Convert energy units between units, using exact SI factors.
A kilowatt-hour is 3,600,000 joules, or 3.6 megajoules. The number falls out directly: a kilowatt is 1,000 watts and an hour is 3,600 seconds. The joule is the SI unit of energy defined in the BIPM brochure, but electricity bills use kilowatt-hours.
A kilocalorie is a thousand calories, and kilocalories are what food packaging calls simply calories. One kilocalorie is 4,184 joules. The calorie is not an SI unit and does not appear in the BIPM brochure, which is why science states energy in joules.
BTU is the British thermal unit, about 1,055 joules — the heat needed to raise a pound of water by one degree Fahrenheit. It describes the output of air conditioners and heat pumps. It is not an SI unit and the BIPM brochure does not recognise it.
A two-kilowatt heater uses exactly two kilowatt-hours in an hour, or 7.2 megajoules in SI units. At 0.20 EUR per kilowatt-hour that is 0.40 EUR an hour. Energy is power times time, which is why a more powerful appliance costs no more if it runs for less time.