Solar panels for a garden room

Garden rooms are sold on the build quality and the glazing. The electricity bill turns up later. Here is what one actually draws, where the money goes, and how far a plug-in solar kit can realistically get you.

What a garden room actually uses

Almost every garden room follows the same pattern: a modest, steady daytime load, and one enormous seasonal load called heating. Get the second one straight in your head and the rest of the numbers fall into place.

Put those together and a garden room used a few days a week for work, lit and lightly heated, tends to land somewhere in the region of 800–2,000kWh a year. The wide range is heating: an unheated summer-months room can sit under 400kWh, while a poorly insulated room warmed by a plug-in fan heater every winter morning can pass 2,500kWh on its own.

Why the supply itself is expensive

Running a proper spur from the house consumer unit to the garden room is a job for a qualified electrician: armoured cable, a trench at the correct depth, a small consumer unit in the room, testing and certification. Depending on distance and what the ground is made of, that is usually a four-figure cost before you have used a single unit of electricity.

That cost is generally worth paying — you need a reliable supply for evenings, for winter and for anything with a heating element. What people often want, having paid it, is a way to stop the meter spinning so hard during the day. That is the gap a plug-in kit fits into.

How a plug-in kit fits a garden room

A plug-in device is panels plus a small microinverter with a plug on the end. It goes into an ordinary socket and feeds up to 800VA into your household circuits. Anything running at that moment uses the solar electricity first; only the shortfall comes from the grid. The detail is in how does plug-in solar work?

The reason garden rooms suit this so well is timing. In a normal house, most people are out when the sun is highest, so a large share of the generation is exported for little or nothing. A garden room is occupied precisely during those hours. The lighting, the laptop, the monitor, the router and the fridge are all running while the panels are working, so more of what you generate is actually used — and that is what the saving depends on.

Roof mounting or ground mounting?

On the garden room roof

• Flat roofs take tilted frames well, and a 20–35° tilt facing broadly south is the best orientation you will get in a garden.
• Check the roof structure. Garden room roofs vary enormously and some are not built for the added weight and wind load of framed panels.
• Anything fixed through a membrane is a potential leak. If you are not confident, get the builder or a roofer to do the fixings.

On the ground beside it

• A low A-frame on ballast or ground screws avoids touching the building entirely — useful for renters and for anyone whose warranty says not to drill the roof.
• Easier to clean, easier to angle, easier to take with you if you move.
• Costs you garden space, and is more exposed to shading from fences and planting.

Shade matters more than most people expect. A panel in dappled shade for part of the afternoon loses a meaningful chunk of its day. Watch the actual spot across a full day before deciding.

What it will and won't cover

It will cover a good share of your lighting, computing, networking and fridge load through spring, summer and autumn, and a smaller share in winter. It will not cover electric heating. A 2kW panel heater draws more than twice the entire output of a plug-in device, and it is needed most in the months when generation is at its lowest. Anyone telling you a plug-in kit will heat a garden room in January is not being straight with you.

The honest framing: insulate first, heat efficiently second — a small air-source unit will deliver several times the heat of a resistive heater for the same electricity — and use solar to chip away at the daytime baseline. On the government's own modelling, a south-facing device at a 30° tilt generates in the region of 690kWh a year, of which roughly 428kWh is typically self-consumed, worth around £110 a year at the assumed price. Figures and assumptions are in the DESNZ plug-in solar impact assessment, and we work through them on our savings page.

Suitability checklist

• Is the garden room, or a spot beside it, unshaded between roughly 10am and 4pm?
• Is the room genuinely used during daylight hours most weeks?
• Is there an existing socket the device can plug directly into — no extension leads?
• Can the roof carry framed panels, or is there ground space instead?
• If you rent or lease, have you asked about fixing anything to the building?
• Are you happy with an offset rather than an off-grid supply?

Answer yes to most of those and a plug-in kit is a sensible addition. Answer no to the daytime-use question and you would be better off looking at a conventional rooftop system with export payments — see plug-in solar vs rooftop solar.