What the evidence says
A 500 kWh nameplate does not promise 500 kWh at the AC load. The permitted state-of-charge window can exclude part of the inventory, while the PCS, transformer, cables, cooling and controls consume or lose another portion.
End-of-life capacity belongs in the commercial definition. A warranty may use retained capacity, throughput, cycles, operating temperature, availability or exclusions that are not equivalent to a guaranteed delivered-energy result.
Power and energy are separate boundaries. A battery may contain enough energy for two hours but still fail the duty because its PCS, cell current, thermal controls or low-SOC voltage cannot support the required kW.
Peak shaving, backup and energy arbitrage use capacity differently. One combined spreadsheet can hide conflicting reserve requirements unless each operating objective has a defined priority and dispatch rule.
- 01NOMINAL DC
- 02SOC WINDOW
- 03AGEING
- 04CONVERSION
- 05AUXILIARIES
- 06USABLE AC
Practical decision guide
A capacity calculation a buyer can challenge
A battery quotation often begins with nominal DC capacity. The facility buys a different outcome: AC energy delivered to defined loads, at a specified power, under agreed conditions. Write that boundary into the specification. A capacity measured at the cell terminals is not directly comparable with energy measured after the inverter and site auxiliaries. Likewise, a beginning-of-life result cannot be compared with an end-of-warranty commitment without accounting for the capacity-retention assumption.
For an explicitly illustrative calculation, take 500 kWh nominal DC capacity, an 80% operating SOC window and 94% discharge-path efficiency. This gives 376 kWh AC before auxiliaries and any additional reserve. If 80% capacity retention is assumed for a future assessment point, the same calculation gives 300.8 kWh. At a constant 100 kW load, the energy-only durations are 3.76 and 3.008 hours respectively. None of these percentages is a manufacturer promise or a measured project result; replacing them with warranted, testable values is the purpose of the assessment.
Do not count the same loss twice. Some warranties already state usable capacity within a specified SOC range, while some efficiency figures include auxiliaries and others do not. Round-trip efficiency includes charging as well as discharging and should not automatically replace the discharge efficiency in an autonomy calculation. Cooling demand also changes with ambient conditions and operation. Ask for a transparent loss ledger showing which terms are included in each supplier figure and where the acceptance meter will sit.
Finally, test the power limit separately. Enough kilowatt-hours do not guarantee that a PCS can start a compressor or support an unbalanced feeder. A low-voltage module may hit its protection threshold before the aggregate SOC estimate reaches the intended endpoint. Review module data, thermal limits and current limits alongside the energy calculation. The resulting decision should state both the supported operating envelope and the events that can reduce available service. That is more useful than a single headline autonomy number.
Victron battery compatibility guidance (model-specific constraints)
Discuss the engineering scope →What a business should do next
- Write a project-specific equation from nominal DC energy to guaranteed usable AC energy at commissioning and at the agreed end-of-life condition.
- State temperature, auxiliary consumption, SOC window, C-rate, power factor, rest periods and reserve assumptions beside every modeled result.
- Use interval load data and simulate the control priority, not only the annual bill or an average daily-energy number.
- Commission with a witnessed capacity and power test, recording meter boundaries, calibration, starting state, ambient conditions and pass/fail criteria.