 3,400 kWh in a year has a capacity factor of 3,400 / (4 x 8,760) = 9.7%. UK solar averaged an 11.1% load factor in 2025 according to DESNZ.
What Is a Solar Capacity Factor Calculator?
A solar capacity factor calculator is a tool that expresses a PV system's actual output over a period as a share of the maximum it could have produced at its nameplate rating, day and night. In plain words: capacity factor = kWh generated / (kWp x hours in the period) x 100.
The Wikipedia article on capacity factor defines it as the unitless ratio of actual electrical energy output over a period to the theoretical maximum over that period. UK government statistics call the same figure the load factor. For solar it is low compared with other generators because panels make nothing at night and little in winter, not because they are faulty.
How Do You Use the Solar Capacity Factor Calculator?
- Choose whether you know the generation and want the capacity factor, or the reverse.
- Enter the array size in kWp, the sum of the panel ratings.
- Enter the inverter's AC rating to see the AC basis as well, or 0 to skip it.
- Pick the period: a year, leap year, month, quarter or a custom number of days.
- Enter the kWh generated in that period from your generation meter or inverter app.
- Compare the result with the UK benchmarks in the solar capacity factor calculator and the tables below.
What Is a Good Capacity Factor for Solar Panels in the UK?
A UK home solar system with an annual capacity factor of around 9 to 11% is in line with measured national figures. DESNZ's Feed-in Tariff load factor analysis for 2024/25 gives a median of 9.2% and a weighted mean of 8.3% for solar installations in the scheme, most of them small systems. Its regional renewable electricity statistics give 11.1% for all UK solar in 2025, a figure that includes large ground-mounted farms.
| DESNZ measurement | Period | Solar load factor |
|---|---|---|
| All UK solar PV | 2025 | 11.1% |
| England / Scotland / Wales | 2025 | 11.2% / 11.1% / 11.2% |
| Northern Ireland | 2025 | 9.4% |
| Feed-in Tariff solar, median | April 2024 to March 2025 | 9.2% |
| Feed-in Tariff solar, weighted mean | April 2024 to March 2025 | 8.3% |
| Feed-in Tariff solar, median | April 2022 to March 2023 | 10.5% |
| Feed-in Tariff solar, April to June | 2024 / 2022 | 13.6% / 15.7% |
| Feed-in Tariff solar, October to December | 2024 / 2022 | 4.7% / 5.3% |
Year to year swings come from the weather, so judge a system over more than one year. The seasonal rows show why a single month tells you little: summer quarters run at roughly three times the winter rate.
How Do You Convert Capacity Factor to kWh per kWp?
Multiply the capacity factor by 87.6 to get annual kWh per kWp: 10% equals 876 kWh per kWp a year, because 8,760 hours x 10% = 876 full-power hours. The same number is the system's specific yield, which our kWh per kWp calculator compares with country benchmarks.
| Annual capacity factor | Specific yield (kWh per kWp a year) | 4 kWp system (kWh a year) |
|---|---|---|
| 8% | 701 | 2,803 |
| 9% | 788 | 3,154 |
| 9.2% | 806 | 3,224 |
| 10% | 876 | 3,504 |
| 11.1% | 972 | 3,889 |
| 12% | 1,051 | 4,205 |
To estimate output for a new system from location, tilt and losses rather than from a capacity factor, use the solar panel output calculator.
Should Capacity Factor Use the DC or AC Rating?
Either can be used, but say which: a DC capacity factor divides by the panel rating in kWp, while an AC capacity factor divides by the inverter rating in kW, which is usually smaller and so gives a higher percentage. NREL's Annual Technology Baseline notes that a capacity factor on a DC basis is lower because the denominator is larger, and that the two are linked by the inverter loading ratio. Home figures in the UK are usually quoted per kWp, so the solar capacity factor calculator leads with the DC basis and shows AC alongside.
Why Is My Solar Capacity Factor Low?
A low annual capacity factor usually comes from the site rather than the panels: shading, a north-facing or steep roof, high latitude, or a cloudier than average year. Check the period first, because a winter month can fall well under 5% on a healthy system. If a full year sits far below the UK benchmarks, look at shading, inverter faults, a tripped isolator, or an inverter much smaller than the array that clips output on bright days. The solar panel calculator helps check whether the array size matches your use.
Capacity factor is not panel efficiency. A 20% efficient panel and a 22% efficient panel of the same kWp rating on the same roof have about the same capacity factor; the more efficient one simply takes less roof area.
How Is Capacity Factor Used in Cost Comparisons?
Capacity factor drives the cost per kWh of any generator, because the same capital cost is spread over more or fewer kWh. DESNZ's Electricity Generation Costs 2023 report assumes an 11% load factor for large-scale solar when it calculates levelised costs. For your own system, a levelised cost calculation uses your measured kWh in the same way.
Worked Example: Checking a 3.5 kWp Roof in Year One
Example: a 3.5 kWp system on a south-east roof in the Midlands, with a 3.0 kW inverter, generated 3,050 kWh in its first full year according to the generation meter.
- Hours in the year: 365 x 24 = 8,760.
- DC capacity factor: 3,050 / (3.5 x 8,760) = 9.95%.
- AC capacity factor: 3,050 / (3.0 x 8,760) = 11.6%.
- Specific yield: 3,050 / 3.5 = 871 kWh per kWp, or 871 full-power hours.
- Against benchmarks: above the 9.2% FiT median for 2024/25 and a little below the 11.1% UK 2025 figure for all solar, so the system is performing normally.
The same system making 420 kWh in a 30-day June would show 16.7% for that month, which is normal for summer and says nothing on its own about the annual figure.
Frequently Asked Questions
How do you calculate solar capacity factor?
Divide the kWh generated by the rated power in kW times the number of hours in the period, then multiply by 100. A 4 kWp system making 3,400 kWh in a 365-day year has 3,400 / (4 x 8,760) = 9.7%. Use the same period for both energy and hours.
What is the capacity factor of solar in the UK?
DESNZ statistics give 11.1% for all UK solar in 2025, including large solar farms. For small installations in the Feed-in Tariff scheme, the median for April 2024 to March 2025 was 9.2%, with a weighted mean of 8.3%. Figures change each year with the weather.
Is capacity factor the same as load factor?
Yes, for practical purposes. UK government statistics use the term load factor for the electricity generated as a proportion of what the installed capacity could produce, which is the same calculation that is called capacity factor elsewhere. Check whether a figure uses DC or AC capacity.
Why is solar capacity factor so low compared with other generators?
Because the calculation counts every hour of the period, including nights, when solar panels produce nothing, and dull winter days, when they produce little. A healthy UK system running at about 10% is not underperforming; it reflects daylight hours and weather at UK latitudes.
How do I convert capacity factor to kWh per kWp?
Multiply the annual capacity factor in percent by 87.6. A 9.2% capacity factor equals about 806 kWh per kWp a year, and 11.1% equals about 972. For a period shorter than a year, multiply by the hours in that period and divide by 100.
Can I compare one month with the annual UK figure?
Not directly. Solar output is highly seasonal: in 2024/25 the Feed-in Tariff solar load factor was 13.6% from April to June but 4.7% from October to December. Compare a month or quarter with the seasonal figures, or wait for a full year of data.
Checked October 2026 by the Solaxyra Editorial Team. Sources: DESNZ: Feed-in Tariff load factor analysis 2024/25, DESNZ: Feed-in Tariff load factor analysis 2022/23, DESNZ: Regional renewable electricity in 2025, DESNZ: Electricity generation costs 2023, NREL ATB: Solar PV AC-DC translation, Wikipedia: Capacity factor.