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Convert between joules, calories, kWh, BTU, and more. Includes food calorie conversions.
Energy is the capacity to do work, measured in joules (J), calories, kilowatt-hours, or BTUs. It can be converted between thermal, electrical, and mechanical forms.
| Description | kJ | kcal | kWh |
|---|---|---|---|
| Apple (medium) | 400 | 95 | 0.11 |
| Daily intake (avg) | 8,400 | 2,000 | 2.3 |
| LED bulb (1 hour) | 36 | 8.6 | 0.01 |
| Laptop (1 hour) | 180 | 43 | 0.05 |
| Gallon of gasoline | 120,000 | 28,700 | 33.4 |
| EV battery (typical) | 216,000 | 51,600 | 60 |
Divide joules by 4.184 to get small calories. Divide by 4,184 for food Calories (kcal).
cal = J ÷ 4.184
Multiply kWh by 3,600,000 to get joules (1 W = 1 J/s, 1 hour = 3,600 s).
J = kWh × 3,600,000
Multiply BTU by 0.000293 to get kWh. Common for HVAC and heating systems.
kWh = BTU × 0.000293
Formula
1 kcal = 4,184 J | 1 kWh = 3,600,000 J | 1 BTU = 1,055.06 JJ (joule) = SI unit of energy — work done by 1 N of force over 1 m
kcal = kilocalorie = 4,184 J — used on food labels as "Calories" (capital C)
kWh = kilowatt-hour = 3,600,000 J — standard unit for electricity billing
BTU = British Thermal Unit = 1,055.06 J — used for HVAC and heating systems in the US
Worked Example
Convert 500 kcal (food) to joules and kWh
Did you know? The joule is named after physicist James Prescott Joule, who demonstrated in 1843 that heat and mechanical work are equivalent. His experiments using a paddle wheel heating water established the mechanical equivalent of heat — a cornerstone of thermodynamics.
Sources
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