A joule is an international unit of energy defined as the energy produced from one watt flowing for one second. A very small unit of energy, there are 3-6-mil joules in a kilowatt-hour. For gas, one gigajoule = 0. 96 Mcf under standard temperature and pressure conditions. Roughly, one gigajoule (Gj) = 1 Mcf; one petajoule (Pj) = 1 Bcf; one exajoule (Ej) = 1 Tcf.