kWp vs kW vs kWh: Solar PV Units Explained
Updated 23 October 2026 · SEO Dons Editorial
What do kWp, kW and kWh each measure?
kWp is a capacity label, kW is a rate of power and kWh is a quantity of energy. Mixing them up is the commonest reason two solar quotes cannot be compared.
kWp (kilowatt-peak) is the direct-current power a manufacturer declares an array can produce under standard test conditions. PVGIS, the European Commission's solar tool, defines those conditions as "a constant 1000W of solar irradiation per square meter in the plane of the array, at an array temperature of 25°C". It is a laboratory benchmark that adds up neatly: 200 modules of 550 W make 110 kWp.
kW (kilowatt) is power at one instant. On a PV job it appears as the inverter's alternating-current output, as an export limit agreed with the network operator, and as the load a building is drawing right now. For solar PV, National Grid Electricity Distribution (NGED) says the figure used to size the connection is the inverter's continuous steady state output, not the panels' capacity.
kWh (kilowatt-hour) is energy: one kilowatt sustained for one hour. It is the unit on the electricity bill and the unit PVGIS returns for annual output.
| Unit | What it measures | Where it appears on a PV job |
|---|---|---|
| kWp | Peak DC capacity of the panels at standard test conditions | Headline size of a quote; module count multiplied by module wattage; the capacity entered into PVGIS |
| kW | Power at one moment | Inverter AC output; export limit; the load a building draws |
| kWh | Energy over time | Annual generation; the saving on a bill; yield per kWp |
How do the three connect?
Annual energy in kWh equals capacity in kWp multiplied by the specific yield in kWh per kWp. Yield is the part that depends on place and layout, so it is the number to challenge in any proposal. PVGIS says its outputs "correspond to the final energy output from a PV system (typically as AC electricity) and include system losses", with a default overall loss of 14%, which is the setting used throughout this page.
There is a physical way to see why yield lands near 1,000. A kWp is defined against 1 kW of sunlight per square metre, so the yearly sunlight reaching each square metre of the array, in kWh/m², is also the number of full-power hours the panels would need to collect it. PVGIS puts the yearly irradiation on a south-facing 35 degree plane at 1,368 kWh/m² in Plymouth, 1,193 in Birmingham and 1,052 in Glasgow. After total losses of about 18% to 19% (the 14% system loss plus angle, spectral and temperature effects), those become 1,122, 968 and 853 kWh per kWp.
What is a good kWh per kWp in the UK?
A good figure is one you can reproduce for your own location and layout. These are PVGIS values for 1 kWp with 14% losses.
| Location | South, 35° tilt (kWh per kWp a year) | East-west, 10° tilt (kWh per kWp a year) | Year-to-year variation, south 35° |
|---|---|---|---|
| Plymouth | 1,122 | 932 | about 29 kWh per kWp |
| Birmingham | 968 | 800 | about 34 kWh per kWp |
| Glasgow | 853 | 707 | about 26 kWh per kWp |
For a south-facing 35 degree array the range across these three cities is 853 to 1,122 kWh per kWp, a gap of about 32%. The yield-by-region guide tabulates 13 locations with the same settings. East-west rows at 10 degrees sit at about 83% of the south figure in all three cities, which is why a flat roof carrying them needs more kWp to reach the same kWh.
Weather adds a few per cent either way. PVGIS reports a year-to-year standard deviation of 26 to 34 kWh per kWp for the south 35 degree cases, roughly 3% of the annual figure, so one year can land above or below the average without anything being wrong. The familiar 900 to 1,000 kWh per kWp rule suits a mid-country south-facing roof. It overstates a Glasgow east-west flat roof, where PVGIS gives 707. How the same yield is converted into a size is covered in the sizing guide, and per-panel figures are on the panel output page.
Why do DC kWp and inverter kW differ?
An inverter smaller than the array's kWp is a design choice, not a mistake, because panels seldom reach their full rated power. The ratio of panel kWp to inverter kW is the DC:AC ratio: 120 kWp of panels on a 100 kW inverter is 1.2.
