Abstract: In A method of energy storage comprises receiving input energy (1) and using the input energy to compress (2) air or other process gas to produce a compressed process gas. The compressed process gas is stored (8). The compressed process gas is expanded (16) to generate output energy (17). Heat is transferred (5) from the process gas, before the process gas is stored (8) as a compressed process gas, to a hydrogen production process (10). The transferred heat is used in the hydrogen production process (10). The hydrogen may be stored (13) and subsequently used to heat to provide heat prior to, during, or after expanding (16) the compressed gas.