Enter your room size, insulation, heater and hours to see what heating will cost a day, a month and a winter. The chart compares an electric heater with a heat pump at each insulation level.
How the calculator works
Cost = heater power (kW) × hours × your unit rate. A thermostat switches the heater on and off, so a well-insulated room runs it about half the time; an average room runs it most of the day; and a poorly insulated room needs about twice the heater, all day. Those are the same assumptions as our running costs guide, so a 4m x 3m room comes out at about £1.05 a day well insulated and £4.21 a day poorly insulated, at 26.32p per kWh.
With “size the heater for me” ticked, the heater is about 1kW for a well-insulated 4m x 3m room (about 83W per m², inside the usual 70–100W per m² guidance), doubled for a poorly insulated room. A heat pump uses its heat output divided by its efficiency (COP). Monitored homes average about 2.5 to 3.0, below the 3.5 to 4.5 on labels.
A month is worked out as days a week × 52 ÷ 12. No standing charge is added, because a garden room on your house supply doesn’t add one. The laptop office option adds about 0.7kWh a day.
Questions
How much does it cost to heat a garden room?
A well-insulated 4m x 3m office heated 8 hours a weekday costs about £20–£25 a month at the October–December 2026 price cap. A poorly insulated one can cost four times as much.
What is the cheapest way to heat a garden office?
Insulation first: it changes the bill more than the heater does. After that, an air-to-air heat pump uses roughly a third to two-fifths of the electricity of a plain electric heater for the same heat, depending on its real-world efficiency. All plain electric heaters cost the same per unit of heat.
Can I claim tax relief for my garden office?
Self-employed people can claim a flat £10, £18 or £26 a month depending on hours worked at home, or a share of actual costs. Employees lost working-from-home tax relief from 6 April 2026. See our garden office tax guide.
Sources
- Ofgem: energy price cap, 1 October to 31 December 2026
- Electrical Deals Direct: what size panel heater do I need?
- Selectra: air-to-air heat pump real savings
- Energy Saving Trust: air-to-air heating overview
- Selectra: working from home electricity cost per day
- GOV.UK: simplified expenses, working from home
- GOV.UK: tax relief for employees working at home