Specific energy consumption

The electrical energy used per unit of hydrogen produced, usually kWh per kg.

The headline efficiency measure for an electrolyser plant, and the number most performance guarantees are written around. It is also routinely quoted at three different boundaries: direct current into the stack, alternating current into the electrolyser package, or alternating current at the plant boundary including all utilities.

Those three differ by several kilowatt hours per kilogram, and comparing a figure at one boundary against a figure at another is the most common source of apparent underperformance.

Specific energy is not constant across load. Cell voltage generally falls at lower current density, which improves the stack figure, while rectifier efficiency falls and fixed auxiliary loads take a larger share, which worsens the system figure. The result is typically a shallow curve with an optimum somewhere in the middle of the operating range.