Battery storage is scaling up. In its Global Energy Review 2026, the International Energy Agency counts 108 gigawatts of new capacity deployed worldwide in 2025, 40% more than in 2024. Around four-fifths of these installations are large-scale systems; the rest serve residential and business consumers.
But a power rating in megawatts does not tell us how long a battery can supply electricity. This second piece of information becomes crucial when shifting solar generation into the evening or relieving a line that is temporarily saturated.
Two figures to read together
A megawatt, or MW, describes power. A megawatt-hour, or MWh, measures a quantity of energy. For storage, the two therefore need to be considered together: the available output on one side, the reserve on the other.
In an educational explainer published in 2019, the US Department of Energy gives the example of a 60 MW system with 240 MWh. The ratio between these two values equates to four hours at full power. At 30 MW, the same amount of energy represents eight hours. This calculation explains a technical feature; it does not guarantee autonomy for every installation or every use.
An announcement giving only “240 MWh” therefore leaves one question open: at what power can this reserve be delivered? Conversely, stating only “60 MW” tells us nothing about how long that power can be maintained. Comparisons between projects are more useful when they include both units.
Keeping solar power for later
The value of storage lies in the gap between generation and consumption. Solar panels can generate power when demand is low, then produce much less when usage rises. Charging a battery and discharging it later makes it possible to shift some of that energy through time.
This process involves losses: converting energy and then returning it never achieves 100% efficiency, the US Department of Energy notes. A battery therefore does not create additional generation. It makes energy that has already been generated available at another time. Storage can also smooth a brief fluctuation, for example when a cloud passes in front of a solar installation.
According to the IEA, many new projects still offer around two hours of storage, but systems with four hours or more are becoming more common. Duration thus distinguishes different services within the same family of technologies.
Relieving a line, not just waiting for evening
France’s Ringo programme illustrates another possible function. RTE designed it to absorb a renewable surplus locally when the grid cannot carry it away. Automated systems control charging and then discharging according to the state of the lines.
The experimental scheme links Bellac, Ventavon and Vingeanne. Its principle is for charging at certain sites to be offset by equivalent discharging elsewhere, in order to preserve the national balance between supply and demand. It therefore addresses a geographical constraint as well as a question of timing.
This approach is a reminder of why a battery and a power line are not interchangeable. Storage can help make better use of the existing grid during temporary congestion; it does not eliminate every need for transmission. To understand a project, three details matter together: its power, its energy reserve and the specific problem it is meant to address.
Sources
- IEA: Global Energy Review 2026, battery storage.
- US Department of Energy: Solar-Plus-Storage 101, 2019.
- US Department of Energy: Solar integration, solar energy and storage basics.
- RTE: overview of the Ringo experiment.
Sources consulted on 20 October 2026.
