A 100W solar panel generates roughly 94Wh on an average December day in Britain. That runs an 80W tubular heater for 57 minutes, or a 2kW fan heater for two and a half. Solar is genuinely good at powering shed lights, a radio, and a phone charger. Heat is a different order of magnitude, and most kits sold as solar shed heaters cannot deliver it. Below is the arithmetic, then what works instead.
We have not tested these products ourselves. Everything here is built from manufacturer specifications, UK retailer listings, published climate and solar yield data, and calculations we show in full so you can check them. Some Amazon links on this page are affiliate links, and we may earn a commission from qualifying purchases at no extra cost to you.
- How much heat a shed actually needs
- What a solar panel delivers in a British December
- Work out the numbers for your own shed
- Solar panels and solar air collectors are not the same product
- If your shed already has mains power
- If your shed has no power at all
- Insulate and draughtproof before you heat anything
- Getting a supply to the shed safely
- Common questions
How much heat a shed actually needs
A typical 8ft by 6ft garden shed loses around 90 watts of heat for every degree it is warmer than outside, so holding it just above freezing on a cold night takes roughly 450 watts of heater.
That figure comes from the fabric of the shed, not from a rule of thumb. A standard 19mm shiplap wall has a thermal resistance of 0.019 ÷ 0.13 = 0.146 m²K/W, using the tabulated softwood conductivity in BRE’s conventions for U-value calculations of 0.13 W/mK. Add the standard surface resistances of 0.13 and 0.04 and the wall comes out at a U-value of 3.16 W/m²K. A cavity wall in a modern house is about 0.28.
An 8ft by 6ft shed has roughly 25.7 m² of wall, roof, and floor, plus about 8.6 m³ of air changing perhaps three times an hour through the gaps around the door.
| Shed build-up | Heat loss | Holding 5°C when it is 0°C out | Holding 15°C when it is 3°C out |
|---|---|---|---|
| Bare 19mm shiplap, draughty | 90 W/K | 449 W | 1,078 W |
| Plus 50mm PIR, draughtproofed | 13 W/K | 64 W | 153 W |
The common mistake here is sizing a shed heater the way you would size one for a spare bedroom. Rules of thumb like “100 watts per square metre” are built around insulated houses. A bare timber shed leaks heat roughly eleven times faster per square metre of wall than a modern cavity wall does, which is why a 2kW fan heater can run flat out in one and still feel like it is achieving nothing.
What a solar panel delivers in a British December
A 1kWp solar array produces about 29 kWh in December in the UK against 119 kWh in May, so the darkest month yields roughly a quarter of the best one.
Those monthly figures come from PVGIS, the European Commission’s solar yield model, averaged across UK postcodes for a south-facing array at fixed tilt. Scale 29 kWh per kWp down to a 100W panel and you get 2.9 kWh for the whole of December, or 94Wh a day. Off-grid, you lose roughly another 19% to battery round-trip and inverter losses, leaving about 76Wh actually available at the socket.
Here is what 76Wh buys you:
| Appliance | Runtime per December day, from one 100W panel |
|---|---|
| LED shed light, 5W | Over 15 hours |
| Radio or phone charging, 10W | 7.5 hours |
| 80W tubular heater | 57 minutes |
| 500W convector heater | 9 minutes |
| 2kW fan heater | 2.3 minutes |
Run that backwards and the picture is starker. To keep a well-insulated shed frost-free through December, cycling on a thermostat around 40% of the time, you need roughly 810 watts of panel and a battery. To do the same in an uninsulated one, closer to 5.7kW, which is a domestic rooftop array pointed at a garden shed.
Work out the numbers for your own shed
Sheds vary, so the figures above will not match yours exactly. This calculator uses the same method: fabric heat loss from the build-up you choose, ventilation loss from how draughty it is, then the December solar yield needed to cover it.
Shed Heating & Solar Panel Size Calculator
Work out how much heat your shed actually needs, what that costs on mains electricity, and how much solar panel it would take to do the same job through a British December.
UK December overnight lows average 1–4°C.
One caveat the calculator cannot fix. The battery figure it gives you covers a single winter night. Britain gets overcast weeks in December where output stays near zero for days, so a system you would actually rely on needs three to five days of autonomy, not fourteen hours. Multiply the battery accordingly.
