Eight Hundred Watts, One Grey Season: What a Balcony PV Kit Really Delivers
Plug-in solar has moved from a continental curiosity to a recurring item in UK retrofit conversations. The pitch is attractive: two panels, a microinverter and a cable, generating into a domestic socket with little more disruption than hanging a curtain rail. But the British winter is not a Bavarian balcony in June. Between 1 October 2025 and 31 March 2026 we instrumented an 800 W kit on a south-east-facing balcony in Leeds to see what the grey months actually deliver.
The rig and the caveats
The test kit used two 400 W monocrystalline panels mounted at a shallow angle on a balcony railing, feeding an 800 W microinverter. The balcony is open to the south-east but shaded by the building after mid-afternoon and by a neighbouring gable in the low winter sun. That is not an unusual UK installation; it is probably more representative than a clean, unshaded roof.
The electrical side was deliberately conservative. The inverter fed a dedicated outdoor socket on its own RCBO, with a calibrated kWh meter and a DC clamp logger upstream. The microinverter's local API supplied one-minute AC power data, and a nearby weather station supplied irradiance, module temperature and wind speed. No export tariff was involved. Readers should not treat the arrangement as a compliance template: the UK position on plug-in solar connected through a standard 13 A plug remains unsettled, and any permanent installation should be designed and commissioned against current wiring regulations.
What the sky gave us
Over 182 days the kit generated 71.8 kWh, a specific yield of 89.8 kWh per kWp. That is modest, and it is consistent with a shaded, near-vertical balcony rather than an optimally inclined roof. The monthly split shows how strongly the British winter is driven by available irradiance rather than by headline panel capacity.
| Month | Yield (kWh) |
|---|---|
| October | 18.4 |
| November | 9.1 |
| December | 5.6 |
| January | 6.8 |
| February | 12.3 |
| March | 19.6 |
The best day, 26 March, produced 2.31 kWh. The highest instantaneous AC output was 604 W, well below the inverter's 800 W ceiling. There were 21 days with effectively zero yield, mostly in December and January, when cloud cover was thick enough that the inverter never sustained useful output. On many other days the kit spent hours between 40 W and 150 W: enough to run a router and a few LED lamps, but not enough to make a meaningful dent in space heating.
Inverter behaviour under low light
The microinverter's behaviour was more interesting than the totals. Start-up occurred at around 12 W of DC input, after which the maximum power point tracker hunted across a narrow voltage window. In diffuse light the tracker was stable but slow to respond to rapid cloud transitions; on broken-cloud days the AC output stepped rather than ramped. Below about 20 W the inverter cycled in and out of standby, and its standby draw measured 0.3 W when the DC side was fully dark.
Clipping was never reached. That is unsurprising for an 800 W inverter on an 800 W array in a UK winter, but it matters for payback claims: the nameplate figure is a ceiling, not a realistic operating point. The inverter also reported a small grid-voltage rise, around 2.1 V at full output, on a dedicated circuit. That is not a problem in itself, but it illustrates why plug-in kits should not be treated as invisible appliances. Protection, labelling and a sensible connection point are part of the engineering, not paperwork.
The payback arithmetic
The kit cost £649, the mounting hardware £90 and the dedicated socket and metering £120. Using a 24.5p per kWh electricity price, the winter's 71.8 kWh is worth £17.59. Extrapolating the measured specific yield to a full year, allowing for better spring and summer performance and the balcony's shading, suggests around 410 kWh annually. At the same tariff that is roughly £100 a year, giving a simple payback of about 8.6 years before degradation, inverter replacement or tariff changes.
That figure is sensitive to things the sales literature tends to flatten. A sunnier site or a less shaded balcony could improve annual yield by 20–30 per cent. A household that self-consumes most of the output avoids the import price; one that exports without a tariff captures much less. And a kit used only in winter, when the balcony is the only available space, will look considerably worse than the annual average suggests.
What it means for a British balcony
An 800 W plug-in kit is a useful base-load generator in the brighter half of the year and a marginal one in December. It will not heat a home, and it should not be sold as if it will. What it can do is reduce standby and daytime import, teach a household how weather and orientation affect real yield, and provide a low-commitment entry point to PV. The engineering case is strongest when the kit is installed safely, measured honestly and judged against a realistic annual yield rather than a sunny afternoon in May.
