Energy storage

Storing recoverable energy for later use. Energy capacity, maximum power, duration, round-trip efficiency and degradation are independent dimensions of performance; a high specific energy need not imply high power or efficiency.

The ratio of useful energy recovered on discharge to the energy supplied during charging for a complete storage cycle. Generation efficiency alone excludes the preceding charging losses.

Storage of hydrogen as a chemical-energy carrier. Comparing electricity-storage systems requires including production, containment and reconversion; the fuel heating value alone does not specify electrical round-trip efficiency.

Water is pumped to an upper reservoir and later discharged through a turbine. For approximately constant head, useful discharge energy is $\eta \rho Vgh$ while power is $\eta \rho Qgh$; reservoir volume and flow rating set different properties.