Two battery suppliers can quote different round-trip efficiency figures and both be describing valid measurements. One may measure the battery’s DC terminals; another may measure the complete system’s AC exchange. If cooling, standby power or transformer losses sit outside one boundary, the percentages cannot be compared directly.
For a factory owner, the relevant question is the energy and cost at the agreed point of connection under the intended duty. Procurement should turn the headline percentage into a measurement method before the commercial comparison begins.
The 100 kW / 215 kWh BESS application guide separates nameplate energy from delivered energy. Round-trip efficiency adds another boundary: how much metered energy must enter the system to recover a measured discharge under defined conditions?
Write the numerator and denominator in the guarantee
At a consistently defined AC boundary, round-trip efficiency is discharged energy divided by charged energy for the agreed test sequence. Both quantities must use the same units, direction conventions and accounting rules. If auxiliary equipment has a separate supply, the method must explicitly account for it rather than allowing an unmetered source to disappear from the calculation.
The Sandia and PNNL energy-storage performance protocol, Revision 2 from April 2016, is a dated methodological reference for comparable testing. It treats measurement, duty and reporting as a connected process and discusses auxiliaries supplied separately. It should not be described as a newly issued 2026 standard.
| Boundary | What enters the measurement | What may remain outside |
|---|---|---|
| Battery DC terminals | DC charge and discharge at the battery connection | PCS conversion, separate cooling and site transformer |
| PCS AC terminals | Battery and converter energy exchange | Auxiliaries on another supply and downstream transformer |
| Project AC point of connection | Complete agreed storage system exchange | Only items expressly excluded in the contract |
| Whole-site revenue meter | Storage plus concurrent site energy flows | Battery performance cannot be isolated without additional measurements |
A whole-site meter is valuable for verifying a bill, but it cannot automatically identify battery loss when factory loads and solar production change at the same time. Specify a storage feeder meter and the auxiliary connections that make the energy balance auditable.
Start and finish in a comparable battery state
A test that begins with a more highly charged battery and ends at a lower stored-energy state can exaggerate the apparent energy recovered. Matching the declared starting and ending conditions is therefore essential. State-of-charge readings are estimates; the procedure should explain conditioning, rest periods and how the reference state is restored within the equipment’s approved limits.
Record temperature and actual power throughout the sequence. A partial-power cycle in a cool factory may not represent full-power operation at a hot outdoor installation. Equally, a low-power application may spend much longer carrying fixed control and cooling loads. The test condition must fit the decision being made.
Cooling architecture affects the auxiliary boundary. The air- versus liquid-cooled BESS comparison explains why buyers should request auxiliary energy and failure response rather than choose a system from the cooling label alone.
| Recorded data | Unit or format | Question it resolves |
|---|---|---|
| Charging and discharging energy | kWh at named meters | Are both sides measured at the same boundary? |
| Auxiliary energy | kWh by supply and operating state | Was energy imported outside the main meter? |
| Power profile | kW with timestamps | Was the test performed at the contracted duty? |
| Battery reference state | Declared conditioning and SOC method | Was stored energy comparable at each end? |
| Temperature | Ambient and available battery measurements | Can the result be compared with another test? |
| Metering quality | Calibration, resolution and uncertainty | Is a small efficiency difference meaningful? |
A cycle result cannot answer the annual operating question
A battery may complete a short efficiency test and then remain ready for a long interval before its next dispatch. Standby control, cooling, heaters and communications still consume energy. A test percentage covering the cycle alone does not report all the energy required to maintain availability between events.
The DOE FEMP BESS evaluation method, published in January 2024, uses metered charge and discharge time series to assess deployed performance. Its long-term assessment serves a different purpose from a factory acceptance cycle: operating records can show whether the real dispatch and losses match the owner’s application.
Keep two contract lines where necessary. The first is a repeatable acceptance efficiency at stated power, temperature and state limits. The second is a duty-specific energy budget with standby and auxiliary treatment. Do not label an annual energy estimate as a laboratory efficiency measurement.
For peak shaving, the financial effect also depends on when recharge occurs and whether it produces another billed peak. A battery can achieve the promised efficiency while delivering disappointing savings because its dispatch schedule ignores the actual tariff. The energy accounting and the billing model must share timestamps.
Reconcile the model with the tested equipment
The System Advisor Model battery-storage documentation describes voltage, temperature and degradation modelling alongside dispatch options. Such tools help explore operating cases, but the selected loss and replacement inputs still need evidence appropriate to the purchased system.
Ask bidders to provide the source of every efficiency input. A PCS peak efficiency may be valid at one loading condition yet unsuitable as a constant for every hour of battery operation. Apply the same scrutiny to assumed cooling demand, standby periods and transformer losses.
When comparing an option such as the HMX 125 kW / 261 kWh liquid-cooled system, request a written guarantee at the required measurement boundary. The product listing alone does not establish the efficiency available at a particular site’s climate, power profile or AC connection.
What to ask the bidder before comparing percentages
Request a one-line energy-flow diagram with meters and separate auxiliary supplies. Ask for the complete test cycle, the state-restoration rule, temperature range, permitted exclusions and retest method. Include an exportable data file and calculation sheet in the acceptance deliverables.
If two bids use different boundaries, obtain comparable results or explicitly price the unmeasured losses. The stronger offer is the one whose conditions, data and exclusions the owner can verify. No generic efficiency percentage is supplied here because equipment performance must come from a tested configuration and a defined method. Sources reviewed on 10 October 2026.