The joule (J) is the SI base unit of energy, defined as the work done when a force of one newton moves an object one meter. It is the universal unit in physics for measuring kinetic energy, heat, and electrical energy, appearing in everything from textbook problems to nutritional science. One joule equals the energy released by dropping a small apple one meter, making it tangible despite its small magnitude.

The kilowatt-hour (kWh) equals 3,600,000 joules and is the standard billing unit for residential and commercial electricity consumption worldwide. Utility companies charge by the kilowatt-hour, and understanding this unit helps consumers evaluate appliance running costs and solar panel output. One kilowatt-hour represents the energy used by a 1,000-watt appliance running for one hour.