Keep the observation year
The IEA reports 108 GW of new battery storage capacity in 2025, 40% more than 2024, with around 80% of additions at utility scale. These are power-capacity additions in a report published in 2026. They are not proof that 2026 deployment will double. Retain the distinction between additions and the installed stock.
Use both MW and MWh
Power describes the rate of discharge; energy describes how much can be delivered under a defined boundary. For fictional nominal specifications of 100 MW and 400 MWh, the ratio is four hours. Useful duration also depends on usable energy, permitted discharge power, reserve settings and losses. Ask whether the stated energy is at the cells or delivered to the grid.
| Ask for | Why |
|---|---|
| Usable energy and delivery point | Nominal cell storage is not necessarily grid-delivered energy |
| Power and operating limits | Check charging, discharging and allowed conditions |
| Losses and reserve | Preserve the stated measurement boundary |
| Availability and degradation | Identify the period and contractual condition |
| Connection and dispatch | A storage asset still needs permission and an operating plan |
A bounded duration example
If a hypothetical 400 MWh system reserves 10%, 360 MWh remains before other losses. At a constant 100 MW, the corresponding simple duration is 3.6 hours. This subtraction assumes the reserve percentage refers to that energy figure. It omits efficiency, state-of-charge limits and degradation. It is not a dispatch guarantee.
A deployment statistic does not establish savings
A project’s value depends on tariffs, market access, cycling, losses, availability and financing. Keep purchase cost, operating revenue and avoided charges separate. A smaller price spread or an unavailable dispatch window can change the result. Obtain the actual contract and operating study; this article does not calculate a universal payback or offer investment advice.
Compare the delivery boundary of two offers
Suppose fictional offer A lists 100 MW and 400 MWh, while B lists 100 MW and 200 MWh. At the same assumed constant 100 MW output, their nominal ratios are four and two hours. Matching power ratings have not made their stored energy or useful duration equal.
| Assumption | Offer A | Offer B |
|---|---|---|
| Nominal power | 100 MW | 100 MW |
| Nominal energy | 400 MWh | 200 MWh |
| Reserve on that energy | 10% | 10% |
| Energy after this subtraction | 360 MWh | 180 MWh |
| Simple duration at 100 MW | 3.6 hours | 1.8 hours |
This arithmetic treats the reserve as a fraction of the stated energy and assumes constant output. It excludes other losses, power limits, degradation and availability. Do not subtract a reserve or efficiency loss again if a quoted usable-energy value already includes it. Ask the supplier what the figure measures and where it is delivered before applying a formula.
Download the blank battery-offer boundary sheet (CSV). Copy each value with its source document and units. Leave usable energy or delivery conditions unresolved when they are not stated; the sheet does not calculate revenue, payback or guaranteed dispatch.
Scoped source check, October 11, 2026: the IEA chapter still describes 2025 capacity additions. Our two-offer example is hypothetical arithmetic, not a project measurement or supplier quotation.
Sources and scope
- IEA, Global Energy Review 2026, Battery storage — Reports 108 GW added in 2025 and 40% growth over 2024; not a full-year 2026 result.
Sources checked October 10, 2026. AI-assisted research and drafting. The examples, calculations and worksheets are Luminesca editorial work; they are not measured trials, first-hand reporting or an independent certification. Retains the October 6 removal of current-year doubling, payback and universal bill-saving claims; verifies the dated additions and calculation boundary. Report a specific correction with its source.