Efficiency and gas purity

The efficiency of an electrolyser is calculated as the ratio between the energy content of produced hydrogen and the electric energy consumed for electrolysis but also to run all the required auxiliary equipments (pumps, gas separator,…). 

η = mH2 * LHVH2 / Eelec

Note that the appropriate energy content to consider is the lower heating value (LHV) of hydrogen, while some rare manufacturers still state efficiencies based on higher heating value (HHV) to display more advantageous numbers. 

Industrial size alkaline electrolysers have an efficiency of 65+% (up to 67%) and produce hydrogen with a gas purity of 99.5+%vol (up to 99.9%vol). They generally operate between 60 and 90°C with a current density of 0.2 to 0.4 A/cm2, leading to a stack lifetime of 60.000 to 90.000 h . Note that an overhaul to replace the diaphragms and to reactivate the electrodes can be an alternative to replacing all the equipment lowering the environmental burden (see Life Cycle Analysis). 

Beyond hydrogen, the electrolyser also produces oxygen with a purity up to 99.7 %vol without an auxiliary purification equipment. This oxygen can either be sold or vented to the atmosphere. Given the relatively low commercial value of oxygen compared to hydrogen, it is generally only sold if there is a local consumption or a pre-existing oxygen dispensing infrastructure.