A cryogenic energy storage system comprising a liquefaction apparatus for liquefying a gas to form a cryogen, wherein the liquefaction apparatus is controllable to draw power from an external power source to liquefy the gas, a cryogenic storage tank in fluid communication with the liquefaction apparatus for storing cryogen produced by the liquefaction apparatus, a power recovery apparatus in fluid communication with the cryogenic storage tank for recovering power from cryogen from the cryogenic storage tank by heating the cryogen to form a gas and expanding said gas, a hot thermal store for storing hot thermal energy, wherein the hot thermal store and the power recovery apparatus are arranged so that hot thermal energy from the hot thermal store can be transferred to the gas before and/or during expansion in the power recovery apparatus, and a charging apparatus which is controllable to draw power from the external power source when the power drawn by the liquefaction apparatus is below a threshold value, and supply the cryogenic energy storage system with thermal energy.