Solar panels and solar air collectors are not the same product
A solar air collector heats air directly with sunlight and blows it into the building, which is a genuinely different technology from a photovoltaic panel that generates electricity.
This distinction gets lost in search results, because both get sold as "solar heaters". The collector is the more interesting of the two for a shed, because it skips the conversion losses entirely. The SolarVenti SV3 is the model most often sold into UK garden buildings: a 0.35 m² panel with a 6W photovoltaic cell that runs nothing but its own fan, pushing 35 m³ of air an hour at a stated temperature rise of about 15K.
You can work out its peak heat output from those two numbers. Air has a density of roughly 1.2 kg/m³ and a specific heat capacity of about 1,005 J/kgK, so:
35 m³/h ÷ 3,600 × 1.2 × 1,005 × 15K = 176 watts
That is 176 watts in full sun, which in December in Britain means a few hours around midday on the days it is not overcast. Against a 449W frost-protection load in a bare shed, it will not hold a temperature. Against damp, it is a different story, because it is ventilating with warm dry air rather than trying to raise the whole shed by 10 degrees. If your actual complaint is rust on tools and mould on the walls rather than cold hands, a collector is the honest match for the problem.
The cheaper version of the same idea is a solar-powered extract fan, which skips the heating element entirely and just moves air whenever the sun is on the panel. A 12W weatherproof solar fan kit is £25.99 and a roof-mounted solar ventilator is £54.90 (both checked August 2026). Neither adds a single degree of heat, and that is fine, because on the damp problem they are the better value per pound than anything on this page. Solar is well matched to ventilation for the same reason it is badly matched to heating: the fan wants power on bright days, which is exactly when a panel has it.
If your shed already has mains power
A thermostatic tubular heater is the cheapest sensible way to keep a shed frost-free, at roughly £5 of electricity for the whole of December.
The Dimplex ECOT2FT 80W tubular heater is £27 at Screwfix (checked August 2026), rated IPX4, 713mm long, with a built-in thermostat. Left on a low setting in an insulated shed it will cycle on maybe 40% of December, drawing 23.8 kWh, which at the Ofgem price cap rate of 26.11p/kWh for 1 July to 30 September 2026 comes to £6.22.
If you would rather order it alongside everything else, the Hylite Ecoheater 500mm 55W in green is £24.95 on Amazon (checked August 2026) and is the same idea in a slightly lower output, sold specifically for greenhouses and sheds. The white slimline version is £24.49 and identical apart from the colour, which matters only if your shed has opinions. At 55W it draws 16.4 kWh over December, or £4.27.
Who should skip both: anyone wanting to work in the shed in comfort rather than keep it above freezing. A 55W or 80W tube holds the fabric above dew point, it does not warm a space you are standing in. For that you want a 500W panel or convector and the running cost that comes with it.
| Heater | Rated output | December cost, 40% duty cycle |
|---|---|---|
| Tubular heater, Hylite Ecoheater | 55 W | £4.27 |
| Tubular heater, Dimplex ECOT2FT | 80 W | £6.22 |
| Small convector | 500 W | £38.86 |
| Fan heater | 2,000 W | £155.42 |
The mistake to avoid is putting a 2kW fan heater on a timer and calling it frost protection. It heats the air, which then leaks out of the shed within the hour, and it costs twenty-five times as much as the tube for a result the tube achieves better. Tubular heaters work because they run constantly at low output against a thermostat, keeping the fabric of the shed above dew point rather than blasting the air. The same logic applies indoors, which is why oil-filled and low-output electric radiators beat fan heaters on running cost for anything longer than a quick blast, and why the numbers on our 2kW electric radiator running cost breakdown look the way they do.
If your shed has no power at all
With no supply to the shed, the real choice is between an off-grid kit that will run lights and tools but not heat, and a permanent mains supply that will run everything.
