<P> Water electrolysis is also used to generate oxygen for the International Space Station . </P> <P> Hydrogen may later be used in a fuel cell as a storage method of energy and water . </P> <P> Efficiency of modern hydrogen generators is measured by energy consumed per standard volume of hydrogen (MJ / m), assuming standard temperature and pressure of the H. The lower the energy used by a generator, the higher would be its efficiency; a 100% - efficient electrolyser would consume 39.4 kilowatt - hours per kilogram (142 MJ / kg) of hydrogen, 12,749 joules per litre (12.75 MJ / m). Practical electrolysis (using a rotating electrolyser at 15 bar pressure) may consume 50 kilowatt - hours per kilogram (180 MJ / kg), and a further 15 kilowatt - hours (54 MJ) if the hydrogen is compressed for use in hydrogen cars . </P> <P> Electrolyser vendors provide efficiencies based on enthalpy . To assess the claimed efficiency of an electrolyser it is important to establish how it was defined by the vendor (i.e. what enthalpy value, what current density, etc .). </P>

In what proportion the two gases are produced during electrolysis of water