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 BTU (British Thermal Unit) equals approximately 1,055.06 joules, defined as the heat needed to raise one pound of water by one degree Fahrenheit. It is the primary unit for rating heating and cooling equipment in the United States, appearing on air conditioner specifications, furnace ratings, and energy efficiency labels. One BTU equals about 0.2931 watt-hours or roughly 0.252 kilocalories.