Both cost money up front, so compare them honestly:
| Off-grid solar for frost protection | Armoured mains supply | |
|---|---|---|
| Kit needed | ~810W of panel, charge controller, inverter, 3 to 5 days of battery | SWA cable, small consumer unit, sockets |
| Up-front cost | Battery alone from £359.99 for 3.36 kWh (Eco-Worthy 12V 280Ah, checked August 2026), before panels and electronics | £800 to £2,000 depending on cable run |
| Running cost, December | £0 | About £6 |
| Works in a dull week | No | Yes |
| Also powers lights and tools | Yes, comfortably | Yes |
If you want lights, a charger, and the odd cordless tool battery topped up, a small off-grid kit is a good purchase and a genuinely satisfying one. Sizing it is easy because those loads are tiny. An Eco-Worthy 25W 12V panel kit at £38.99 or a Topsolar 20W kit with a 10A charge controller at £29.99 (both checked August 2026) will run LED lighting in a shed comfortably, and you add a leisure battery to suit. Skip both if you were hoping to run a heater from them, for every reason set out above.
It is only when heat enters the specification that the numbers fall apart, and no amount of clever kit selection fixes that, because the limit is how much sunlight lands on a shed roof in December.
Worth knowing: if you are having a supply run anyway, the marginal cost of also fitting a proper shed alarm and decent lighting on the same circuit is small, and it is far cheaper than a second visit.
Insulate and draughtproof before you heat anything
Adding 50mm of PIR board and sealing the gaps cuts a shed's heat loss from about 90 watts per degree to about 13, a seven-fold reduction that no heater choice can match.
PIR board has a thermal conductivity of 0.022 W/mK, so a 50mm layer adds 2.27 m²K/W of resistance to a wall that currently has 0.146. Celotex GA4050 in 2400mm by 1200mm sheets is £23.75 a board excluding VAT (checked August 2026). An 8ft by 6ft shed needs roughly 21.4 m² of wall and roof coverage, which is eight boards, or about £228 including VAT.
That is more than the tubular heater and less than the off-grid kit, and it is the only spend on this page that improves every other option at once. It cuts the mains running cost, shrinks the solar array you would need, and makes the solar air collector's 176 watts start to look meaningful rather than symbolic.
Leave a ventilation gap and do not seal the shed completely airtight. A shed with no air movement and a heat source is a shed with condensation on the inside of the cladding, which is the problem you were trying to solve.
Getting a supply to the shed safely
Running a new circuit to an outbuilding is notifiable work under Part P of the Building Regulations in England and Wales, and it is not a job to do informally.
Safety and regulations
Installing a new circuit to supply a detached outbuilding, and installing or replacing a consumer unit, is notifiable work under Part P. Buried supplies must use SWA armoured cable under BS 7671 Regulation 522.8.10. Use an electrician registered with a competent person scheme, who can self-certify the work and issue an Electrical Installation Certificate. Uncertified outbuilding wiring can cause problems at the point of sale and is not covered by most home insurance.
If you are running the cable yourself as far as first fix, knowing which conductor is which is the bare minimum before you touch anything, and the final connection still needs to be someone else's signature.
Common questions
Can you run an electric heater from a solar panel?
Yes, but only a very small one for a very short time. A 100W panel produces about 94Wh on an average UK December day, which is enough to run an 80W tubular heater for under an hour, and a 2kW fan heater for about two and a half minutes.
What size solar panel do I need to heat a shed?
Around 810 watts of panel plus a battery bank to keep an insulated 8ft by 6ft shed frost-free through a UK December, and roughly 5.7kW to do the same in an uninsulated one. Both figures assume the heater cycles on about 40% of the time.
Do solar shed heaters work in winter?
Not as heaters. UK solar output in December is roughly a quarter of the May figure, which is precisely when heating demand peaks, so the supply and the demand are out of phase by six months. Solar handles shed lighting and charging in winter without difficulty.
Is a solar air collector worth it for a shed?
For damp and stale air, yes. For temperature, no. A SolarVenti SV3 delivers around 176 watts of heat at peak sun by calculation, which ventilates and dries a shed effectively but will not hold a working temperature in midwinter.
What is the cheapest way to keep a shed warm?
Insulate with 50mm PIR, draughtproof the door, and fit an 80W thermostatic tubular heater on a mains supply. That combination costs roughly £6 of electricity for the whole of December against £155 for a 2kW fan heater doing the same job badly.
Will a solar panel stop my shed getting damp?
Not on its own, but a solar-powered extract fan or a solar air collector will help, because damp is a ventilation problem more than a heat problem. Moving air through the shed on sunny days is more effective per pound than trying to raise its temperature, and a 12W solar fan kit costs around £26.
