The SI unit of energy; the work done by a 1 newton force over 1 meter. Everything converts through joules.
Calorie (cal)
Energy to raise 1 gram of water by 1 °C. The food Calorie (kcal) is 1,000 times larger — always check context.
Kilowatt-hour (kWh)
3.6 MJ of energy. Your electricity bill is in kWh because it makes household consumption numerically readable.
BTU
British Thermal Unit — energy to heat 1 lb of water by 1 °F. Used in US HVAC, furnaces, and air conditioners.
Therm
100,000 BTU — the standard unit for US residential natural gas billing. One therm ≈ 105.5 MJ.
Electronvolt (eV)
1.602 × 10⁻¹⁹ joules. Used in particle physics, chemistry, and semiconductor science for atomic-scale energies.
Real-World Examples
Example 1
Monthly Home Electricity
900 kWh
= 3.24 × 10⁹ J = 3.24 GJ — average US household monthly electricity consumption
Example 2
Daily Dietary Intake
2,000 kcal
= 8,368,000 J = 2.32 kWh — the recommended daily caloric intake for an average adult
Example 3
Natural Gas Heating
10 therms
= 1,000,000 BTU = 1,054.8 MJ — typical monthly natural gas use for home heating
Energy Unit Equivalents
Unit
Joules
kcal
kWh
BTU
1 cal
4.184
0.001
1.163×10⁻⁶
0.003969
1 BTU
1,055.06
0.252
2.931×10⁻⁴
1
1 kWh
3,600,000
860.4
1
3,412
1 therm
105,480,400
25,200
29.3
100,000
1 MJ
1,000,000
239.0
0.2778
947.8
1 GJ
1,000,000,000
239,006
277.8
947,817
Energy Units Across Disciplines
Why So Many Units?
Energy is one of the most fundamental quantities in science and engineering — yet it is measured in over a dozen different units depending on context. Physicists use joules and electron-volts. Nutritionists use kilocalories. Engineers designing heating systems use BTU. Utility companies bill in kilowatt-hours. Gas suppliers use therms. Each unit evolved to keep the numbers human-scale and meaningful within its domain.
Comparing Fuel Sources
Converting to a common unit reveals the dramatic differences in energy density between fuels. One gallon of gasoline holds about 132,000 BTU (139 MJ), while a standard AA battery stores only about 9,000 joules (0.0025 kWh). A therm of natural gas packs 105.5 MJ — roughly the same as running a 100-watt light bulb for 293 hours. Understanding these conversions is essential for comparing heating costs, evaluating renewable energy systems, or analyzing transportation efficiency.
The Calorie Confusion
The most common source of confusion is the calorie vs. Calorie distinction. The small calorie (cal) is a laboratory unit. The food Calorie (always capitalised on US nutrition labels) is actually a kilocalorie — 1,000 times larger. A 200-Calorie granola bar contains 200,000 small calories, or 836,800 joules. When comparing food energy to electrical energy, note that a 200-Calorie snack represents about 0.23 kWh — roughly enough to run a laptop for 2–3 hours.
Frequently Asked Questions
What is the difference between a calorie and a Calorie?
A lowercase calorie (cal) is the energy needed to raise 1 gram of water by 1 °C, equal to 4.184 joules. A food Calorie (with a capital C) is actually a kilocalorie — 1,000 small calories or 4,184 joules. Nutrition labels always use the larger unit.
Why is electricity billed in kilowatt-hours (kWh)?
A watt measures power (joules per second). Multiplying power by time gives energy in joules — but joules are tiny. A kilowatt-hour equals 3,600,000 joules and keeps billing numbers in a human-readable range (tens to hundreds per month).
What is a therm of natural gas?
A therm equals exactly 100,000 BTU or approximately 105.5 MJ. It is the standard billing unit for residential natural gas in the US. One therm is roughly equivalent to burning about 100 cubic feet of natural gas.
How many joules are in a kilowatt-hour?
Exactly 3,600,000 joules (3.6 MJ). The calculation: 1 kilowatt = 1,000 joules/second × 3,600 seconds/hour = 3,600,000 J.
What is an electronvolt used for?
An electronvolt (eV) equals 1.602 × 10⁻¹⁹ joules — extremely small, making it ideal for describing atomic and subatomic energies. It is used in particle physics, semiconductor band-gap energies, and chemical bond energies.
How much energy is in a gallon of gasoline in kWh?
One US gallon of regular gasoline contains approximately 132,000 BTU, which equals about 38.6 kWh of chemical energy. For comparison, a Tesla Model 3 long-range battery holds about 82 kWh — equivalent to roughly 2.1 gallons of gasoline in raw energy content.