The PVGIS hourly series shows why. For a south-facing 35 degree array in Birmingham, using the 2023 data and 14% losses, the best hour delivered 885 W per kWp (on 18 April); no hour reached 900 W, and only 51 of 8,760 hours reached 800 W. Capping that series at different inverter limits gives the share of annual energy an inverter would clip:
| DC:AC ratio | Inverter limit per kWp of panels | Annual energy clipped, south 35° | Annual energy clipped, east-west 10° |
|---|---|---|---|
| 1.0 | 1,000 W | under 0.01% | under 0.01% |
| 1.2 | 833 W | 0.03% | under 0.01% |
| 1.3 | 769 W | 0.36% | under 0.01% |
| 1.4 | 714 W | 1.00% | under 0.01% |
| 1.5 | 667 W | 1.90% | 0.18% |
This is the arithmetic of this page applied to PVGIS hourly values for one place and one year, not an inverter model. Hourly averages smooth out short peaks, so real clipping will be somewhat higher, and the PVGIS output already includes the 14% loss allowance. The pattern still holds: east-west rows peak lower, so they tolerate a higher ratio before clipping matters.
Which figure counts depends on the rule. NGED's G99 page sizes the connection from the inverter, and the Energy Networks Association (ENA) splits G98 from G99 at 3.68 kW per phase. Section 82 of the Energy Act 2004 sets microgeneration at "50 kilowatts" of capacity to generate electricity, without saying whether that means the DC array or the AC inverter. For a design near 50, ask the planning authority and the network operator in writing which figure each applies; the 50 kW page sets out the thresholds.
Two worked conversions for commercial sizes
Both are illustrations, not designs. Multiply the capacity by the PVGIS yield for the location and layout.
- A small south-facing array. 90 modules of 550 W make 49.5 kWp. In Birmingham, on a south 35 degree roof, 49.5 × 968.06 = 47,919 kWh a year, an average of 131 kWh a day.
- A mid-sized flat-roof array. 250 kWp on east-west rows at 10 degrees in Manchester: 250 × 740.72 = 185,180 kWh a year. The same 250 kWp facing south at 35 degrees would give about 221,500 kWh, but it would not fit the same roof, because tilted rows need spacing.
Going the other way, divide. A target of 150,000 kWh a year in Birmingham needs 150,000 ÷ 968.06 = 155 kWp on south 35 degree modules, or 187 kWp on east-west rows at 10 degrees. There is no fixed kWp-to-kWh factor: the yield for the site and layout is the conversion.
How should a quote state the units?
- Panel capacity in kWp, with module count and wattage shown separately.
- Inverter capacity in kW AC, and the resulting DC:AC ratio.
- Annual generation in kWh and the kWh per kWp it implies.
- The yield basis (tool, database, tilt, orientation, losses) so it can be rerun.
The commercial PV systems guide shows where these figures sit in a specification. To have a contractor check a roof, you can send an enquiry through the site's form.
Sources
- PVGIS user manual, European Commission Joint Research Centre, pvgis-user-manual, checked 22 October 2026.
- PVGIS 5.3 PVcalc runs (PVGIS-SARAH3, crystalline silicon, 1 kWp, 14% losses; south at 35 degrees, east and west at 10 degrees), European Commission Joint Research Centre, checked 22 October 2026: Plymouth south, east, west; Birmingham south, east, west; Glasgow south, east, west; Manchester south, east, west.
- PVGIS 5.3 seriescalc hourly data for Birmingham, 2023, same settings, European Commission Joint Research Centre, checked 22 October 2026: south, east, west.
- Connecting commercial generation to the electricity networks, Energy Networks Association, energynetworks.org, checked 22 October 2026.
- G99 connection procedures, National Grid Electricity Distribution, connections.nationalgrid.co.uk, checked 22 October 2026.
- Energy Act 2004, section 82, legislation.gov.uk, legislation.gov.uk, checked 22 October 2026.
Common questions
Is kWp the same as kW?
No. Both measure power, but kWp is the panels' declared DC output under standard test conditions, while kW is power at a given moment, usually the inverter's AC output. Ask which figure a quote means.
How many kWh does 1 kWp produce in a day?
In Birmingham, on a south-facing 35 degree array, PVGIS gives a yearly average of 2.65 kWh a day per kWp, from about 1.07 in December to 3.9 in June. Other places and layouts differ.
Does a 100 kW system mean 100 kWp?
Not necessarily. A quote might mean 100 kWp of panels, a 100 kW inverter, or an agreed export capacity. The three can differ, and the grid and planning rules do not all measure the same one, so ask for panel kWp and inverter kW separately.
Why would a quote show more than 1,100 kWh per kWp?
The highest figure in the table above, for a south 35 degree array in Plymouth, is 1,122. A higher number needs a stated reason, such as a different radiation database, lower assumed losses or a bifacial gain. PVGIS does not make specific calculations for bifacial modules, so ask how any gain was worked out.