Multiply Efficiency Factors

Example: if the efficiency factor of a fuel cell is 60 percent, the same for power to gas, then the energy storage with hydrogen using those two methods has an efficiency factor of 36 percent. The energy loss then is 64 percent.

Efficiency factor is a term for how much of the energy used does exactly what you want it to. Energy is lost in every process, for example through waste heat, sound waves or other things; this loss is unavoidable. A machine without such a loss would be a perpetual motion machine of the first kind, which cannot exist; it is physically prevented by the law of conservation of energy (first law of thermodynamics). The efficiency is therefore always less than 100 percent. Often the aim is to achieve the highest possible efficiency, i.e. the lowest loss. However, this is sometimes technically complex and not always economically viable. In practice, therefore, efficiencies often result from a compromise between what is technically feasible and what is economically worthwhile.

Sometimes you come across efficiency factors of over 100 percent. This is possible when the energy is used to extract energy from the environment, as a heat pump does. Since the energy (temperature) of the environment drops more than the energy is made available, this is not a violation of physics.