How to compare efficiency claims during electrolyzer evaluation

Electrolyzer efficiency claims become useful for equipment selection only after they describe the same energy inputs, hydrogen output, and operating conditions. A supplier quoting a higher percentage may still require more electricity to deliver usable hydrogen at your project’s specified pressure and purity. Begin the evaluation with a common measure: electrical energy consumed per kilogram of hydrogen delivered across an agreed system boundary.

For a continuously operated plant, that comparison may center on performance near its intended operating load. For a plant following variable renewable generation, it must also account for partial load, standby consumption, startup losses, and hydrogen rejected during transitions. The strongest proposal is the one supported by credible performance data across the operating conditions your project will actually impose.

Convert headline percentages before comparing equipment. Hydrogen efficiency percentages depend on whether the calculation uses the lower heating value (LHV) or higher heating value (HHV). These represent different treatments of the energy associated with water vapor condensation. An efficiency percentage without its heating-value basis leaves a material ambiguity.

Using rounded hydrogen heating values of approximately 33.3 kWh/kg on an LHV basis and 39.4 kWh/kg on an HHV basis, electrical efficiency can be expressed as:

Electrical efficiency (%) = hydrogen heating value (kWh/kg) ÷ specific electricity consumption (kWh/kg) × 100.

For an illustrative electrolyzer consuming 50 kWh/kg, the result is approximately 67% LHV efficiency or 79% HHV efficiency. Both percentages describe the same electricity consumption. Comparing one supplier’s HHV figure with another supplier’s LHV figure would create an apparent performance difference that comes entirely from accounting.

Convert each claim into kWh/kg H₂ using its stated basis, then retain the original percentage and calculation in the evaluation record. This makes the comparison traceable and exposes inconsistencies between a supplier’s percentage, power rating, and hydrogen production rate. If those numbers do not reconcile, request the underlying definitions before using the claim in a financial model.

Check the hydrogen denominator as carefully as the energy numerator. Production expressed in normal cubic meters requires a declared reference temperature and pressure before conversion to mass. A flow measurement also needs a clear treatment of moisture and gas composition. Gross hydrogen generated inside the stack can exceed the dry, specification-compliant hydrogen available at the delivery point after purging, crossover-related losses, and processing.

For high-temperature electrolysis, electricity consumption alone requires additional interpretation because externally supplied heat can contribute to the process. Request electrical and thermal inputs separately, including heat temperature, supply conditions, and the boundary used to account for steam generation. A low electrical kWh/kg figure does not establish a lower total energy requirement or operating cost when the required heat supply differs between proposals.

Draw the comparison boundary around the hydrogen your plant needs. Stack efficiency describes only part of the installation. Depending on the supplier’s definition, a stack figure may use DC electricity at the stack terminals and exclude power conversion, cooling, water circulation, gas processing, controls, and other auxiliaries. A project purchasing AC electricity needs visibility into those additional loads.

Ask each bidder to provide an energy balance that identifies the electrical metering point and every included subsystem. “System efficiency” is insufficient as a definition: one package may include drying and cooling equipment, while another assigns them to the customer’s balance of plant. Even identical package consumption figures can therefore represent different project energy requirements.

A practical comparison boundary runs from the agreed electrical supply point to dry hydrogen meeting the specified delivery pressure and purity. Where equipment sits outside the supplier’s package, add its consumption consistently. Record assumptions for external cooling, water treatment, and compression so that differences remain visible rather than disappearing inside a single adjusted number.

Pressure deserves particular attention. A proposal producing hydrogen at a lower pressure may require additional downstream compression. Another may deliver at a higher pressure while consuming more electricity within the electrolysis package. Neither figure alone determines which arrangement uses less energy at the required delivery point. Compare the complete pressure path, including intermediate treatment and compression operating conditions.

Consider a deliberately simplified example. Supplier A declares 49 kWh/kg and Supplier B declares 52 kWh/kg. If A’s boundary excludes equipment consuming 5 kWh per kilogram delivered, while B’s figure already includes equivalent services, their adjusted values become 54 and 52 kWh/kg. This illustrative reversal shows why an apparently precise stack comparison can support the wrong purchasing decision.

Such adjustments need supporting engineering data. Compressor consumption depends on inlet and outlet conditions, equipment configuration, and utilization; cooling demand depends on the heat rejection arrangement and ambient conditions. Applying one generic auxiliary allowance to every proposal can hide meaningful design differences. Use a common calculation method, with proposal-specific inputs where the equipment actually differs.

Define how shared utility consumption will be allocated. A cooling system serving several electrolyzer trains may have fixed and variable loads, so its consumption per kilogram can change as trains enter or leave service. Report package performance and the adjusted project figure separately, preserving a clear route from the supplier’s measurement to the evaluator’s comparison.

