Understanding different energy units and when to use them:
Joule (J) - SI Base Unit
The standard unit of energy in the International System of Units (SI). Named after James Prescott Joule. 1 J = 1 kg⋅m²/s². Used in physics, engineering, and science.
Calorie (cal) - Thermal Energy
Originally defined as the energy needed to raise 1 gram of water by 1°C. The "Calorie" (capital C) in nutrition is actually 1 kilocalorie (kcal). 1 cal = 4.184 J.
Kilowatt-hour (kWh) - Electrical Energy
The standard unit for measuring electrical energy consumption. 1 kWh is the energy consumed by a 1,000-watt appliance running for 1 hour. 1 kWh = 3.6 MJ.
British Thermal Unit (BTU) - HVAC
Commonly used in heating, ventilation, and air conditioning (HVAC) systems, especially in the United States. 1 BTU = 1,055.06 J.
Electronvolt (eV) - Particle Physics
A very small unit used in atomic and nuclear physics. 1 eV = 1.602 × 10⁻¹⁹ J. For reference, visible light photons have energies of 1.8-3.1 eV.