Long-Duration Electricity Storage and the LDES Cap and Floor Scheme

Long-duration electricity storage is storage that holds energy not for minutes or hours but for days, weeks or whole seasons, and Britain is building very little of it. The country's existing long-duration capacity is 2.8GW of power and 23.9GWh of stored energy, almost all of it pumped hydro built decades ago¹. Grid-scale batteries, by contrast, reached 7.5GW of power by the end of 2025, but they store electricity for up to about eight hours¹. That gap is the problem the government's cap and floor scheme exists to close.

The scheme, announced in 2025 for technologies such as pumped storage hydro, compressed air energy storage, liquid air energy storage and flow batteries, offers revenue support to projects that can bridge the hours and days when wind and solar output collapses³. As of June 2026, cap and floor investment support has been offered to 7.6GW of new long-duration electricity storage projects, and the 16 projects involved collectively represent 137GWh of new electricity storage capacity¹. That is roughly six times the stored energy the country has today.

For a household, none of this is visible at the meter. What it changes is how the system copes with a still, cold February week, how much it has to pay for imported power at those moments, and how much of the bill is exposed to global gas prices. This page sets out what long-duration storage is, what has been committed, how the support scheme works, how it differs across the four nations, and what it does and does not do for a home's energy independence.

Defining long-duration storage and how it stands apart from batteries

Electricity storage is not one thing. The batteries most people now know, in a phone, an electric car or a grid-scale site, store energy as chemical energy which is converted back to electrical energy when the battery is discharged⁴. At grid scale in Britain these batteries are fast and increasingly numerous, but they are short-lived in a specific sense: they hold electricity for up to eight hours². That is enough to shift solar power from midday into the evening peak, or to cover a spike in demand, but not to carry the country through a three-day wind drought in January.

Long-duration electricity storage, usually shortened to LDES, is the class of technology that goes beyond those hours. The government's cap and floor scheme names the technologies in the running: pumped storage hydro, compressed air energy storage, liquid air energy storage and flow batteries³. Each works differently, but they share one property, the ability to hold large amounts of energy for long enough to bridge days rather than hours. Hydrogen sits at the far end of the spectrum, storing very large amounts of energy for very long time periods⁵.

The distinction matters because the two kinds of storage do different jobs. Batteries trade, they buy cheap and sell dear within the same day, and they provide the fast balancing services that keep the grid stable. Long-duration storage insures, it takes energy generated in one period and releases it in another, sometimes weeks later. A system with only batteries is a system that can smooth a day but not a fortnight. NESO's own scenario work makes the scale of the need clear: its Consumer Transformation and Leading the Way scenarios have more than 115GWh of electricity storage in 2035, against less than 30GWh in Great Britain in 2022⁶.

There is also a third category that rarely gets counted as electricity storage but does the same job in a different form: thermal storage. Energy assessment guidance treats thermal and electrical energy storage together as flexibility solutions to reduce peak demands⁷. Britain's hot water cylinders alone represent around 27GW of existing installed capacity⁸, which is a reminder that storing heat is often cheaper than storing electricity.

The numbers: 137GWh of extra capacity backing 7.6GW

The headline numbers are straightforward. Cap and floor investment support has been offered to 7.6GW of new long-duration electricity storage projects¹. The 16 projects involved collectively represent 137GWh of new electricity storage capacity¹. Existing long-duration capacity stands at 2.8GW and 23.9GWh as of June 2026¹.

Two things stand out. First, the pipeline is large relative to what exists: 137GWh of new capacity against 23.9GWh today is close to a sixfold increase in stored energy, if all of it is built. Second, power and energy are different measures and the gap between them is the point of the technology. The existing fleet's 2.8GW of power against 23.9GWh of energy means it can run for roughly eight and a half hours at full tilt, which is what pumped hydro plants like Dinorwig were designed for. The new projects are being selected precisely because they hold much more energy per unit of power, so they can run for a day or more.

The comparison with batteries is also worth making precisely. Batteries reached 7.5GW of power by the end of 2025, more than double the long-duration fleet, with a record 2.3GW energised in 2025 alone¹. But their stored energy is measured in gigawatt-hours at most, and their duration tops out at eight hours². The two fleets are complements, not competitors: batteries handle the daily cycle, long-duration storage handles the weeks the batteries cannot cover.

