Energy is the capacity to do work or transfer heat. Power is how fast that happens. Mixing the two is why “kW” and “kWh” cause so much confusion on bills and appliance labels.
A heater rated 2 kW draws energy at two kilowatts while it is on. Run it for three hours and it uses about 6 kWh of energy. Horsepower and kilowatts of engines are power; our Power converter handles those. Food “Calories” are also energy, but culinary labelling has its own conventions — see Nutritional Energy.
Numbers used by our Energy converter:
Charging an EV with 40 kWh packs about 144 MJ into the battery (before charging losses). A small portable heater using 1.5 kWh overnight transfers about 5.4 MJ as heat to the room (idealised). An air-conditioner marketing “12,000 BTU” is usually quoting a rate (BTU per hour) — multiply by hours to get energy transferred.
Electricity statements multiply kWh by a unit price, then add standing charges and tax. Two appliances with the same wattage use similar energy only if they run for similar hours. A 10 W LED left on all day can rival a 1,000 W kettle used for a few minutes. Convert standby watts × hours into kWh with the same “power × time” idea, then use the Energy converter if you need joules or BTU for a physics-style estimate.
US room air-conditioner boxes often print BTU/h. That is a cooling power marketing figure. Divide by 3.412 approximately to sketch kilowatts, remembering real-room performance depends on insulation and climate. Gas boiler leaflets that mention kWh refer to energy delivered or gas energy content over time — read the small print for net versus gross calorific values.
SI definitions of the joule and watt; International Table BTU relationship used in engineering conversions.
Last updated: July 2026