Efficiency, or Heat Efficiency, is the ratio of energy output to energy input in a process. One of the most frequently encountered uses of this ratio in the gas industry is in the use of gas for power generation, where the electricity send out is expressed as a percentage of the gas consumed, measured on the basis of a common unit e.g. kWh. Care needs to be taken to distinguish between net and gross efficiency. See also Gross Calorific Value, Net Calorific Value.