raise the temperature of one gram of water by one degree Celsius (~4.184 J). Erg (erg) – unit of energy and mechanical work in the centimetre-gram-second (CGS) system of units (10−7 J). Imperial or US Customary units British thermal unit (BTU) – equal to the energy need to raise the temperature of one pound of water by one degree Fahrenheit (~1055 J). Therm (thm) – unit of heat energy. In the US gas industry it is defined as exactly 100,000 BTU59 °F. It is approximately the heat equivalent of burning of natural gas (~105.5 MJ). Quad – unit of energy equal to 1015 (a short-scale quadrillion) BTU. Foot-pound (ft·lbf or ft·lbf) – unit of mechanical work, or energy, although in scientific fields one commonly uses joule (~1.356 J). Other units Electronvolt (eV) – the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt (~1.60 × 10−19 J). Planck energy () – natural unit of energy common in particle physics (~). Barrel of oil equivalent (BOE) – energy unit equal to the energy released when burning one barrel () of oil (~6.12 GJ). Tonne of oil equivalent (toe) – energy unit equal to the energy released when burning one tonne of oil (~42 GJ). Related units and concepts Volt Ampere Coulomb Enthalpy EU energy label Fill factor – defined as the ratio of the maximum power (Vmp x Jmp) divided by the short-circuit current (Isc) and open-circuit voltage (Voc) in light current density – voltage (J-V) characteristics of solar cells. Gigaton – Metric Unit of mass, equal to 1,000,000,000 (1 billion) metric tons, 1,000,000,000,000 (1 trillion) kilograms Any of various units of energy, such as gigatons of TNT equivalent, gigatons of coal equivalent, gigatons petroleum equivalent. Gray (unit) – (symbol: Gy), is the SI unit of energy for the absorbed dose of radiation. One gray is the absorption of one joule of radiation energy by one kilogram of matter. One gray equals 100 rad, an older unit. Heat Mass-energy equivalence – where mass has an energy equivalence, and energy has a mass equivalence Megawatt Net energy gain Power factor – of an AC electric power system is defined as the ratio of the real power to the apparent power. Energy industry Energy industry Worldwide energy supply, outline by country/region World energy resources and consumption List of energy resources, substances like