Energy Unit Conversions: kWh, Joules, BTU, Calories, and Therms
October 21, 2026
Energy shows up in many units, depending on context: joules in physics, kilowatt-hours on electricity bills, British thermal units (BTU) and therms on gas bills and heating equipment, and calories on food labels. Converting between them lets you compare, for example, the energy in a slice of cake with the electricity needed to run a lamp, or the price of heating with gas versus electricity.
The central relationship: 1 kilowatt-hour (kWh) = 3.6 million joules = 3,412 BTU = 860 kilocalories. A food Calorie (capital C, as on nutrition labels) is a kilocalorie, equal to 4,184 joules.
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The factors below are rounded to useful precision. To convert, multiply by the number given.
| From | To | Multiply by |
|---|---|---|
| kWh | Joules (J) | 3,600,000 |
| kWh | BTU | 3,412.14 |
| kWh | Food Calories (kcal) | 860.42 |
| BTU | Joules | 1,055.06 |
| Therm | BTU | 100,000 |
| Therm | kWh | 29.307 |
| kcal | Joules | 4,184 |
| Horsepower-hour | kWh | 0.7457 |
Food Energy in Everyday Terms
A 2,000-Calorie daily diet equals 2,000 × 4,184 = 8.37 million joules, or 2.32 kWh. That is the energy a 100-watt light bulb uses in 23 hours. A 300-Calorie muffin has 0.35 kWh, enough to run a 1,000-watt microwave for 21 minutes. A person's resting power of about 80 watts is roughly 1,650 kcal per day, about the same as a dim incandescent bulb. These comparisons show why human muscle power is small compared with machines: a day of heavy manual labor yields around 1 kWh of mechanical work, worth a few cents of electricity.
A liter of gasoline contains about 9.5 kWh (34 MJ) of chemical energy, about 8,200 Calories, which is why fuel is such a compact store.
Comparing Electricity and Natural Gas
US natural gas is sold in therms (100,000 BTU) or hundreds of cubic feet; Europe uses kWh. To compare costs, convert to the same unit. If electricity costs $0.16 per kWh, a therm-equivalent costs 29.307 × 0.16 = $4.69 for resistance heat. If gas costs $1.50 per therm and the furnace is 95% efficient, the delivered heat costs $1.58 per therm-equivalent. Gas is cheaper by about threefold in this example, though a heat pump delivering 3 units of heat per unit of electricity brings the electric cost down to about $1.56, comparable to gas.
Efficiency matters as much as price, so always compare the cost per unit of useful heat delivered.
Other Common Equivalents
1 horsepower (mechanical) = 745.7 watts. 1 watt = 1 joule per second. 1 BTU per hour = 0.293 watts. A 12,000 BTU/h air conditioner (one ton) removes heat at 3.52 kW. A typical household uses about 10,000 to 12,000 kWh of electricity a year in the United States, or about 30 kWh per day. One barrel of oil contains about 5.8 million BTU, or 1,700 kWh. A ton of TNT is defined as 4.184 billion joules, or about 1,162 kWh.
Tips for Conversions
Pay attention to prefixes: kilo (10³), mega (10⁶), giga (10⁹). Distinguish calories (small c, 4.184 J) from Calories (capital C, 1,000 small calories or 4,184 J). Keep track of whether you are talking about energy (joules, kWh) or power (watts, BTU per hour): power is the rate of using energy. Energy = power × time.
Frequently Asked Questions
How many joules are in a kilowatt-hour?
3.6 million joules.
How many BTU are in a kWh?
About 3,412 BTU.
Is a food Calorie the same as a calorie?
A food Calorie is a kilocalorie, 1,000 of the small calories used in physics.
How many kWh are in a therm?
About 29.3 kWh.
What is the difference between energy and power?
Energy is the amount of work done (kWh, joules). Power is the rate of doing work (watts).