How the figure is calculated
An electric heater turns every watt it draws into heat, so the rated wattage is the maximum it can cost per hour. We multiply that wattage by the duty cycle, the share of time the element is actually on, then by your hours and your price per kWh. The duty cycle starts at 100 %, the worst case, because we have no tested figure for how often a thermostat clicks off in your room. Lower it if you know yours cycles.
What drives the cost
Wattage and hours, almost entirely. Ceramic, oil-filled and fan heaters of the same wattage cost the same per hour at full power; they differ in how quickly they warm you and how long they hold the heat after the thermostat cuts out. US plug-in models top out at 1,500 W because of household circuits, while European models such as the Trotec TDS 20 M run at 3,000 W.
Worked example
US: the Lasko 754200 ceramic heater on high draws 1,500 W, which is 27.47¢ an hour at 18.31¢/kWh. Four hours a day for 30 days is $32.96. If the thermostat keeps it on half the time, the duty cycle at 50 % brings that to $16.48. On its 900 W low setting it is 16.48¢ an hour.
GB: the Trotec TDS 20 M at 3,000 W costs 78.96p an hour at 26.32p/kWh. Four hours a day is £3.16, or £94.75 over 30 days at full power and £47.38 at a 50 % duty cycle.
Heating a person, not a house
A heater makes sense for one room for a few hours. Close the door, use the thermostat, and compare the monthly figure with your central heating before running it all evening.
Questions
Is an oil-filled radiator cheaper than a fan heater?
Not per hour at the same wattage. It may cycle off sooner because the oil holds heat, which the duty-cycle slider lets you model.
Why does the default assume 100 % on?
That is the rated maximum. We will not assume your thermostat behaviour without telling you, so the slider is visible and starts at the top.
Does GB electricity include VAT right now?
Ofgem states there is no VAT on electricity from 1 October 2026 to 31 March 2027, and the 26.32p cap reflects that.