Sizable Energy, a climate-tech startup headquartered in Milan and Palo Alto, has raised US $8 million for the development of its offshore pumped-hydro storage system. This is an innovative long duration energy storage solution designed to operate at sea using gravity and saturated brine. Announced in October 2025, the company says its system has already passed wave-basin testing and is now moving toward sea trials off Reggio Calabria, Italy. Once realized, the project will supply multi-gigawatt-hour storage in marine environments where land-based systems are constrained.
The offshore battery storage system technology builds on pumped-hydro concepts but relocates them offshore. Here, saturated sea-salt brine is pumped from deep-sea reservoirs to floating surface reservoirs when electricity is in excess, and released back down through turbines when supply is needed. Sizable also claims this marine approach can deliver significantly lower cost of storage than lithium-ion battery.
The new funding round, led by Playground Global with participation from Exa Ventures, Verve Ventures, Satgana, EDEN/IAG and Unruly Capital, continues to signals investor confidence in the potential of ocean-based storage.
Factsheet for Sizable Energy Offshore Storage Project
Developer: Sizable Energy
Project type: Ocean-based pumped hydro storage
Latest Funding raised: US $8 million
Technology Use: Two reservoirs, one floating at surface, and one on seabed. Brine is pumped up and released through turbines. This is designed for depths greater than 500 meters. Modular scaling is also possible.
Demonstration sites: Wave-basin test completed at Maritime Research Institute Netherlands (MARIN) in September 2025. Sea trial deployment started off Reggio Calabria, Italy.
Target commercial rollout: Starting 2026 across multiple global sites.
Pros: Lower cost of storage compared to other long duration energy storage technologies. Modular design. Zero land use. Scalable to hundreds of GWh.
Timeline for Sizable’s Offshore Pumped Hydro Energy Storage Technology
2023-2024: Concept development and early engineering.
September 2025: Successful wave-basin validation of offshore pumped-hydro system at MARIN.
October 2025: $8 million funding round announced. Sea-trial deployment also begins off Reggio Calabria, Italy.
2026: Expected first megawatt-scale demonstration projects. Preparation for commercial site development also expected.
2026 onward: Scaling to multi-gigawatt-hour deployments globally expected.
What to Expect of Sizable’s Ocean-based Pumped Hydro Energy Storage Technology
The renewable energy transition landscape is increasingly being challenged by the need for long duration storage. This is with the need to balance surging solar and wind generation. On-shore pumped hydro and large battery storage face limitations of geography, land use, cost and material supply. Sizable’s ocean-based system offers an alternative by tapping the vast resource of ocean depth. This allows for large-scale storage offshore where land constraints are inconceivable, permitting is less contentious, and co-location with offshore renewables is very much possible.
Regional Impact
Regionally, the testing in Mediterranean waters also highlights Europe’s ambition to accelerate storage capacity in renewables-dense areas. The Italian trial site off Reggio Calabria puts the technology in a deep-water environment around renewables with potential connection into the European grid.
Impact on the Renewable Energy Storage Market
From a market perspective, if Sizable can achieve the promised lower cost of storage and modular scalability, the technology may challenge traditional lithium-ion battery manufacturers. This also has the potential to shift economics for utility-scale storage as well as business models in grid services tied to offshore renewables.
However, the offshore storage technology also introduces new engineering and operational risks. These range from deep-sea mooring and maintenance, salt-water corrosion, marine environmental permitting, to interconnection logistics.
Also taking shape is Estonia’s pumped hydro energy storage plant in Ida-Virumaa. Here, state-owned Eesti Energia is using a former oil-shale mine for infrastructure needed for the storage system.
