Table of Contents
Technology and Infrastructure
Energy storage
Energy storage encompasses technologies that capture energy at one point in time for use at another, enabling electricity systems to manage the mismatch between variable generation and demand. Storage operates across timescales from seconds to seasons and at scales from individual households to grid-level installations, and its role in smart grid transitions extends from frequency regulation to long-duration balancing of renewable energy systems.
Why this matters
Electricity systems built around dispatchable generation can balance supply and demand in real time by adjusting output. Systems with high shares of variable renewables cannot do this without either curtailing generation or storing surplus energy. The quantity of storage required, and its duration, depends on the generation mix and the degree of interconnection available. Analysis of a net-zero UK system suggests total storage requirements in the range of 60 to 100 terawatt-hours — equivalent to roughly a third of annual UK electricity consumption — with the specific figure depending on the ratio of wind and solar to storage capacity.1) In a scenario where wind and solar are sized 33% larger than peak demand, with 15% of supply coming from storage, modelled costs range from approximately £52 to over £90 per megawatt-hour depending on discount rate and storage cost assumptions, with a central estimate around £64 per megawatt-hour including transmission.2)
Storage duration matters as much as storage capacity. Short-duration storage (minutes to hours) addresses daily balancing; medium-duration (days to weeks) covers weather variability; long-duration (months) is needed only in systems with very high renewable penetration or limited interconnection.
Shared definitions
Energy storage is the conversion of electrical energy into another form — chemical, mechanical, thermal, or gravitational potential — for later reconversion to electricity or direct use as heat or cooling.
A storage classification by discharge duration distinguishes the following operational roles:
Table 1. Storage duration classes and system function. Source: adapted from Llewellyn Smith (2020).
| Duration class | Typical discharge period | Primary system function |
|---|---|---|
| Short-duration | Seconds to hours | Frequency regulation, peak shaving, daily balancing |
| Medium-duration | Hours to days | Weather-driven variability, multi-day balancing |
| Long-duration | Weeks to months | Seasonal balancing, security of supply |
| Very long-duration (VLS) | More than 180 days | Extreme events, annual renewable variability |
Figure 1. Very long store (VLS, >180 days) requirements. Source: Llewellyn Smith (2020).
Figure 2. Candidate storage technologies. Source: Llewellyn Smith (2020).
Figure 3. Energy storage conclusions with UK focus. Source: Llewellyn Smith (2020).
Figure 4. Questions about candidate storage technologies. Source: Llewellyn Smith (2020).
Perspectives
Actors and stakeholders
Storage assets are owned and operated by a range of actors: transmission and distribution system operators deploying grid-scale storage for system services; utilities and independent power producers operating large battery or pumped hydro assets in wholesale markets; commercial and industrial customers using behind-the-meter storage for peak demand management; and households combining rooftop solar with residential batteries. Aggregators can pool dispersed small-scale storage into portfolios capable of market participation.
Technologies and infrastructure
Candidate storage technologies span several physical principles, with different characteristics for power capacity, energy capacity, round-trip efficiency, cost, and discharge duration.3) Key questions for each technology include: What is the energy-to-power ratio? What are the self-discharge characteristics? Can it be deployed at grid scale? What are the infrastructure requirements for installation?
Institutional structures
Whether storage assets can participate in electricity markets, and which services they can provide, depends on regulatory classification. In many jurisdictions, storage has historically been classified either as generation or as consumption, but not both — creating a regulatory barrier to its operation as a flexibility resource. Revisions to market rules to accommodate storage as a distinct category are underway in several regulatory frameworks.
Distinctions and overlaps
Energy storage vs flexibility
Flexibility is the broader capability of the power system to manage variability and uncertainty; storage is one of several resources that provide flexibility alongside demand response, interconnection, and dispatchable generation. Not all storage provides flexibility in a market sense, and not all flexibility comes from storage.
Short-duration vs long-duration storage
The distinction matters for system planning and technology choice. Short-duration storage (batteries, flywheels) addresses daily and sub-daily balancing at competitive cost; long-duration storage (hydrogen, pumped hydro, compressed air) is required for seasonal balancing but faces higher capital costs and lower round-trip efficiency. Policy and market design must account for both.
Behind-the-meter storage vs grid-scale storage
Behind-the-meter storage is installed at user premises and primarily serves the owner's energy management needs; its contribution to the wider system depends on whether aggregation and market access mechanisms exist. Grid-scale storage is directly connected to transmission or distribution networks and operated explicitly as a system asset.
Related topics
1)
, 3)
Llewellyn Smith, C. (2020). The need for energy storage in a net zero world. ERA Technology. https://www.era.ac.uk/write/MediaUploads/Other%20documents/Need_for_Storage_in_a_Net_0_World_Chris_Ll_S_23_3_20.pdf
2)
Llewellyn Smith, C. (2023). Solving for storage [podcast transcript]. Cleaning Up, Episode 122. https://www.cleaningup.live/ep122-sir-chris-llewellyn-smith-solving-for-storage/