kWp vs kWh: the two solar numbers, and why mixing them up costs money

The confusion, and why it is expensive

Solar is quoted in kWp because that is what an installer supplies: a quantity of panel capacity. Electricity is billed in kWh because that is what you consume. Export payments under the SEG are paid per kWh. Nothing you ever receive or pay is denominated in kWp.

So a quote for “a 4 kWp system” tells you the size of the thing being installed and nothing at all about what it will do. Two 4 kWp systems — one on a south-facing roof in Brighton, one on an east-facing roof in Glasgow — differ by more than half in annual output. Same kWp. Very different investment.

What kWp actually measures

A panel’s rating is measured at Standard test conditions : 1,000 watts per square metre of Irradiance , a cell temperature of 25°C, and a defined light spectrum. A 400 W panel produces 400 W at that instant, in those conditions.

Here is the part that is rarely said plainly: those two main conditions are close to mutually exclusive in the UK. Sunshine bright enough to deliver 1,000 W/m² also heats a roof-mounted panel well past 25°C, and silicon loses output as it warms. On the clearest June day in London, our modelling puts in-plane irradiance on a 35° south roof at a peak of 602 W/m² — around 60% of the test condition, at the moment of the year when it is highest.

That is not a defect in the rating. It is a rating designed for comparing panels with each other under identical conditions, being read as though it were a prediction of output.

Specific yield: the number that does the work

Specific yield is annual generation divided by installed capacity, in kWh per kWp. In MCS’s MIS 3002 method it appears as Kk, and the standard calculation is:

annual AC output = kWp × Kk × shade factor

We have modelled Kk ourselves for 25 UK locations. At a typical 35° pitch facing due south:

Source: our own modelling from PVGIS 5.3, European Commission Joint Research Centre, at 20% system losses following the method in MCS MIS 3002. Full dataset: data/irradiance.yaml.

The spread is a factor of 1.5 from Lerwick to Brighton. Change the orientation as well as the location and the range widens to roughly 3.5×: a vertical north-facing wall in Lerwick against a 35° south roof in Brighton.

The worked example

A 4 kWp array, quoted identically, in three places at 35° facing south:

Two lessons sit in that table, and the second is the bigger one.

Location matters. The same purchase produces 50% more electricity in Brighton than in Shetland.

What you do with the electricity matters far more. The gap between the best and worst location is £365 a year. The gap between using a Brighton system’s output and exporting it at a common SEG rate is £917. Nobody chooses their latitude; everybody has some say over self-consumption. That is why we treat Self-consumption as the central variable and location as background.

Rates used above: 26.11p is Ofgem’s price cap unit rate for 1 July to 30 September 2026; 4.1p is a rate several suppliers in Ofgem’s latest SEG report pay. Both are illustrative and neither is a recommendation.

The other units you will meet

Why the inverter is smaller than the panels

An array’s kWp is direct current at the panels. The Inverter is rated in kW of alternating current it can deliver. It is entirely normal for the first to exceed the second — a DC:AC ratio above 1, sometimes called overpanelling.

The reason is in the numbers above. A UK array spends almost no time near its rating, so an inverter sized for a peak that occurs a handful of hours a year is oversized for the other several thousand. Sizing it below the array captures more of the year’s energy at lower cost, and gives up a small amount at the very top, which is called clipping.

Two consequences worth knowing:

• A quote listing “4 kWp” with a 3.68 kW inverter is not a mistake or a short-change. It is a deliberate choice, and 3.68 kW is a common figure because it corresponds to the 16 A single-phase limit that G98 notification is built around.
• If your system is subject to Export limitation , the limit is in kW of export, not kWp of panels. You can install more panel capacity than you are permitted to export, and use the difference yourself.

How to sanity-check any quote

Take the annual kWh figure, divide by the kWp, and look at the result.

None of those bands is a verdict. A low figure on a north-facing Glasgow roof is honest modelling; the same figure quoted for a south-facing Cornish roof is a question worth asking. What matters is that the two numbers in a quote are consistent with each other, and with where your house is.

Degradation, and why year one is not every year

The annual figures here and on our calculators are year-one output. Degradation means output declines slowly over the panels’ life, at a rate the manufacturer states in a performance warranty. Those warranties differ between products, so the honest thing to do is read the one you are being offered rather than apply a generic figure — which is why we do not apply one.