Energy Converter — Convert Joules, Calories, kWh, BTU

How Energy Conversion Works

Energy is the capacity to do work or produce heat. The SI unit is the joule (J), named after physicist James Prescott Joule. One joule is approximately the energy needed to lift a 102-gram object (about the weight of a large egg) by one meter. Because the joule is small for most real-world applications, practical energy measurement uses kilojoules (kJ), megajoules (MJ), kilowatt-hours (kWh), and other larger units.

The calorie creates significant confusion because two different units share the name. The small calorie (cal) is the heat needed to raise 1 gram of water by 1°C, equal to 4.184 joules. The food calorie (Cal, or kcal — kilocalorie) is 1,000 small calories, or 4,184 joules. Nutrition labels in the United States list "Calories" (with a capital C) — these are kilocalories. European labels often show both kilocalories and kilojoules. A 200 Calorie food item contains 200 kcal = 836,800 joules.

Electricity is measured and sold in kilowatt-hours (kWh). One kWh equals the energy consumed by a 1,000-watt device running for one hour, which equals 3.6 megajoules. Your electricity bill is denominated in kWh. A kilowatt-hour also equals 3,412 BTU. The BTU (British Thermal Unit) is used extensively in the United States for air conditioner output, furnace capacity, and natural gas billing — 1 BTU is the energy needed to raise 1 pound of water by 1°F.

Energy conversions matter when comparing food labels from different countries, understanding electricity bills, sizing HVAC equipment (air conditioners are rated in BTU/hour), evaluating solar panel output, calculating fuel costs across different fuel types, and comparing the energy density of batteries, gasoline, and other energy storage systems.

Key Facts About Energy

Human body power: A resting human body burns roughly 80 watts of power (basal metabolic rate), consuming approximately 1,920 watt-hours (1.92 kWh) per day — similar to leaving a 80-watt light bulb on continuously.
Gasoline energy density: One liter of gasoline contains about 34 megajoules (MJ) of chemical energy. An internal combustion engine converts only 20–30% of that into motion; an electric motor converts about 90%, which is why EVs use far less energy per kilometer.
Nuclear energy scale: The bomb dropped on Hiroshima released approximately 63 terajoules (TJ) of energy — equivalent to about 15 kilotons of TNT, or roughly 17.5 million kWh, enough to power 1,500 average US homes for a year.
AA battery: A single AA battery stores about 3–4 watt-hours (10,800–14,400 joules). At 100% theoretical efficiency, that energy could lift a 1 kg weight to over 1,000 meters altitude.

Frequently Asked Questions

How many joules are in a calorie?

One calorie (small calorie, cal) equals exactly 4.184 joules. Note: the "Calorie" used in food nutrition (with a capital C) is actually a kilocalorie (kcal), which equals 4,184 joules.

How do I convert kilowatt-hours to joules?

To convert kilowatt-hours (kWh) to joules, multiply by 3,600,000. For example, 1 kWh = 3,600,000 joules = 3.6 megajoules. This is because 1 watt = 1 joule/second, and 1 hour = 3,600 seconds.

What is a BTU?

A BTU (British Thermal Unit) is the amount of heat needed to raise the temperature of 1 pound of water by 1°F. 1 BTU equals approximately 1,055 joules or 0.293 watt-hours. BTUs are commonly used for air conditioning and heating ratings.

What is the difference between a calorie and a kilocalorie?

A kilocalorie (kcal) equals 1,000 small calories (cal). Food energy is measured in kilocalories, but they are often labeled as "Calories" (capital C) on nutrition labels. So "200 Calories" on a food label means 200 kcal or 200,000 cal.

How many watt-hours are in a kilowatt-hour?

1 kilowatt-hour (kWh) equals exactly 1,000 watt-hours (Wh). Your electricity bill is measured in kWh — one kWh is the energy used by a 1,000-watt appliance running for 1 hour.