Why multi-day storage is needed

The driver is the shape of a decarbonised system. Britain's electricity comes increasingly from wind and solar, whose output is set by weather rather than demand. The security of supply standard that governs the system is expressed as a Loss of Load Expectation of three hours per year for Great Britain², a measure of how rarely the system should fall short. Meeting that standard with weather-dependent generation means having something to draw on when the weather does not cooperate, and the risk is concentrated in winter, when demand is highest and a high-pressure system can sit over the country for days, suppressing wind output while cold pushes demand up.

The government's own security analysis points to deploying 4 to 6GW of LDES by 2030 as part of the answer². The British Energy Security Strategy, published in April 2022, committed to amending National Policy Statements to enable investment in long-duration energy storage⁹, which was the planning unlock that let projects of this scale move forward. The strategy was written in the shadow of a gas crisis, and its logic still holds: the less firm, dispatchable generation the system retains, the more storage it needs to replace it.

The scale of the remaining dependence explains the urgency. Fossil fuels still accounted for 75.2% of UK energy consumption in 2024, down 1.4 percentage points on 2023, when the figure was 76.6%¹⁰. Electricity is decarbonising faster than the energy system as a whole, which means the gas used to generate electricity at peak times is one of the last and most expensive fossil uses left. Every gigawatt-hour that long-duration storage can release in a tight week is a gigawatt-hour that does not have to be bought from gas plants running at high prices, or from interconnectors importing from neighbours who are often short at exactly the same moment. You can read more about that exposure in UK gas storage and electricity interconnectors.

There is a second, quieter driver: money. Storage that can wait weeks can buy electricity when it is nearly free and sell it when it is scarce. The growth of negative electricity prices in periods of surplus wind is a signal that the system is already throwing away energy it could be keeping. Long-duration storage is the technology that can catch it.

Inside the cap and floor support mechanism

The cap and floor is a revenue support scheme, and its shape is what makes it suited to projects of this size and life. Announced in 2025, it was designed to introduce a cap and floor revenue support scheme for long duration energy storage technologies such as pumped storage hydro, compressed air energy storage, liquid air energy storage and flow batteries³.

The principle works like this. The floor is a guaranteed minimum level of revenue. If a project's market earnings fall below it, consumers top up the difference, through a levy on bills. That floor is what makes a multibillion-pound pumped hydro scheme bankable: an investor lending against a 60-year asset needs to know the income will not disappear if market prices shift. The cap is the counterpart. If a project earns above the cap, the surplus is returned to consumers rather than kept as profit. The scheme therefore shares risk in both directions.

This is not the only support structure in the energy system, and the comparison is instructive. The Contracts for Difference scheme used for renewable generation guarantees a set price per MWh of electricity for 15 years, indexed to inflation¹¹. A CfD is a two-sided contract around a single strike price. A cap and floor is a band: the operator keeps everything between the floor and the cap, loses nothing below the floor, and returns everything above the cap. The band structure suits storage because storage's income is inherently volatile, it depends on price spreads that vary enormously year to year, and a fixed strike price would either overpay in cheap years or underpay in expensive ones.

The scheme's lineage is worth knowing. A cap and floor has already been used for electricity interconnectors, and the storage version follows the same logic: large, long-lived infrastructure whose value to the system is real but whose merchant income is uncertain. The Public Accounts Committee has scrutinised how such government support for energy projects is accounted for and delivered¹², and the Spending Review 2025 set the fiscal context in which Great British Energy and related bodies operate¹³.

What the scheme does not do is pick technologies by decree. It names the eligible classes, pumped hydro, compressed air, liquid air and flow batteries³, and lets projects compete within them. The 16 projects offered support, representing 7.6GW and 137GWh¹, are the outcome of that competition. Not all will be built: consent, construction and connection still lie ahead, and the grid connection queue remains a constraint on when any of this capacity actually arrives.

The four nations of the UK

Long-duration storage is not evenly spread across Britain, and the reasons are physical. Pumped hydro needs height and water, which is why the existing fleet sits where it does. Wales contributes the majority of the UK's existing 2.8GW of pumped hydropower storage through Ffestiniog and Dinorwig¹⁴, and Scotland holds most of the rest, with the potential for more.

Scotland's position is distinctive in two ways. First, its planning framework has already designated pumped hydro storage as a national development: National Planning Framework 4 lists Pumped Hydro Storage as a national development in the North, Central, and North and West Coast and Islands areas, on the grounds that it extends hydro-electricity capacity to support the transition away from fossil fuels¹⁵. That designation matters because it signals consent at the national level for schemes that would otherwise face years of local determination. Second, Scotland generates a far higher share of renewables than the rest of the UK, 29.3% of electricity generation for the UK as a whole in the figures cited in Scotland's 2019 energy statement, against 25.6% for the rest of the UK excluding Scotland¹⁶, and the pattern of surplus generation in the north is one of the things long-duration storage in Scotland can absorb.

