What is the SI unit for energy?

The SI unit for energy is the joule. Its symbol is J, and one joule equals one kilogram metre squared per second squared: 1 J = 1 kg·m²·s⁻².

The joule is also the work done when a force of one newton moves an object one metre in the direction of the force. Because energy, work, and heat are measured in the same unit, the joule is used across mechanics, thermodynamics, electricity, and chemistry. One watt sustained for one second is also one joule, so a 1-watt device uses 1 joule of energy each second.

The unit is named after English physicist James Prescott Joule, whose experiments helped establish the mechanical equivalent of heat and the connection between mechanical work and thermal energy. The joule was officially adopted at the second International Electrical Congress in 1889 and became part of the modern SI system. A frequent mix-up is confusing energy with power: the watt measures the rate of energy transfer, while the joule measures the amount of energy itself.