Plug-In Solar for Greenhouses and Polytunnels: What You Can Actually Run

With the 27 August legalisation days away, a lot of the interest so far has focused on houses — cutting the daytime import bill, running the kettle and dishwasher off free generation. But there is a smaller, keener audience that has been asking a more specific question: can a plug-in solar kit run the greenhouse?

It is a good question with a genuinely useful answer, and it lands at the right time of year. Late August is when serious UK gardeners start thinking about extending the season — heated propagators for autumn sowing, automatic vent openers before the first cold snap, and eventually frost protection once the clocks change. This piece covers what a legal plug-in microgenerator can realistically power in a greenhouse or polytunnel, what it can't, and how to wire it up without breaking the rules.

The regulatory reality: your greenhouse needs its own socket

This is the part people trip over. A plug-in solar unit — under SI 2026 No. 848 — must plug directly into a standard 13A BS 1363 domestic socket, with no extension leads permitted between the unit and the wall. One unit per household, 800VA/3.5A maximum output, protected by a 5A fuse in the plug.

That means you cannot run a cable from your kitchen socket, out through a window, across the garden and into the greenhouse, then plug your solar unit in at the far end. The unit has to sit at an existing outdoor or garage socket that is already part of your house wiring — not one improvised with an extension reel.

In practice this rules out most freestanding allotment greenhouses and polytunnels with no mains connection at all — the same limitation covered in the allotments guide. If your greenhouse is in the back garden and already has a proper armoured cable run to an outdoor socket (the kind an electrician would install, not a trailing extension), you're in business. If it doesn't, plug-in solar isn't the fix; a hardwired outbuilding supply is, and that's a different job entirely.

The good news is that a very large share of UK greenhouses sit within a few metres of the house — against a fence line near the back door, or on a patio — exactly the situation plug-in solar was designed for. The shed guide covers the same wiring logic for a workshop or shed, and it transfers directly to a greenhouse with its own socket.

What an 800W kit can comfortably run

Panel output feeds into your existing household ring, offsetting whatever is drawing power nearby — it doesn't create a dedicated greenhouse circuit. So the practical question isn't "can the greenhouse run entirely on solar" but "does solar generation offset the appliances plugged into that circuit while the sun's up." For most greenhouse gear, the answer is comfortably yes on a decent day, because the loads are small and intermittent rather than continuous.

Heated propagators. A typical windowsill or greenhouse propagator draws 10–70W depending on size and thermostat setting. Even a single 400W panel generating a fraction of its rated output on an overcast September morning comfortably covers this.

Automatic vent openers. Wax-cylinder openers need no electricity at all, but the electric motorised versions used on larger polytunnels draw only a few watts intermittently — negligible against an 800W kit's typical daytime output.

Irrigation timers and low-flow pumps. A small 12V transformer-fed drip irrigation pump or timer typically pulls 20–50W. Fine.

Circulation fans. Greenhouse circulation fans that prevent fungal disease in humid polytunnels draw 15–40W continuously — well within range, and usefully these tend to run during daylight hours when generation is happening anyway.

Frost protection heaters are the outlier. A fan heater sized to keep a greenhouse frost-free overnight typically draws 1–2kW — two to three times the entire output ceiling of a legal plug-in kit, and it needs to run at night when solar isn't generating at all. Solar offsets the daytime draw of smaller kit; it does not replace a heater's overnight running cost. The winter preparation guide has realistic expectations for what solar can and can't cover once the clocks change.

Timing matters more here than in a house

A house has appliances running around the clock — fridge, router, standby loads — that solar offsets whenever the sun is up. A greenhouse setup is more time-specific: most of what you're powering (propagators, irrigation, vent motors) tends to cluster around daytime hours anyway, which is a better match to a solar generation curve than most household use cases. The best appliances to run guide covers the same load-shifting logic for the house, and the principle here is identical — run what you can while the panel is producing, and don't expect coverage after dark.

Late August through September is a reasonable generation window in the UK — not peak summer, but still solid mid-afternoon output on clear days. The September generation guide has typical daily kWh figures for this exact period, useful for sanity-checking what a greenhouse propagator setup will actually see.

A polytunnel-specific problem: shading and panel siting

Polytunnels are usually sited for maximum sun exposure on the crop, which is good news for a nearby panel — but the polytunnel itself can become the shading problem if you're not careful. A ground-mounted panel placed on the wrong side of a tall polytunnel will lose several hours of morning or afternoon sun to the tunnel's own shadow. Site the panel on the south side of the structure where possible, and check the shadow line at different times of day before committing to a fixing point. The shading solutions guide has a more general walkthrough of diagnosing and fixing this.

If your only viable socket is on the shaded side of the house or a north-facing garden wall, don't assume it's not worth doing — even east- or west-facing generation covers small greenhouse loads adequately across a full day, just with a different peak time. The north-facing garden guide covers realistic output expectations for less-than-ideal orientations.

Should you consider a portable power station instead?

For freestanding greenhouses or allotment polytunnels with no mains socket at all — which, as covered above, rule out a plug-in microgenerator kit entirely — a separate solar panel plus battery power station is the correct alternative, not a workaround of the plug-in rules. These systems are legally distinct: they're not wired into your house's mains at all, so the 800VA plug-in cap and no-extension-lead rule simply don't apply. A portable power station paired with a foldable panel can run a propagator or irrigation pump entirely off-grid, charged by its own dedicated panel, with battery storage carrying you through an overcast day or a few hours after dark — something a legal plug-in kit, by design, cannot do.

FAQ

Can I run an extension lead from the house to a greenhouse socket for my plug-in solar unit?

No. The regulations require the plug-in solar unit to connect directly to a fixed domestic socket, with no extension lead in between. If your greenhouse only has power via a trailing extension reel, you'll need a proper hardwired supply installed before a plug-in kit becomes an option — or use a separate off-grid power station instead.

Will an 800W plug-in kit keep my greenhouse frost-free overnight?

No. Frost protection heaters typically need 1–2kW and have to run overnight, when a plug-in kit generates nothing. Plug-in solar offsets daytime loads like propagators, fans and irrigation pumps; it is not a substitute for a properly sized overnight heater on its own circuit.

Does a polytunnel need a different setup to a rigid greenhouse?

Not electrically — the same 800W kit and wiring rules apply to both. The main practical difference is shading: a polytunnel's own structure can shade a nearby panel more than a smaller greenhouse would, so check the shadow pattern before fixing your panel in place.