Electricity storage: the technologies compared

Electricity storage makes it possible to produce at one time and consume at another: keep the midday solar surplus for the evening, ride through a consumption peak without exceeding the contracted capacity, or help the grid stay balanced. Several technologies do this, at very different scales, durations and efficiencies. This article compares them, to work out which one fits a business site.

What does "storing electricity" mean?

The French Energy Code defines storage in Article L. 352-1, as recalled by the Energy Regulatory Commission (CRE) in its deliberation of 7 April 2022: it is « the deferral of the final use of electricity to a moment later than the one at which it was produced ».

In practice, electricity is very hard to store as it is. It is therefore converted into another form of energy, which is turned back into electricity when needed. Each conversion costs some energy: this is what efficiency measures, the ratio between the electricity returned and the electricity absorbed.

The technologies fall into four families:

• mechanical: pumped-storage hydro (STEP), compressed air, flywheels;
• electrochemical: batteries (lithium, sodium, flow batteries);
• chemical: hydrogen produced by electrolysis;
• thermal: heat stored in molten salts, water or solid materials.

Which electricity storage technologies should be compared?

The criterion that settles things fastest is the duration to cover. A technology designed to store a few seconds is no use for shifting solar production, and a seasonal storage technology loses too much energy for daily use.

How does pumped storage work, and why does it lead in France?

A pumped-storage plant links two reservoirs at different heights. When the grid has a surplus, water is pumped from the lower reservoir to the upper one; when power is needed, it flows back down through turbines. It is the oldest large-scale storage method.

According to EDF (page updated on 25 August 2026):

• the French fleet has 6 pumped-storage plants, with an installed capacity of 5 GW;
• Grand'Maison, in Isère, reaches 1,800 MW in three minutes;
• Montézic reaches its maximum output of 920 MW in under two minutes.

These plants require relief, two reservoirs and years of works. They serve the transmission grid, not an industrial site. Their strength is balancing the national power system.

Why are batteries growing so fast?

Because they can be installed almost anywhere, quickly and at any size: a cabinet in a factory yard, a few containers next to a solar plant, dozens of containers connected to the grid. The renewable electricity overview published on 16 July 2026 by SER, RTE, Enedis and Agence ORE counts 1,617 MW of battery storage connected in France at the end of 2025, including 410 MW commissioned in 2025, with 195 MW in the third quarter and 154 MW in the fourth.

Which chemistry for stationary storage?

The chemistry most used for stationary storage is lithium iron phosphate (LFP), chosen for its thermal stability and cycle life. It is the one used in every RelyEZ range, with 314 Ah prismatic cells (see the RelyEZ technology page). Other chemistries exist:

• lithium NMC (nickel-manganese-cobalt), denser, mainly used in electric vehicles;
• sodium-ion, which does without lithium;
• flow batteries (known as "redox flow"), where energy is stored in liquid electrolytes held in tanks.

What the datasheets say

A battery is judged on a few quantities, all stated on the datasheet of the exact model:

The VenturePro 250 datasheet states a maximum AC efficiency of 90%, excluding auxiliaries: out of 100 kWh drawn from the grid, at most 90 kWh come back out, before the consumption of cooling and control. These figures are in the catalogue, range by range.

The ratio between energy and power determines the use. Two hours suit peak shaving on a site; four hours or more, shifting most of a solar plant's output to the evening, as explained in our article on negative prices and solar power.

Can hydrogen store electricity?

Yes, but at a high energy cost. An electrolyser splits water to produce hydrogen; the gas is compressed and stored, then turned back into electricity in a fuel cell. ADEME, the French environment and energy agency, studied this "Power-to-H2-to-Power" chain: its overall efficiency, from the electrical source to the end use, is in the order of 25%, or even 30% with the best equipment. The agency recommends favouring electrochemical storage where it is technically and economically feasible.

Hydrogen's value lies elsewhere: it can be stored for several days, weeks or months, and used directly as a feedstock or fuel, in industry or transport. It is a long-duration storage tool, not a substitute for a battery in daily use.

What about compressed air, flywheels and heat?

These technologies meet specific needs:

• compressed air stores energy by compressing air in an underground cavern; it requires suitable geology;
• flywheels store energy in a rotor spinning very fast, for a few seconds to a few minutes: they are used to stabilise voltage or ensure continuity of supply;
• supercapacitors absorb and deliver very high power for a few seconds;
• thermal storage keeps heat, rather than electricity, for a process or a heat network.

None of them replaces a battery for storing a few hours of electricity on a site, but they complete the storage landscape.

Which storage technology for a business site?

For a company, a local authority, a farm or a solar plant, the question is rarely pumped storage versus battery: the battery is what gets installed behind the meter. The real question is which one, and what size. It is settled with a few pieces of data:

1. The intended use: shaving peaks, keeping the solar surplus, powering a fast-charging station, taking part in flexibility markets through an aggregator.
2. The duration to cover: it sets the ratio between energy and power.
3. The site's load curve, and its solar production if any.
4. The grid connection capacity and the space available: cabinet or container.

Our article on storage batteries for businesses details this sizing method step by step. For a business site, the VenturePro range covers all-in-one cabinets; for a power plant or a high-power site, GridUltra containers are added according to the grid connection.

Where to start?

The right storage is chosen from the site: what it consumes, what it produces, what its grid connection allows. That is the starting point of our project assessment, which leads to a detailed configuration and a justified sizing. The datasheets for each range are in the catalogue.