Wales hosts the two flagship plants, Ffestiniog and Dinorwig, which between them contribute the majority of the UK's existing 2.8GW of pumped hydropower storage¹⁴. England has little pumped hydro potential and its contribution to the long-duration fleet is expected to come from the newer technologies, compressed air and liquid air, which can be sited near industrial storage caverns or liquefaction plants.

Northern Ireland is a separate case entirely. Its electricity system is part of the Single Electricity Market with the Republic of Ireland, not the GB market, so the cap and floor scheme as announced for Great Britain does not apply there in the same form. The broader pattern of UK-wide schemes operating with national differences is familiar: the Energy Price Guarantee, for instance, operated for the most part in the same way for households across the whole of the UK, with differences for Northern Ireland¹⁷. Anyone assessing storage policy in Northern Ireland needs to look at the all-island market rather than GB instruments.

Electricity storage versus gas storage

Britain has one large, mature form of long-duration energy storage already, and it is not electricity: it is gas. The comparison is worth making because it shows what a storage system that can genuinely cover a season looks like, and how far electricity is from matching it.

Gas storage works because the energy is stored as itself, molecules in a depleted field or a salt cavern, with no conversion losses on the way in or out. Electricity storage has to convert: a battery converts to chemical energy and back⁴, pumped hydro lifts water and lets it fall, liquid air chills and re-expands. Every conversion costs energy, which is one reason electricity storage is harder and dearer per unit stored than gas storage. You can explore the gas side in UK gas storage and Rough, Britain's largest gas storage site.

The two systems also fail differently. Gas storage covers weeks of cold weather because the stored volume is huge relative to daily demand. Electricity storage, at 23.9GWh of long-duration capacity¹, covers hours of national demand, not weeks, because electricity demand is measured in tens of gigawatts around the clock. Even the 137GWh pipeline¹, if fully built, is measured against a country whose peak demand is in the tens of gigawatts. This is why inter-seasonal electricity storage, moving summer surplus into winter use, remains mostly an aspiration in Britain, and why hydrogen, which can store very large amounts of energy for very long time periods⁵, is discussed as the eventual answer at that timescale.

There is a household-scale version of the same comparison, and it is closer than it looks. A hot water cylinder is thermal storage, and the UK's hot water storage capacity is around 27GW of existing installed capacity⁸, comparable to the whole grid-scale battery fleet. A home with solar and a battery maximises its use of self-generated electricity and exports the surplus to the grid¹⁸, which is daily storage, not seasonal. The lesson from the national picture applies at home too: storing heat is easier than storing electricity, and the cheapest inter-seasonal storage most households can actually get is a well-insulated house that holds the heat it is given.

The impact on a home's self-sufficiency

The honest answer is that long-duration storage is national infrastructure, and its effect on a single home is indirect. It does not change what a householder can install, and it does not remove any of the dependencies a home has: the grid, a supplier, a network operator, and the wholesale market that sets most of the price of a unit of electricity.

What it does change is the character of those dependencies. A Britain with 137GWh of long-duration storage¹ is a Britain that has to burn less gas in a tight week, import less at peak, and pay less for the privilege. It is also a Britain whose electricity supply is less hostage to weather, which is the specific fragility that a renewables-heavy system introduces. The security of supply standard, three hours per year of expected loss of load², is the promise the system makes to households, and long-duration storage is one of the tools for keeping it.

The limits deserve equal weight. The 7.6GW of supported projects¹ are offered support, not delivered capacity, and the gap between the two is measured in years of construction. The 4 to 6GW by 2030 deployment figure² is a target, not a forecast. And even at full build, long-duration electricity storage covers days, not months: the inter-seasonal problem, summer surplus against winter scarcity, remains open, with hydrogen the only technology on the table that stores at that timescale⁵.

For a household thinking about its own position, the practical reading is this. National storage reduces the volatility and the fossil exposure in the price of grid electricity, which lowers the risk in staying connected and relying on the grid. It does nothing by itself for a home's ability to stand alone. That still rests on the things covered across this site: energy security and household independence, household demand flexibility, and the generation mix described in UK electricity generation mix and wind power on the UK grid. Long-duration storage is the system-level counterpart of what a home battery does at the street level, and the two work best together.