Every figure on this site comes from one of the rules below. If a rule changes, every page that uses it updates at the same time.
Energy prices
We use the Ofgem price cap, because it is what most households on a standard variable tariff pay. For 1 October to 31 December 2026 that is 26.32p per kWh for electricity and 7.97p per kWh for gas (Ofgem price cap, Direct Debit average for England, Scotland and Wales). We update these each time the cap changes, on 1 January, 1 April, 1 July and 1 October. If you're on a fixed or time-of-use tariff, put your own rate into the running cost calculator.
Running costs
Cost per hour = power in kW × price per kWh.
A 1,000W (1kW) appliance running for an hour uses 1kWh, so it costs 26p an hour. Power is the rated figure from the manufacturer, which is the most the appliance draws. Anything with a thermostat or humidistat usually draws less over time, so our figures are the most you'd pay rather than the least. We leave out the daily standing charge, because you pay it whether or not the appliance is on.
"Per month" is 30.4 days. "Winter" is 26 weeks, roughly October to March.
Dehumidifier size
Manufacturers rate dehumidifiers by how many litres they extract a day in warm, very humid test conditions, which no UK home matches in winter. So the size calculator uses a rule of thumb rather than a water calculation:
| Where it runs | Starting size, heated home | Starting size, cold or unheated |
|---|---|---|
| One room | 10 litres a day | 7 litres a day |
| 1 to 2 bed flat | 10 litres a day | 7 litres a day |
| 2 to 3 bed house | 12 litres a day | 8 litres a day |
| 4 or more bedrooms | 20 litres a day | 10 litres a day |
From the starting size it moves one step up the size ladder for damp patches or a musty smell, two steps for mould or wet walls, and one more step for drying washing. The compressor ladder is 10, 12, 16, 20 and 25 litres; the desiccant ladder is 7, 8 and 10 litres. It recommends a desiccant model when the room is often below 15°C, because compressor models collect much less water in the cold.
Humidity and mould risk
The mould risk checker works out the dew point, the temperature at which the air in your room would start to leave water on a surface, using the Magnus formula with the Sonntag constants (6.112 hPa, 17.62 and 243.12°C).
It also works out the surface temperature at which the air touching a wall would reach 80% relative humidity. 80% is the surface humidity that the European standard for assessing condensation and mould risk on building surfaces (BS EN ISO 13788) uses as the limit for mould growth, taken as a monthly average.
The bands on our humidity scale are rules of thumb: 40% to 60% is a comfortable range for most homes, above 60% is damp, and above 70% room humidity, cold surfaces in the room are likely to pass 80%. For comparison, Approved Document F, the government guidance on ventilation for new homes in England, says a ventilation system should keep average humidity in living spaces below 65% over a month and 75% over a week.
Heating comparisons
The heating cost comparison uses typical power for each kind of plug-in heater: 100W for a double electric blanket, 120W for a heated throw, 1.2kW for a halogen heater, 1.5kW for an oil-filled radiator and 2kW for a fan heater.
Central heating is compared by the cost of delivering the same amount of heat to the whole house. We use 5kW as a round example; the calculator lets you change it. Each system then needs:
- Gas boiler: heat ÷ 0.84 efficiency × gas price.
- Oil boiler: heat ÷ 0.84 efficiency × oil price per kWh. Kerosene holds about 9.82kWh per litre, so 120.13p a litre is about 12.2p per kWh.
- Air source heat pump: heat ÷ 2.93 × electricity price, because it moves about 2.93 units of heat for every unit of electricity.
- Electric heaters or an electric boiler: heat × electricity price, because they turn electricity into heat one for one.
The heat pump figure, 293%, is the real-world average from the government-funded Electrification of Heat field trial. The boiler figure, 84%, is the typical fossil fuel heating efficiency it is compared with. A new, well set-up boiler or heat pump can do better; an old one can do worse. Heating oil isn't covered by the price cap, so we use a recent national average and update it with each cap change. Economy 7 and storage heaters aren't included, because their rates vary by supplier and region.
Cooling
Fan, air conditioner and air cooler costs use the same formula as everything else: rated power in kW × price per kWh. A fan's rated power is close to what it draws on its top setting. A portable air conditioner's compressor cycles on and off once the room reaches the set temperature, so its real use depends on the weather, the room and the setting. Our figures show the most it would cost at full power. Cooling power in BTU is the manufacturer's rating; the room size each model suits is the manufacturer's guide, not our own.
Products
Product specifications come from the manufacturer's own pages and manuals, and each product links to where its figures came from. We recommend products by those specifications. We don't use or summarise Amazon customer reviews or star ratings, and we don't claim to have tested anything we haven't.
Sources
- Ofgem: changes to energy price cap between 1 October and 31 December 2026
- GOV.UK: Approved Document F, ventilation
- Heat Pump Association: Electrification of Heat demonstration report
- Heating oil price: BoilerJuice Great Britain average, 22 September 2026, via Selectra
- SEAI conversion factors (kerosene energy content)
Checked 14 October 2026.