A home battery's round-trip efficiency is the share of the energy put into the pack that comes back out on discharge. If a datasheet quotes 90% for a 10.24 kWh pack, roughly 9.2 kWh comes back at the same measurement boundary. The catch is the boundary: many quoted figures are DC-to-DC, while a household draws AC through an inverter. Add conversion and standby losses and the energy the customer actually receives is lower.
What round-trip efficiency measures
Round-trip efficiency (RTE) is discharged energy divided by charged energy over a cycle. It is a ratio, not a rate and not a capacity. The same pack can produce different percentages depending on the power level it was cycled at, the temperature of the test, and whether the measurement stopped at the battery terminals or continued through the inverter.
That is why two batteries can carry the same headline percentage and still behave differently in a house. The number is only useful when it travels with its conditions.
DC round-trip, AC round-trip, and the number the customer experiences
The industry uses the same phrase for at least three different measurements. They are not interchangeable:
| Figure | Measured at | Includes | Leaves out |
|---|---|---|---|
| Cell coulombic efficiency | Cell terminals | Charge accepted versus charge returned by the cell | BMS, wiring, inverter, standby |
| DC round-trip efficiency | Battery DC terminals | Cells, internal wiring, BMS electronics (depending on the test) | Inverter conversion, AC wiring, household auxiliaries |
| AC round-trip or system efficiency | AC terminals of the system | The above plus inverter charge and discharge conversion, and standby if the test includes it | Losses beyond the system boundary, such as building wiring |
A DC figure above an AC figure for the same product is not a contradiction. It is the same energy measured at two different points, with two conversion stages added in the AC case.
Where the energy actually goes
Loss mechanisms sit at different places in the chain, and only some of them are covered by a cell-level measurement:
- Cell losses. Coulombic inefficiency and internal resistance turn a small share of each cycle into heat. The effect is continuous and grows at higher currents.
- Battery electronics. Measurement, cell balancing, communication and contactor control all draw power from the pack.
- Conversion, twice. The inverter converts AC to DC when charging and DC to AC when discharging. An AC-coupled retrofit can add further conversion stages — our guide to AC- versus DC-coupled retrofits covers the topology side of that decision.
- Standby and auxiliary loads. Idle electronics, displays, fans and any thermal management run whether or not the pack is cycling.
- Cabling and protection. Conductor resistance and protective devices add a small, fixed share of loss that grows with cable length and current.
Research on installed systems separates these paths rather than reducing them to a single number. HTW Berlin characterised the efficiency of 26 residential photovoltaic battery systems and their individual loss mechanisms in an open-access Journal of Energy Storage paper, and KIT evaluated the resulting electrical and economic losses of PV home storage systems in a separate study. For how these figures are defined and compared across markets, the IEA 4E Technology Collaboration Programme's report on home energy storage efficiency is a useful primary reference.
Why the same battery can be quoted at two percentages
Before comparing two figures, establish four things:
- The boundary. DC terminals or AC terminals?
- The power level. Efficiency at low part-load is not efficiency at rated power.
- The temperature. A figure is attached to a test temperature, commonly 25 °C.
- What is included. Standby consumption and auxiliary loads are sometimes inside the number and sometimes excluded.
A percentage without those conditions cannot be compared with another percentage. Our guide to reading a home battery datasheet covers the rest of the specification page in the same way.
A worked example on a 10.24 kWh pack
The Storage Wall range runs from 2.56 to 14.34 kWh, so the 10.24 kWh model (SEN-51200) is a useful illustration. The arithmetic below is deliberately generic: the percentages are assumptions, not Woolii specifications, and the point is how far the delivered energy moves when the boundary changes.
| Assumed DC round-trip | Delivered at the DC boundary | Delivered as AC after an assumed 96% charge and 96% discharge conversion |
|---|---|---|
| 95% | 9.73 kWh | 8.97 kWh |
| 90% | 9.22 kWh | 8.49 kWh |
| 85% | 8.70 kWh | 8.02 kWh |
| 80% | 8.19 kWh | 7.55 kWh |
The AC column applies the same converter assumption to each row: 0.96 × 0.96 = 0.9216, multiplied by the DC figure. Real inverter efficiency varies with load and model, and the usable depth-of-discharge window can reduce the deliverable figure further. That is why the datasheet's capacity and the datasheet's efficiency answer two different questions.
Standby consumption: small per hour, continuous over a year
Standby is easy to ignore because it is measured in watts rather than kilowatt-hours. It also runs when the battery is idle. For illustration, assume 5 W of continuous standby: over a year that is about 44 kWh (5 W × 8,760 h). Against 8 kWh of daily cycling — roughly 2,920 kWh of annual throughput — it is about 1.5% of the energy moved. Those are assumed figures, not a product claim, but the structure holds: the lighter the cycling, the larger standby looms in the annual result.
Efficiency is not self-consumption
A high round-trip figure does not tell you how much of a household's solar generation is stored rather than exported. Self-consumption depends on the ratio of generation to evening load, the usable capacity, and how the system is dispatched. Our explainer on what home battery storage can save covers the dispatch side; efficiency simply sets the ceiling on what survives the round trip. The two numbers answer different questions and should not be used interchangeably in a customer conversation.
What to ask a supplier before you quote a percentage
- Is the figure DC-to-DC or AC-to-AC, and at which terminals was it measured?
- At what power level and temperature was it measured, and is there a part-load figure?
- Does it include BMS and inverter standby consumption, and any thermal management?
- Is the number a measured test result or a calculated one, and to which method?
- What is the usable capacity for the exact SKU, separate from nameplate kWh?
- What efficiency and capacity retention should the installer expect at the temperature the site actually reaches?
How to check the claim after commissioning
Efficiency can be measured on site with the energy counters already in most hybrid inverters, provided the boundaries are consistent:
- Compare energy in and energy out over the same window, at the same boundary. Do not divide AC output by DC input and call it round-trip efficiency.
- Record the temperature and the average charge and discharge power alongside the result.
- Exclude days with abnormal events — a full backup discharge or a firmware update can distort a short measurement.
- Treat inverter-reported counters as indicative rather than calibrated instruments.
For the relationship between power and energy on the same system, see kW versus kWh on a home battery.
What we publish, and what we will confirm in writing
The Storage Wall series is a wall-mounted LiFePO4 line in four capacities from 2.56 to 14.34 kWh, with 48 V and 51.2 V variants and CAN/RS485 communication for pairing with the hybrid inverters in a project. Where we quote cycle life, we quote its condition with it — for example 6,000 cycles at 80% DoD and 25 °C. Where a figure is SKU-specific, including efficiency, we return it in writing rather than approximate it in a brochure.
Send us the SKU, the destination market and the inverter you intend to pair, and we will return the datasheet with the measurement boundary and test conditions marked, plus the usable capacity for that model. Tell us about the project, or browse the full storage range and the Storage Wall series.