Drop a small apple from waist height. It hits the floor with roughly one joule of kinetic energy. The joule is the standard unit of energy in the International System, named after James Prescott Joule, a British physicist who spent decades measuring heat and mechanical work.

One joule equals the energy needed to push with one newton of force over one meter. It also equals one watt of power for one second. These definitions connect mechanics and electricity, which is why the joule is so useful. A 60-watt bulb left on for one second uses 60 joules.

Joules are small. A typical human eats about 8 million joules a day. A lightning bolt carries around a billion. Because the numbers get unwieldy, engineers often use kilojoules, megajoules, or kilowatt-hours. The joule remains the base unit underneath all of them.

Common conversions

• 1 kilojoule = 1,000 joules
• 1 megajoule = 1 million joules
• 1 kilowatt-hour = 3.6 million joules
• 1 calorie = 4.184 joules
• 1 BTU = about 1,055 joules