Energy It is equal to the energy expended (or work done) in applying a force of one newton through a distance of one metre. However energy is also expressed in many other units not part of the SI, such as ergs, calories, British Thermal Units, kilowatt-hours and kilocalories, which require a conversion factor when expressed in SI units. The SI unit of energy rate (energy per unit time) is the watt, which is a joule per second. Thus, one joule is one watt-second, and 3600 joules equal one watt-hour. The CGS energy unit is the erg and the imperial and US
Rad (unit) in the latest NIST definition. Even where the standard medium is fully defined, the ionizing energy is often not precisely known. In 1940, British physicist Louis Harold Gray, who had been studying the effect of neutron damage on human tissue, together with William Valentine Mayneord and John Read published a paper in which a unit of measure, dubbed the ""gram roentgen"" (symbol: gr) defined as "that amount of neutron radiation which produces an increment in energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one roentgen of radiation" was proposed.