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What does kWp mean on a solar quote?

Short answer

kWp (kilowatt peak) is the combined label rating of all panels under standard test conditions. MCS uses it to size your estimate: 10 × 450 W panels = 4.5 kWp. It is not typical output. UK systems rarely exceed about 85–90% of kWp, and the inverter's kW can be lower than the panel kWp (MCS 032:2025).

Last checked 24 September 2026 · 4 sources

In more detail

  • MCS: kWp = sum of module data-plate Wp at STC (value printed on the module label).
  • Inverter AC rating can be below array kWp (DC oversizing); modest clipping on the brightest hours.
  • DESNZ now publishes cost per kW on Total Installed Capacity (TIC); older series used Declared Net Capacity (DNC), which is often the export/inverter limit.
  • EST: average home system ~4.5 kWp, about 12 panels, 20-30 m² of roof.

The facts, dated

  1. 01
    MCS kWp definition: sum of module data-plate Wp at STC

    standard · UK · checked 24 September 2026 · MCS

  2. 02
    EST typical system size — 4.5 kWp

    market_data · UK · checked 24 September 2026 · Energy Saving Trust

  3. 03
    PVGIS 2023 hourly-mean peak for 1 kWp south 35° Portsmouth — 0.89 kW

    market_data · England · checked 24 September 2026 · PVGIS (EU Joint Research Centre)

  4. 04
    DESNZ: TIC-based £/kW typically 6-12% lower than DNC-based — 6-12 %

    market_data · GB · checked 24 September 2026 · GOV.UK

What we saw in the field

[QA review 2026-09-24] I-8 AGREES (proxy). C2-2 AGREES (DC:AC ~1.23). | Earlier notes: Installer claim that panels went from 300-350 W to 450-480 W in ~5 years: not tested (no dated panel-size statistics opened). Current UK plug-in kits use 430-520 W panels (Which? and EcoFlow listings, Sept 2026), which is consistent. Customer 2's deliberate DC oversizing (13.5 kWp into ~11 kW) is a DC:AC ratio of ~1.23. Expected clipping is small because a mixed south/north array rarely reaches full kWp.

Sources

Last checked: 24 September 2026