The term ‘energy storage’ refers to a diverse group of technologies that work according to different principles: mechanical (e.g. pumped hydro storage, flywheels, compressed air energy storage), chemical (e.g. power-to-gas), electrochemical (e.g. batteries), thermal (e.g. hot water storage), and electrical (e.g. supercapacitors). Some provide very fast responding, short-duration balancing power (such as flywheels or supercapacitors) while other technologies provide longer duration storage for balancing at a timescale of hours, days or even for seasons (for instance, pumped hydro or hydrogen storage).