kWh vs kVA in Energy Storage: Reading a BESS Datasheet Correctly

Short answer. kWh, kW and kVA answer three different questions. kWh is energy — how much work can be delivered over time. kW is real (active) power — the rate at which useful energy flows. kVA is apparent power — defined by the U.S. EIA as the product of volts and amperes in a circuit — and it includes reactive power that does no useful work. Real power equals apparent power only when the power factor is 1, because power factor is the ratio of kW to kVA. A kVA number on a storage datasheet is therefore not, by itself, a rated kW number.

Key terms

Table note: definitions from the U.S. EIA glossary; the var as the unit of reactive power (1 var = 1 V·A) from the BIPM SI Brochure, 9th edition, Table 8.

The relation that follows from these definitions is: kW = kVA × power factor. It can only be applied when the power factor and the conditions under which it holds are stated.

Note on the example: below we use the apparent-power (kVA) figures exactly as printed by the manufacturer and do not recompute them from the datasheet's voltage and current values.

First check the port, then the number

A storage datasheet often lists values for different points in the system: the DC battery side, the low-voltage AC side of the power conversion system, and sometimes the medium-voltage or grid-connection point. Numbers measured at different ports should not be compared in one column. A DC battery capacity is not the energy delivered at the AC terminals, because conversion, auxiliary loads and transformers sit in between. (CNWattHub editorial method.)

Worked example: one real datasheet

We use one manufacturer datasheet as an evidence sample, not as a product recommendation or ranking: Sungrow, ST5015kWh-2500kW-2h / ST5015kWh-1250kW-4h, English, Version 3, page 2 "Technical Data".

Table notes: values as printed on page 2 of the Version 3 datasheet; SHA256 of the PDF e244c8208bedfc5c14191f60f8edd7c608ba83fc7c004aef696a214629d0e8de; re-read 2026-10-19 and independently re-verified 2026-10-23. The document does not state a publication date; "© 2023" and "Version 3" are its only version markers. The datasheet lists DC-side and LV AC-side sections and "Transformerless"; CNWattHub classifies the example as AC-integrated on that basis — this is our interpretation, not a manufacturer label.

Nominal capacity is not usable energy

5015 kWh is a nominal figure listed under the DC side. The datasheet gives no usable DC energy at a stated SOC window, state of health or temperature, and no usable AC energy after losses. It therefore cannot be used to calculate deliverable duration or round-trip efficiency.

To compare two systems, put energy on the same port (for example the LV AC terminal or the grid-connection point) and the same SOC window, SOH, charge/discharge power and temperature. Leave missing conditions as "unknown", not zero.

kVA is not rated kW

The AC power row multiplies out to 2,520 kVA and 1,260 kVA. Those are apparent-power figures. The datasheet gives a power factor of "> 0.99" at nominal power, which is a bound, not a defined test power factor with complete conditions. We therefore leave rated active power as unknown, and we do not treat the "2500kW" or "1250kW" strings in the model names as independently verified ratings.

The −100% to 100% reactive-power range is a reminder of why kVA and kW differ: when a converter is asked to supply reactive power, part of its kVA capacity is used for kvar rather than kW.

Operating range is not full-power range

The datasheet allows operation up to 50 °C and states derating above 45 °C. With no derating curve, output at 46, 48 or 50 °C cannot be inferred. Record the missing items separately: rated-power test temperature, humidity and altitude conditions, temperature measuring point, derating start point, and derated power and duration.

Questions to send to the manufacturer

1. Usable AC energy and test report: which port (LV AC or MV/grid point), SOC window, SOH, temperature, C-rate and end conditions? How does it map to the DC nominal capacity?
2. Rated active power (kW) for each model, with duration, power factor, reactive-power requirement, temperature, voltage and altitude. What equipment does each factor in "210 kVA × 12" represent?
3. A versioned derating curve or point table, especially from above 45 °C to 50 °C, for charge and discharge.
4. Are cooling, control, heating and other auxiliary loads deducted from usable AC energy or efficiency? Where are they supplied from, and how are standby and operation metered?
5. The supply scope of transformers, switchgear and other station equipment, and whether transformer losses are included in any MV-side figure.

Scope limits

One manufacturer, one historical datasheet version. This example does not show current sales configurations, regional certifications, or the equivalence between these Version 3 models and current web listings. It is not a procurement recommendation or a cross-supplier ranking.