An electrolyzer uses input electrical power to split water into hydrogen and oxygen: 2H₂O → 2H₂ + O₂. Not all input energy ends up as stored chemical energy — some is lost as heat, set by the electrolyzer efficiency. The hydrogen produced fills a storage tank. A fuel cell then draws hydrogen from the tank and runs the reaction in reverse, 2H₂ + O₂ → 2H₂O, releasing electrical power plus more waste heat — real PEM fuel cells convert only around half the hydrogen's energy content back into electricity.
H2 power in = P_input · η_electrolyzer
round-trip η = η_electrolyzer · η_fuelcell
net power out = draw · η_fuelcell (0 if tank empty)
- Input power — electrical power fed into the electrolyzer; more power splits water faster and fills the tank quicker.
- Electrolyzer efficiency — how much of that input becomes stored hydrogen energy rather than heat; real PEM electrolyzers run roughly 60–85%.
- Fuel cell draw — how hard the fuel cell is drawing on the stored hydrogen; draw stalls once the tank empties.
Real-world relevance: this electrolysis → storage → fuel-cell chain is exactly how green hydrogen, hydrogen fuel-cell vehicles and grid-scale hydrogen energy storage work — and the round-trip efficiency, typically only 35–45%, is why hydrogen storage competes with batteries mainly for long-duration or portable use cases rather than everyday grid buffering.