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The 800W limit, explained

Quick answer: The UK plug-in solar cap is 800 watts AC inverter output, not panel kWp. It comes from Germany’s Balkonkraftwerk rule and is harmonised across most of Europe. The limit exists so a microgeneration unit can sit on a normal household ring final circuit without overloading it — keeping the safety case simple enough that BS 7671 Amendment 4 can class the install as a portable appliance. Panel capacity behind a clipping 800W inverter can legally be 1,000–1,200W+.

Why 800 watts specifically

The 800W cap was set in Germany’s VDE-AR-N 4105 (revised 2024) and has since become the de facto European number. Three things converge on it:

  • A standard ring final circuit is rated at 30A (~7.2 kW) total load. Pushing 800W back through one outlet doesn’t overload anything, even on a fully-loaded circuit.
  • 800W happens to be a useful amount of solar. One or two 400–500W panels behind a clipping 800W inverter is exactly the “south-facing balcony in a UK city” sweet spot.
  • It’s small enough to be a portable-appliance question, not a building-services question. That’s the regulatory hinge that makes the whole DIY model work.

Inverter watts vs. panel watts — the bit people get wrong

The 800W limit applies to the inverter AC output, not the panels. So you can run:

  • 2 × 500W panels = 1,000W DC behind an 800W AC inverter. The inverter clips at 800W. On a perfect noon-summer-day moment you lose a small fraction of peak; over a full year you typically gain 5–10% more total kWh because the panels stay closer to their optimal output during all the non-peak hours.
  • 2 × 410W panels = 820W DC — mainstream “balcony kit” spec. Inverter rarely actually hits 800W in UK conditions.
  • 1 × 450W panel + 800W inverter. Simplest one-panel install.

If you see a kit listed as “800W kit” check whether the spec sheet calls it 800W inverter or 800W panel — reputable UK sellers will be explicit. EcoFlow STREAM, Anker SOLIX, Zendure SolarFlow, APsystems EZ1-M and Hoymiles HMS-800 all cap inverter output at 800W.

What happens if you go over 800W

Above 800W inverter AC output, your install falls outside BS 7671 Amendment 4’s portable-appliance pathway. You’d be in the territory of a normal grid-tied solar PV install: MCS-registered installer, DNO connection agreement (G99, not G98), full Part P notification. That’s a meaningful step up in cost and admin and only makes sense if you have the roof or space to justify 2 kW+ of capacity.

Three-phase homes (rare in UK flats)

Almost all UK domestic supplies are single-phase — the 800W limit applies to single-phase microgeneration. Three-phase supplies (occasionally found in large houses or mixed-use buildings) have higher per-phase headroom, but the same 800W aggregate is the relevant cap for the portable-appliance classification. If you have a three-phase supply, talk to your DNO — they often have specific guidance.

Want to model your specific setup? Use the UK savings calculator — cap inverter output at 800W and adjust panel-derate via the per-watt yield slider.