September 2026
As the UK’s electricity system decarbonises rapidly, ensuring security of electricity supply will require new forms of flexible capacity that can complement variable wind and solar power. At the same time, the growing electrification of the economy means that security of electricity supply will become even more important than it is now.
This briefing paper, developed with University College London, explores the growing role of long-duration energy storage (LDES) in Britain’s electricity system and assesses the policies needed to support sufficient investment in, and efficient operation of, LDES of different durations.
While short-duration batteries are already being deployed rapidly in the UK, LDES remains significantly underdeveloped despite its increasing importance for managing periods of surplus renewable generation and maintaining security of supply during prolonged periods of low wind and solar output.
The UK is a world-leader in introducing a dedicated investment policy for LDES, the Revenue Cap-and-Floor. However, the current conception of LDES in UK policy is very broad. Policymaking could be aided by a more detailed framework distinguishing different types of LDES based on the services and markets addressed and the relevant technologies. We propose a storage duration taxonomy of four duration categories: short-duration (0-10 hours), inter-day (10-48 hours), inter-week (48-300 hours) and seasonal (more than 300 hours). Drawing on modelling of future GB power system needs, we find that storage of different durations will be needed with varying frequency. The four types of storage perform different functions, face different investment and operational risks, and therefore require different policy approaches.
Current policy to support LDES deployment risks falling short by not incentivising investment in enough storage capacity of longer durations – particularly inter-week or seasonal LDES – and potentially by failing to ensure these assets are operated in line with system needs, particularly the need for security of supply during rare but high impact scarcity events.
The paper compares market-based approaches to LDES deployment with state-ownership and operation of a strategic energy storage reserve. We argue that while the new LDES Revenue Cap-and-Floor policy is an important step forward, it should be reformed to support a wider range of technologies and storage durations. A reformed Revenue Cap-and-Floor mechanism would be suited to inter-day LDES, encouraging adequate investment and efficient operation. For inter-week storage, we identify a need for further work to determine the most effective balance between market participation and central coordination to ensure security of supply. For seasonal storage, which may only be needed during rare but high-impact system stress events, we propose a new approach: a state-owned strategic energy storage reserve, analogous to strategic oil and gas stocks, operated in the public interest to ensure security of supply.
By examining how storage requirements evolve as renewable penetration increases, and how electricity market design must also evolve, this paper contributes to growing debates on electricity market reform, system resilience and the infrastructure needed to support a secure and affordable clean energy transition. It highlights that further policy development on LDES should be a priority, as decisions taken today are likely to determine whether sufficient long-duration storage capacity is available in the 2030s and beyond.
