Photo: Sizable Energy

Ocean energy storage startup Sizable Energy just raised $8 million to bring its long-duration offshore pumped hydro system to market.

The round was led by Playground Global, with backing from Exa Ventures, Verve Ventures, Satgana, EDEN/IAG, and Unruly Capital. The funding will help Sizable move from successful wave basin testing at the Maritime Research Institute Netherlands (MARIN) to full sea trials off the coast of Reggio Calabria, Italy.

Turning ocean depth into long-duration energy storage

Sizable Energy is reimagining pumped hydro storage for the sea. Instead of using freshwater and dams, the company’s system uses saturated sea-salt brine – about 20% denser than seawater – that moves between a floating reservoir and a deep-water reservoir. By harnessing gravity and ocean depth, it can store and release gigawatt-scale power without the land use or environmental impact of onshore hydro projects. Here’s a video explaining how it works:

CEO and co-founder Dr. Manuele Aufiero says the tech could help stabilize the grid as renewables surge: “Without cost-effective long-duration storage, the grid cannot keep up, regardless of energy source. Our ocean-based system stores gigawatt-scale power affordably, making the grid more stable, resilient, and ready for the future.”

The company claims its system offers the lowest Levelized Cost of Storage (LCOS) among long-duration storage technologies, even under optimistic lithium-ion cost projections. It combines readily available materials that can be made, assembled, and installed at depths of 500 meters or more, using existing maritime infrastructure.

Playground Global general partner Bruce Leak called ocean depth “a practically unlimited resource,” adding, “Sizable Energy is leveraging it to deliver long-duration energy storage at a fraction of the cost of batteries.”

What’s next

After proving its concept in the lab and at MARIN, where it functioned successfully in harsh ocean environments, Sizable Energy is now testing a new prototype in the Mediterranean. The pilot near Reggio Calabria, Italy, will validate its floating system, assembly, and deployment process, and pave the way for a multi-megawatt demonstration plant in the Mediterranean Sea.

If all goes to plan, the company expects to begin commercial project development in 2026 at multiple global sites, working with governments, energy providers, and local manufacturers to bring long-duration ocean energy storage to scale.

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