Replace the single rated point with the expected operating profile. A rated efficiency claim normally refers to a particular combination of load, temperature, pressure, and equipment condition. It cannot establish annual consumption for a plant that spends substantial time elsewhere. Request a performance map covering the expected operating range, with hydrogen output and total electrical input reported together at each point.

Partial-load behavior requires system-level evidence. Reduced stack current can improve electrochemical efficiency over part of the operating range, while fixed auxiliary loads become larger relative to hydrogen output. The combined effect can differ from the stack trend. Minimum stable load, gas quality restrictions, and the way multiple stacks are dispatched can also change usable production and energy consumption.

Ask suppliers to state the conditions behind each performance point, especially:

• The load definition, including whether it refers to stack current, electrical input, or hydrogen production.
• Operating temperature, hydrogen outlet pressure, cooling conditions, and relevant ambient assumptions.
• Whether measurements represent stabilized operation or include transitions.
• Whether auxiliary consumption changes with load or has been treated as a constant allowance.
• The equipment configuration, including the number of active stacks and shared utility loads.

Then apply those data to a representative dispatch profile. Calculate annual specific consumption by dividing total modeled electricity use by total modeled kilograms of accepted hydrogen. Include energy consumed during periods with no saleable output. A simple average of efficiency percentages, or even of kWh/kg values at different loads, can misrepresent the result because each operating period contributes a different quantity of hydrogen.

Startup and standby definitions can materially affect that calculation. Establish whether “ready” means the equipment is cold, warm, pressurized, or already maintaining process conditions. Obtain the energy requirement and time to produce hydrogen that meets the delivery specification. A short startup time has limited value in an energy model if the associated consumption and off-specification production remain unspecified.

Use consistent external conditions across bids. If site cooling temperatures or seasonal ambient conditions differ from the supplier’s reference case, request supported corrections or additional performance data. Avoid extrapolating a favorable rated point across a year when the equipment’s thermal limits or utility requirements could change its operating behavior.

Separate initial performance from performance you can rely on over time. Beginning-of-life efficiency is useful for acceptance testing, but operating costs accumulate as the equipment ages. Increasing cell voltage can raise electricity consumption for a given hydrogen output. Changes in auxiliary performance, maintenance condition, and operating strategy can also affect the delivered result.

Request a clear degradation definition: the measured parameter, its units, the reference operating conditions, and whether it applies to the stack or complete system. A voltage increase per operating hour cannot be inserted directly into a project kWh/kg forecast without relating it to current, hydrogen output, and the relevant operating profile. Steady-operation evidence also needs careful interpretation for a project expecting frequent starts or extended standby periods.

Compare beginning-of-life, expected operating-period, and end-of-life consumption on a consistent basis. Where suppliers provide only limited aging evidence, carry that uncertainty into a sensitivity analysis. Keep stack replacement timing visible because it changes both the energy trajectory and the cost model. A favorable initial number can lose commercial significance if its advantage disappears early in the planned operating period.

The evidence supporting a claim should match the strength of the conclusion drawn from it. Measured results from the offered configuration provide a different level of confidence from a simulation or an extrapolation from a smaller unit. Request the test report, equipment identification, measurement locations, operating conditions, and calculation method. Independent witnessing adds confidence, but its scope still needs to cover the boundary and conditions relevant to the procurement.

Measurement uncertainty matters when competing claims are close. Electrical metering and hydrogen mass determination both contribute to uncertainty in kWh/kg. Check instrument calibration, sampling intervals, gas conditioning, and whether the test period was long enough to represent stable operation. A small declared advantage may be inconclusive if it is comparable to the combined uncertainty of the measurements.

Turn the evaluation basis into an acceptance requirement before contract award. Specify the electrical boundary, hydrogen delivery conditions, agreed test loads, stabilization criteria, test duration, treatment of interruptions, and permitted corrections. Define how compliance will be judged, including measurement uncertainty and any repeat-test provisions. A guarantee expressed only as “up to” an efficiency percentage provides little support for a firm operating-cost assumption.

Finally, translate the normalized difference into a project consequence. Under a simplified constant electricity-price assumption, every 1 kWh/kg difference adds electricity cost per kilogram equal to the price of 1 kWh. For variable electricity prices, calculate consumption and production by time period; an annual average price can obscure the effect of dispatch decisions.

Use that result to judge whether an efficiency advantage is large enough, well supported enough, and relevant enough to influence selection. A technically defensible recommendation can state the expected annual energy demand, the conditions under which one proposal performs better, and the uncertainties that could change the ranking. That gives the project team a usable decision basis—and gives suppliers a precise performance obligation to address.