How many kWh does a kWp of solar panels make in a year?

What is the difference between kWp and kWh?

A kilowatt (kW) is a rate — how fast energy is being made or used at one moment. A kilowatt hour (kWh) is an amount — that rate kept up for an hour, and the unit your electricity bill and your export payment are counted in. A kilowatt peak (kWp) is neither: it is the rated power of your solar panels under one fixed test condition, printed on the label at the factory. Think of a tap and a bath: the tap’s flow rate is the kW, the water in the bath is the kWh, and kWp is the flow the tap was rated for on the test bench — not what it does under your sky. A quote gives you kWp because that is the one number the installer can state with certainty; the kWh is a forecast.

What does the “p” in kWp stand for?

Peak. The figure is the panels’ DC output under Standard Test Conditions — light of 1,000 watts per square metre falling on the panel, a cell temperature of 25 °C, and a standard spectrum called AM1.5 — as set out in the international measurement standard IEC 60904-3. A UK roof meets all three at once rarely and briefly, which is why a 4 kWp array is seldom seen making 4 kW. The installers’ own standard, MCS MIS 3002, defines a system’s kWp plainly as “the sum of the data plate value (Wp at STC) of all modules installed” — add up the labels, and that is your kWp.

How do I convert kWp to kWh?

You multiply by two factors: one for where the roof is and how it faces, and one for shade. That is not a rule of thumb; it is the Standard Estimation Method every MCS-certified installer must use, set out in Appendix B of MIS 3002: Annual AC output (kWh) = kWp × Kk × SF. Kk is “kWh per kWp”, looked up from a table for your postcode zone, roof pitch and orientation; SF is the shade factor, which is 1.00 where “there is an obvious clear horizon and no near or far shading” and less where something blocks the sun. The standard requires the installer to give you this estimate “before the point that the contract is awarded” (clause 4.1.3), so if a quote shows a kWh figure with no Kk and no shade factor behind it, ask for the working.

How many kWh does 1 kWp produce in the UK?

On an unshaded south-facing roof pitched at 35°, between roughly 740 and 1,130 kWh a year, depending on where you live — and those are the installers’ own table figures, not ours. MCS publishes the Kk values as the Irradiance Datasets workbook, one sheet for each of its 21 postcode zones (some split east and west), and MIS 3002 notes that the data “is drawn from the Climate-SAF-PVGIS dataset and multiplied by 0.8” — PVGIS being the European Commission’s solar radiation model, and the 0.8 a built-in allowance for real-world losses. Read directly from that workbook (v2.0), south-facing at 35°:

The zone is decided by the outward part of your postcode, not the town’s name; MIS 3002 carries the full lookup, and your installer’s estimate should state which zone it used.

So what will a 4 kWp system generate in a year?

Multiply the zone figure by four. From the table above, an unshaded south-facing 4 kWp array at 35° comes to about 3,900 kWh a year in the London zone, about 3,500 in the Manchester zone and about 3,300 in the Glasgow zone. Now turn the same roof to face due east or due west, at the same pitch, and the same workbook gives 783, 698 and 669 kWh per kWp for those three zones — roughly a fifth less, so about 3,100, 2,800 and 2,700 kWh. That is the whole point of the method: the kWp did not change, the kWh did, and the difference is orientation, which is a fact about your roof rather than a choice about your panels.

Is that what systems actually produce?

Broadly, and a little less. The independent check is the government’s own metering: DESNZ’s Feed-in Tariff load factor analysis, published in Energy Trends on 18 December 2025, reads the meters of 282,047 solar PV installations and finds a median load factor of 9.2% for 2024/25, with a weighted mean of 8.3%. A load factor is output as a share of what the rated capacity could make if it ran flat out all year, so 9.2% of the 8,760 hours in a year is about 806 kWh for every kW installed. It sits below the south-facing table figures above because the real fleet includes every orientation, every shaded gable and panels installed as long ago as 2010, when the scheme opened — and because, as DESNZ says, 2024/25 had the fewest sun hours since 2012/13 and was “the lowest recorded median in the time series”. The South West, East of England and North East came out highest at around 9.5%; London lowest, which DESNZ puts down to dirt settling on panels and shading from tall buildings.

Why does my kWh change with the season when my kWp doesn’t?

Because kWp is fixed at the factory and kWh follows the sun. The same DESNZ analysis breaks the year into quarters: in 2024/25 the April-to-June quarter ran at a 13.6% load factor and the October-to-December quarter at 4.7% — nearly three times the output per kWp in late spring as in late autumn. An annual kWh figure averages that away, which is fine for a yearly comparison and useless for guessing a December bill. If a monthly generation figure looks low against the annual estimate, check which month it is before checking the panels.

Why is my inverter smaller than my panels’ kWp?

Because they are rated in different currencies. Panels are rated in kWp of DC; the inverter that turns their output into mains electricity is rated in kW of AC, and on a single-phase home it is usually held at or below 3.68 kW — the 16 amps per phase that National Grid’s G98 connection procedure covers on a connect-and-notify basis, with no application to wait for. UK installers therefore routinely put more kWp of panels behind a 3.68 kW inverter and accept that on the brightest few hours of the year the inverter trims the peak, because the panels almost never reach their kWp anyway. A 4 kWp array on a 3.68 kW inverter is ordinary practice, not a mistake, and the mismatch on your quote says nothing about paperwork.

Is the kWh figure DC or AC?

Ask, because they differ and only one of them reaches your sockets. The MCS method above returns AC — what the inverter delivers to the house — and its 0.8 factor is where the losses live. Some estimates start from a DC figure, which is larger because it is what the panels make before the inverter. This site’s own roof card does exactly that: it takes Google’s DC estimate for your roof and applies the 0.85 DC-to-AC derate Google documents in its Solar API methodology before showing an annual figure, and labels the step “inverter and system losses” rather than hiding it. Any estimate that does not say which it is has left out the one word that moves the number by around 15%.

Which number does the export payment use?

kWh, and only kWh that a meter has counted. Under the Smart Export Guarantee, Ofgem says generators “are paid by their chosen SEG Licensee for the electricity which they export back to the National Grid”, that payments “are calculated by using export meter readings”, and that the supplier sets the rate, which “must always be above zero”. Your kWp never appears on a payment. What matters to the export line is how many kWh you did not use while the sun was on the roof — which is a question about your household, not your panels, and the reason this site asks about your usage before it says anything about export.

What should I check on a quote?

Four things, all of which follow from the method above. That the kWp is the sum of the panel labels, with the panel model named so you can check the label yourself. That the kWh estimate shows its Kk — or at least its postcode zone, pitch and orientation — and its shade factor, as MIS 3002 requires. That it says whether the figure is AC or DC. And that it is one year’s figure, not a lifetime total: the wording MIS 3002 obliges the installer to attach says the estimate “is given as guidance only for the first year of generation” and “should not be considered as a guarantee of performance”. A kWh figure with no method behind it is a number without a basis, and the standard exists so that you never have to accept one.