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How do I know if my solar system is underperforming?

Short answer

Compare yearly or monthly kWh with the MCS performance estimate in your handover pack, not with the peak kW figure. In the UK a well-sited south system typically makes about 850 to 1,100 kWh per kWp a year. A shortfall over 10% across a whole year, or one inverter input reading zero, is worth an engineer check.

Last checked 24 September 2026 · 2 sources

In more detail

  • PVGIS (14% losses, south 35 degrees): Manchester 886, Edinburgh 904, London (30 deg) 1014, Portsmouth 1119 kWh/kWp/yr.
  • Monthly shape (London south 30): Dec 34, Jan 38, Jun 125, Jul 126 kWh/kWp, so winter months are roughly a quarter of summer ones.
  • Performance ratio of UK domestic systems averaged 83% (SD 7%) in the Sheffield Solar analysis.
  • Weather varies +/-10% between years; compare to the same month last year and to neighbours with the same orientation.
  • Checks you can do: inverter display vs app, per-MPPT/string readings, shading changes, dirt/droppings, error log.

The facts, dated

  1. 01
    PVGIS annual yield, Manchester, south 35 degrees — 886 kWh/kWp/yr

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

  2. 02
    PVGIS annual yield, Edinburgh, south 35 degrees — 904 kWh/kWp/yr

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

  3. 03
    Mean performance ratio of UK domestic PV — 83 %

    market_data · UK · checked 24 September 2026 · researchgate.net

What we saw in the field

[QA review 2026-09-24] C2-4 AGREES (compare with MCS 1,021 S / 552 N zone 3, not PVGIS which reads ~10% higher). | Earlier notes: Customer 2 (Portsmouth, 13.5 kWp including north and garage strings, ~10.5 MWh/yr = ~780 kWh/kWp) is plausible: PVGIS gives ~1,119 for south-only there, and north/garage strings pull the average down. Customer 1 5.87 MWh in ~11 months cannot be checked without kWp.

Sources

Last checked: 24 September 2026