Convert between joules, calories, kWh and BTU instantly.
Energy appears in nutrition, electricity billing, heating, fuel and physics, and each field brought its own unit. All of them convert through the joule, defined as the work done when a force of one newton acts over one metre.
| Unit | In joules | Typically used for |
|---|---|---|
| Joule (J) | 1 | SI unit; physics |
| Kilojoule (kJ) | 1,000 | Nutrition labels outside the US |
| Calorie (cal) | 4.184 | Chemistry |
| Kilocalorie (kcal) | 4,184 | Nutrition — the dietary "Calorie" |
| Watt-hour (Wh) | 3,600 | Battery capacity |
| Kilowatt-hour (kWh) | 3,600,000 | Electricity billing |
| BTU | 1,055.06 | Heating and air conditioning |
| Therm | 105,505,600 | Natural gas billing |
There are two units called the calorie, differing by a factor of a thousand. The small calorie (cal) is the energy needed to raise one gram of water by one degree Celsius. The large Calorie (kcal, sometimes written with a capital C) is a thousand of those, and it is what every nutrition label means by "calories".
So a 500-calorie meal is really 500 kilocalories, or 2.09 million small calories, or 2,092 kJ. Nutrition labels in Europe, Australia and much of Asia print kilojoules, sometimes alongside kcal. To convert a label, multiply kcal by 4.184 to get kJ, or divide kJ by 4.184 to get kcal. A useful shortcut is that kJ is roughly four times kcal.
| Food energy | kcal | kJ |
|---|---|---|
| Apple | 95 | 397 |
| Slice of bread | 80 | 335 |
| Chocolate bar | 250 | 1,046 |
| Daily intake, adult | 2,000-2,500 | 8,368-10,460 |
This is the distinction most worth getting right. Power is a rate — energy per unit time — measured in watts. Energy is the total delivered, measured in joules or kilowatt-hours.
A 2,000 W heater running for 3 hours uses 2 x 3 = 6 kWh. The kilowatt-hour is itself a compound unit, which is why "kilowatts per hour" is meaningless and always a mistake when you see it on a bill or in an article.
Since electricity is billed per kWh, running costs follow directly. At 8 per kWh, that heater costs 48 for the three hours. A 10 W device left on permanently uses 0.24 kWh a day, or 87.6 kWh a year — worth knowing before worrying about standby power on small electronics.
The British thermal unit is the energy to raise one pound of water by one degree Fahrenheit. Air conditioners and heaters are commonly rated in BTU per hour, which is a power rating despite often being written as just "BTU".
The "ton" in air conditioning refers to the cooling power of a ton of ice melting over 24 hours, which is a pleasingly literal origin. A 1.5 ton unit is 18,000 BTU/h or about 5.3 kW of cooling — though that is cooling delivered, not electricity consumed. A unit's electrical draw is the cooling output divided by its efficiency ratio, so an efficient 1.5 ton unit might consume only 1.5 kW to deliver 5.3 kW of cooling.
Multiply dietary calories (kcal) by 4.184. A 500 kcal meal is 2,092 kJ. As a mental shortcut, kJ is a little over four times the kcal figure.
No. The dietary calorie is a kilocalorie — 1,000 physics calories, or 4,184 joules. Nutrition labels have used the larger unit for so long that the prefix is usually dropped, which is the source of the confusion.
A kilowatt is a rate of energy use; a kilowatt-hour is a quantity of energy. A 2 kW appliance running for 3 hours consumes 6 kWh. Electricity bills charge per kWh, so the total matters rather than the rate.
A rough guide is 20 BTU/h per square foot of floor area, adjusted upward for strong sun, high ceilings, poor insulation or a hot climate. A 150 square foot room typically needs around 3,000 BTU/h, though a proper load calculation is better than a rule of thumb for anything expensive.
12,000 BTU per hour, or about 3.517 kW of cooling. The name comes from the cooling produced by a ton of ice melting over 24 hours. It measures cooling delivered, not electricity consumed.