Energy converter for electricity, food, heating and physics
This energy converter covers 12 units that come from very different fields: joules and their multiples, watt-hours and kilowatt-hours from electricity bills, calories and kilocalories from nutrition, BTUs and therms from heating, and electronvolts, ergs and foot-pounds from physics and engineering. All of them are converted through the joule.
Definitions used
| Unit | Joules |
|---|---|
| Calorie (cal) | 4.184 (thermochemical calorie) |
| Kilocalorie / food Calorie (kcal) | 4184 |
| Watt-hour (Wh) | 3600 |
| Kilowatt-hour (kWh) | 3,600,000 = 3.6 MJ |
| BTU | 1055.05585262 (International Table BTU) |
| US therm | 105,480,400 |
| Electronvolt (eV) | 1.602176634 × 10⁻¹⁹ (exact) |
| Foot-pound (ft·lbf) | 1.3558179483314004 |
| Erg | 10⁻⁷ |
Calorie versus Calorie
The "Calories" on a food label are kilocalories. A snack with 250 Calories contains 250 kcal, which is 1046 kJ, not 250 small calories. The tool labels the kilocalorie as the food Calorie to make this clear. Several calorie definitions exist: this converter uses the thermochemical calorie of exactly 4.184 J, while the International Table calorie is 4.1868 J, a 0.07% difference.
1 kcal = 4.184 kJ
2000 kcal = 8368 kJ
2000 kcal = 2.324444444 kWh
100 kJ = 23.90057361 kcalThat third line is a nice perspective: a whole day's food energy is about the same as running a 1 kW heater for 2.3 hours.
Electricity and heating
A kilowatt-hour is the energy used by a 1 kW load for one hour, so 1 kWh = 1000 W × 3600 s = 3.6 MJ. In heating terms, 1 kWh = 3412.141633 BTU.
Gas bills in the US often use the therm. The US therm is defined as 105,480,400 J, which works out to 29.30011111 kWh. It was set as 100,000 BTU using a slightly smaller BTU than the International Table one used here, so the converter shows 1 therm as 99,976.13 BTU rather than a round 100,000.
Worked example: comparing heating costs
Gas is billed per therm and electricity per kilowatt-hour, so the two prices cannot be compared until they are in the same unit. With example prices of $1.50 per therm and $0.16 per kWh:
1 therm = 29.3 kWh
gas: $1.50 ÷ 29.3 = $0.051 per kWh of heat
95% efficient furnace → $0.051 ÷ 0.95 = $0.054 per useful kWh
electric resistance heater → $0.16 per useful kWh
heat pump, COP 3 → $0.16 ÷ 3 = $0.053 per useful kWhConverted this way, the gas furnace and the heat pump cost almost the same per unit of heat, while plain resistance heating costs three times as much. Swap in the prices from your own bills and the converter handles the unit step; efficiency and COP come from the equipment datasheet.
Energy is not power
A kilowatt (kW) is a rate; a kilowatt-hour (kWh) is an amount. Mixing them is the most common error in energy math. To convert rates such as horsepower or BTU per hour, use the power converter. Heat and temperature are also different quantities, so the temperature converter cannot tell you how much energy it takes to warm something up.