Electrolysis Energy Calculator
Calculate energy required for electrolysis, Faradaic efficiency, and electricity cost.
Why Real Cells Need More Than the Theoretical Voltage
Thermodynamics sets a minimum voltage below which electrolysis simply cannot proceed. For water splitting that is 1.23 V at standard conditions. Yet no practical electrolyser runs at 1.23 V — commercial units operate between 1.8 and 2.2 V, and the difference is entirely wasted as heat.
The excess is called overpotential, and it comes from several independent sources that add together:
| Source | Cause | How it is reduced |
|---|---|---|
| Activation overpotential | Slow electron transfer kinetics at the electrode | Better catalysts — platinum, iridium oxide |
| Ohmic loss | Resistance of electrolyte, membrane and wiring | Higher conductivity, thinner membranes, shorter gaps |
| Concentration overpotential | Reactant depletion at the electrode surface | Stirring, flow, gas removal |
| Bubble resistance | Gas bubbles blocking active electrode area | Electrode design, forced circulation |
Activation overpotential is usually the largest term, which is why catalyst development dominates electrolyser research. Oxygen evolution at the anode is particularly sluggish because it requires four electrons and the formation of an O–O bond — it accounts for most of the loss in a water electrolyser.
Energy Cost of Hydrogen
Because energy scales directly with applied voltage, efficiency translates immediately into cost. One kilogram of hydrogen is 496 mol, requiring 2 × 496 × 96,485 C of charge — about 26.6 kAh. Multiplying that charge by the cell voltage gives the electrical energy, so voltage alone determines consumption per kilogram.
| Cell voltage | Reference point | kWh per kg H2 | Cost at $0.05/kWh |
|---|---|---|---|
| 1.23 V | Reversible minimum (ΔG) | 32.7 | $1.64 |
| 1.48 V | Thermoneutral (ΔH) | 39.4 | $1.97 |
| 1.80 V | Typical alkaline cell | 47.9 | $2.39 |
| 2.05 V | Higher current density | 54.5 | $2.73 |
| 2.46 V | Poor efficiency | 65.4 | $3.27 |
This is why electricity price and cell efficiency dominate green hydrogen economics far more than capital cost does. A few hundred millivolts of overpotential translates directly into dollars per kilogram.
Worked Examples
Common Mistakes
It is a thermodynamic floor, not an operating point. Every real cell needs overpotential to drive current at a useful rate, and running closer to 1.23 V means vanishingly small production.
Voltage efficiency compares theoretical to applied voltage. Faradaic efficiency asks what fraction of the current produced the desired product rather than side reactions. Both are needed for total energy efficiency.
Energy in kilowatt-hours requires time in hours. Using seconds inflates the figure by 3,600 times — a common slip when the current is given per second.
Pushing more current through the same electrode area increases every overpotential term. Running faster is always less efficient per unit of product, which is the central design trade-off.
Frequently Asked Questions
Formula Explorer connections
Interpretation: This formula links electron transfer, charge, potential, current or ionic transport in an electrochemical system. Assumption: Balance electron count and half-reactions, preserve sign conventions, and use consistent concentration, temperature and electrical units. Real cells include losses and overpotential.