The problem
As Britain and continental Europe build more wind and solar, the hard part is no longer generating clean electricity on a good day. It is covering long, still evenings and the windless spells that can last for days. Great Britain’s existing long-duration storage is almost entirely pumped hydro, about 2.8 GW, according to Ofgem (Ofgem, 26 June 2026). New pumped hydro sites are slow to build and limited by geography.
Lithium-ion batteries are cheap per kilowatt and fast to deploy, but they lose capacity as they cycle, carry a fire risk that worries planners, and get expensive when built for many hours. Ofgem defines long-duration electricity storage (LDES) as at least eight hours, which is where alternative chemistries hope to compete.
The product
ENDURIUM is a grid-scale vanadium redox flow battery. Instead of storing energy in solid electrodes, it stores it in tanks of vanadium electrolyte that is pumped through stacks of cells. Each module holds 300 kWh, and modules are combined into strings and then into projects from 10 MWh up to 1 GWh, with durations from 4 to 18 hours (Invinity).
At launch, Invinity said ENDURIUM offers up to twice the site energy density of its previous VS3 product, performance verified by DNV, and certification to UL and CE standards (Invinity, 3 December 2024). The datasheet describes a four-module string storing 1.2 MWh (ENDURIUM datasheet).
The company traces its roots to redT in the UK and Avalon in North America, which merged in 2020 (Invinity). Its stacks are made in Bathgate, West Lothian, and it has expanded assembly in Motherwell to more than 500 MWh of annual capacity (Energy-Storage.news). Invinity says it has deployed more than 80 projects worldwide.
How it works
A vanadium flow battery uses one element in four different oxidation states:
- Two tanks: one holds a positive electrolyte and one a negative electrolyte, both vanadium dissolved in a sulphuric acid solution.
- Charging: pumps push both liquids through a stack of cells separated by a membrane. Electricity drives ions across the membrane and changes the vanadium’s charge state on each side.
- Discharging: the reaction runs in reverse and releases electricity.
- No cross-contamination: because both sides use vanadium, any mixing across the membrane does not poison the electrolyte, which is why it does not degrade with cycling.
Power (the size of the stacks) and energy (the volume of electrolyte) can be scaled separately, which suits long durations. The electrolyte is non-flammable and can be recovered at end of life.
Timeline
| Date | Milestone |
|---|---|
| Apr 2020 | redT energy and Avalon Battery merge to form Invinity |
| Dec 2024 | ENDURIUM launched as fourth-generation product |
| Feb 2026 | ENDURIUM shown at India Energy Week in Goa with shareholder Atri Energy |
| Jun 2026 | Ofgem’s minded-to list for LDES Window 1 includes Frontier Power’s Legacy flow battery project |
| H1 2026 | 20.7 MWh delivered to Copwood, East Sussex, Europe’s largest flow battery |
| Sep 2026 | 43 MWh order from Dairyland Power in the US takes 2026 orders to 77.2 MWh |
| Q1 2027 | Target date for deliveries at a cost at least 66% below VS3 |
Impact and numbers
- Orders: 34.2 MWh of orders in the first half of 2026, against 11.7 MWh a year earlier, and 77.2 MWh for 2026 to date after the Dairyland order (LSE RNS, 21 September 2026).
- Copwood: 20.7 MWh delivered in East Sussex, which Invinity describes as Europe’s largest flow battery.
- Switzerland: a design contract for FlexBase in Laufenburg, sized at 1.5 GWh and rising to 2.1 GWh.
- Track record: 11 GWh dispatched by Invinity batteries in the field, and a pipeline of about 12.5 GWh.
- Policy: Ofgem’s 16-project minded-to list covers pumped hydro, compressed air, lithium-ion and vanadium flow, with final decisions due later in 2026 (Ofgem). Frontier Power’s Legacy project could reach about 260 MWh.
Honest caveats. Invinity is still loss-making: in the first half of 2026 it reported revenue and grant income of £1.7 million against a loss of £12.1 million, with £10.5 million of net cash, so it depends on new orders, grants and possibly fresh capital. Ofgem’s cap-and-floor awards were provisional at the time of writing, and projects can still fall away. Vanadium is mined mostly in China, Russia and South Africa, and its price swings affect battery costs. Round-trip efficiency of flow batteries is generally lower than lithium-ion, and the claims of “no degradation” and 25-year life rest on manufacturer data and DNV verification rather than decades of field history at this scale. The large FlexBase figure is a design contract, not a firm order.
What’s next
Invinity’s main lever is cost. It is targeting a reduction of at least 66% against VS3 for systems delivered from the first quarter of 2027, which would bring flow batteries closer to lithium-ion for long durations. It has proposed a US factory to serve utilities such as Dairyland, and in the UK its fortunes are tied to Ofgem’s cap-and-floor scheme, which gives investors a guaranteed revenue floor for long-duration storage.
Why it matters for Europe / green buyers
For European grid operators and developers, ENDURIUM offers a long-life, non-flammable storage option manufactured in Scotland rather than Asia, which matters for supply chain resilience and local jobs. It suits sites where batteries will cycle hard every day, such as solar parks, industrial microgrids and grid support near wind clusters. Buyers should compare levelised cost over 25 years, not upfront price alone, and check the vanadium supply terms in each contract.
India is a natural market. Grid-India projected a need for about 4 GW / 17 GWh of battery storage by 2025 to 2026, and Invinity showcased ENDURIUM at India Energy Week in Goa in February 2026 with its shareholder Atri Energy (Gulf Oil & Gas). Hot climates and heavy daily solar cycling favour chemistries that do not wear out.
Sources & image credits
- Invinity Energy Systems, “Introducing ENDURIUM: transforming grid-scale energy storage”, 3 December 2024: https://invinity.com/introducing-endurium-transforming-grid-scale-energy-storage/
- Invinity Energy Systems, “ENDURIUM” product page: https://invinity.com/endurium/
- Invinity Energy Systems, “ENDURIUM Data Sheet”, January 2025: https://invinity.com/wp-content/uploads/2025/01/Invinity-ENDURIUM-Data-Sheet-MAR000020-2025-01.pdf
- Invinity Energy Systems, “Creating the leading vanadium flow battery company”, 2020: https://invinity.com/creating-leading-vanadium-flow-battery-company/
- Energy-Storage.news, “Invinity expanding UK flow battery assembly to more than 500MWh annual capacity”: https://www.energy-storage.news/invinity-expanding-uk-flow-battery-assembly-to-more-than-500mwh-annual-capacity/
- Ofgem, “Ofgem boosts long-duration storage to secure more homegrown energy for customers”, 26 June 2026: https://www.ofgem.gov.uk/press-release/ofgem-boosts-long-duration-storage-secure-more-homegrown-energy-customers
- London Stock Exchange RNS via LSE.co.uk, Invinity Energy Systems “Trading and commercial update” and interim results, 21 September 2026: https://www.lse.co.uk/rns/trading-and-commercial-update-xnfe0m6cadl282t.html
- Gulf Oil & Gas, “Invinity showcases ENDURIUM at India Energy Week”, February 2026: https://www.gulfoilandgas.com/webpro1/main/mainnews.asp?id=1143044
Images:
- “RedT energy vanadium redox flow machines” by Opel2236, licensed CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0), via Wikimedia Commons: https://commons.wikimedia.org/wiki/File:RedT_energy_vanadium_redox_flow_machines.jpg
