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How much electricity will my solar panels generate in a year?

Short answer

MCS installers estimate first-year output as system size (kWp) × a kWh-per-kWp figure for your postcode zone, roof direction and pitch × a shading factor. A south-facing 35° roof gets about 1,021 kWh per kWp near Southampton and 833 near Glasgow, so a 4 kWp system makes roughly 3,300–4,100 kWh a year (MCS tables, checked September 2026).

Last checked 24 September 2026 · 3 sources

In more detail

  • Formula: kWh/yr = kWp × Kk × SF (MCS 032:2025 s.3; MIS 3002 issue 6 App. B).
  • kWp = the total of the panel label ratings; Kk comes from the MCS table for 25 zones (1° pitch, 5° azimuth steps).
  • MCS values = PVGIS satellite irradiation × 0.8, so they are deliberately cautious; PVGIS 5.3 gives 3-10% more.
  • The estimate covers the first year only and is not a guarantee; year-to-year weather can move output by around ±10%.
  • Ask for the MCS performance table (MCS 032 Table 5) with every quote so offers can be compared.

The facts, dated

  1. 01
    Brighton zone 2, south, 35° pitch: MCS Kk — 1130 kWh/kWp/yr

    standard · England · checked 24 September 2026 · MCS

  2. 02
    Lerwick zone 20, south, 35°: MCS Kk — 736 kWh/kWp/yr

    standard · Scotland · checked 24 September 2026 · MCS

  3. 03
    London zone 1, south, 35°: MCS Kk — 984 kWh/kWp/yr

    standard · England · checked 24 September 2026 · MCS

  4. 04
    Belfast zone 21, south, 35°: MCS Kk — 845 kWh/kWp/yr

    standard · Northern Ireland · checked 24 September 2026 · MCS

  5. 05
    PVGIS 5.3 (SARAH3, 14% loss) Portsmouth south 35° — 1119 kWh/kWp/yr

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

  6. 06
    MCS irradiance data = Climate-SAF PVGIS × 0.8 — 0.8 factor

    standard · UK · checked 24 September 2026 · MCS

What we saw in the field

[QA review 2026-09-24] C1-2 AGREES (plausible, ~7-8 kWp implied, size unknown). C2-4 AGREES (778 kWh/kWp vs MCS zone 3 half S/half N ~787). C2-3 CONTRADICTS as AC (12 kW > 11.04 kW). | Earlier notes: Customer 2 (Portsmouth, zone 3, 13.5 kWp across south, north and garage roofs): 10.5 MWh in year one = 778 kWh/kWp. MCS zone 3 at 35°: S 1,021, N 552; a roughly half-south/half-north split gives 13.5 × 787 ≈ 10.6 MWh, so the figure is PLAUSIBLE and agrees closely with MCS if about half the panels face north (with two-thirds facing south, MCS gives ~11.7 MWh). His 'peak ~12 kW' is higher than an ~11 kW inverter can export as AC. It is plausible only as DC or a combined reading. With half the array facing north, PVGIS hourly peaks suggest ~8-9 kW AC. Customer 1 (east-west, ~5.87 MWh in ~11 months): size not stated. E/W at 35° gives 783-862 kWh/kWp in zones 1-4, which implies ~7-8 kWp. PLAUSIBLE for 16-20 modern panels.

Sources

Last checked: 24 September 2026