but as Special says, cost is power multiplied by time.
For convenience, it is expressed as kWh, meaning the amount that a 1kW appliance would use in one hour.
Appliances are described in the power they use, in kW.
A powerful plug-in heater is 3kW (that's the most you can have, though a domestic UK plug. Other countries may be less). Which is enough for a medium-sized room with reasonable insulation. A larger room will be more evenly heated with a heater at each end.
A 3kW fan heater costs the same to run per hour as a 3kW convector or a 3kW oil-filled radiator, and gives out exactly the same amount of heat. A 3kW storage radiator also gives out the same amount of heat and uses the same amount of electricity in an hour, but it normally uses cheap overnight tariff, at about half price, and the heat leaks out during the day.