A system for reversibly storing electrical energy as thermal energy comprising A reversible subcritical vapour-liquid cycle having a working fluid flow path comprising: a hot storage fluid heat exchanger configured and arranged to exchange thermal energy between the working fluid, as it changes phase, and a hot storage fluid; a vapour pressure changing apparatus, configured to change the pressure of the working fluid in a vapour phase and arranged fluidly adjacent the hot storage fluid heat exchanger; a cold storage fluid heat exchanger,fluidly adjacent the vapour pressure changing apparatus configured and arranged to exchange thermal energy between the working fluid, as it changes phase, and a cold storage fluid; a liquid pressure changing apparatus (36,38), fluidly adjacent the cold storage fluid heat exchanger, configured to change the pressure of a liquid phase of the working fluid; an inter storage heat exchanger located fluidly between the pressure changing apparatus (36,38) and the hot storage fluid heat exchanger, configured and arranged to exchange sensible heat of the working fluid with a storage fluid, the system further comprising: a hot storage fluid tank for storing the hot storage fluid a hot storage fluid flow path passing through the hot storage fluid heat exchanger and the hot storage fluid tank; a cold storage fluid tank for storing the cold storage fluid; a cold storage fluid flow path passing through the cold storage fluid heat exchanger and the cold storage fluid tank, characterised by an inter storage tank flow path fluidly connecting the hot storage fluid tank to the cold storage fluid tank via the inter storage heat exchanger. a hot storage fluid heat exchanger configured and arranged to exchange thermal energy between the working fluid, as it changes phase, and a hot storage fluid; a vapour pressure changing apparatus, configured to change the pressure of the working fluid in a vapour phase and arranged fluidly adjacent the hot storage fluid heat exchanger; a cold storage fluid heat exchanger,fluidly adjacent the vapour pressure changing apparatus configured and arranged to exchange thermal energy between the working fluid, as it changes phase, and a cold storage fluid; a liquid pressure changing apparatus (36,38), fluidly adjacent the cold storage fluid heat exchanger, configured to change the pressure of a liquid phase of the working fluid; an inter storage heat exchanger located fluidly between the pressure changing apparatus (36,38) and the hot storage fluid heat exchanger, configured and arranged to exchange sensible heat of the working fluid with a storage fluid, the system further comprising: a hot storage fluid tank for storing the hot storage fluid a hot storage fluid flow path passing through the hot storage fluid heat exchanger and the hot storage fluid tank; a cold storage fluid tank for storing the cold storage fluid; a cold storage fluid flow path passing through the cold storage fluid heat exchanger and the cold storage fluid tank, characterised by an inter storage tank flow path fluidly connecting the hot storage fluid tank to the cold storage fluid tank via the inter storage heat exchanger. a hot storage fluid tank for storing the hot storage fluid a hot storage fluid flow path passing through the hot storage fluid heat exchanger and the hot storage fluid tank; a cold storage fluid tank for storing the cold storage fluid; a cold storage fluid flow path passing through the cold storage fluid heat exchanger and the cold storage fluid tank, characterised by an inter storage tank flow path fluidly connecting the hot storage fluid tank to the cold storage fluid tank via the inter storage heat exchanger.