kWp (Kilowatt Peak)

kWp means kilowatt peak: the rated power output of a solar panel or array measured under standard test conditions. kW is the same unit of power without the reference conditions, used for anything from an inverter's output to a heat pump's draw. kWh is energy rather than power - the amount of electricity actually produced or consumed over time. A 5 kWp array does not make 5 kWh every hour; it reaches close to 5 kW only under conditions that a UK roof meets for a handful of hours a year.

Standard test conditions are laboratory conditions, and the reference solar spectrum they use for rating PV devices is defined in BS EN IEC 60904-3. Because every manufacturer rates against the same reference, a 440 Wp panel from one brand is directly comparable with a 440 Wp panel from another. That comparability is the reason the unit exists.

How to calculate kWp

System size in kWp = (number of panels x panel rating in watt-peak) / 1,000

A worked example for a domestic roof:

1. The design fits 12 panels, each rated at 440 Wp.
2. 12 x 440 = 5,280 watt-peak.
3. 5,280 / 1,000 = 5.28 kWp.

Swap the panel for a 500 Wp module and the same 12 positions give 6.0 kWp, which is why two quotes for the same roof can differ by 14% in system size without either being wrong. The panel count and the watt-peak rating both belong on the proposal, not just the total.

Why the kWp figure decides more than price

In the UK the kWp number is not just a sizing convention; it selects the regulatory route. MCS certification for electricity-generating technologies covers installations up to 50kW, so anything larger falls outside the MCS certificate scope entirely - and outside the customer's route to Smart Export Guarantee payments.

Below that, the thresholds are set in amps per phase rather than kWp, and the conversion matters. Engineering Recommendation G98 covers fully type-tested micro-generators up to and including 16A per phase, notified to the network operator within 28 days of commissioning. Above 16A per phase, G99 applies and written permission is needed before the system is installed. Adding battery storage moves the goalposts again: over 3.68kW with storage, the application goes in before installation.

So the kWp figure fixed at survey decides whether the job has a notification or an application in it, and that decision sets the programme. Solar and battery installers who size the system and price it in one place can carry the kWp figure, the panel schedule, the grid route and the expected annual yield through from proposal to commissioning without retyping any of them. The guide to running a solar PV installation project sets out where each of those steps falls.

One caution on proposals. A kWp figure with no annual generation figure beside it is not a comparable quotation, and a customer holding three quotes at 4 kWp, 5.28 kWp and 6 kWp is not comparing like with like on price per kWp either, because inverter sizing, scaffolding and roof type do not scale with the array.