Sizable Energy raises €7.4M for ocean energy storage

Italian climate tech startup is taking energy storage to the deep. Sizable Energy has raised €7.4M ($8M) to commercialize its ocean energy storage system that stores gigawatt-scale power using gravity and seawater. Playground Global led the round, with participation from Exa Ventures, Verve Ventures, Satgana, EDEN/IAG, and Unruly Capital.

The Milan and Palo Alto-based company is reimagining pumped hydropower for the ocean. Traditional pumped hydro requires specific geography and massive concrete dams. Sizable’s offshore approach sidesteps those constraints entirely.

How Offshore Pumped Hydro Tackles Europe’s Storage Gap

The technology works like an underwater hourglass. Sizable’s system pumps saturated sea-salt brine—20% denser than seawater—between reservoirs on the seabed and floating at the surface. When the grid needs power, gravity pulls the heavy brine down through turbines, generating electricity. When power is cheap, the process reverses.

“Without cost-effective long-duration storage, the grid cannot keep up, regardless of energy source,” said Dr. Manuele Aufiero, CEO and co-founder of Sizable Energy. “Our ocean-based system stores gigawatt-scale power affordably, making the grid more stable, resilient, and ready for the future.”

The company recently completed wave-basin testing at the Maritime Research Institute Netherlands, confirming the system can withstand harsh marine conditions. Sea trials are now underway off Reggio Calabria, Italy, ahead of a multi-megawatt demonstration plant in the Mediterranean.

Mass Production Meets Energy Transition Urgency

Europe’s renewable energy buildout faces a storage bottleneck. The International Energy Agency estimates the world needs 120 terawatt-hours of long-duration storage by 2040—ten times current capacity. Onshore pumped hydro can’t scale fast enough due to geography, permitting, and environmental concerns.

Sizable’s modular design enables mass production impossible with land-based systems. Each installation is identical regardless of location, requiring only coastal waters deeper than 500 meters. The standardized approach dramatically reduces deployment time and cost uncertainty.

“Ocean depth is a practically unlimited resource, and Sizable Energy is leveraging it to deliver long-duration energy storage at a fraction of the cost of batteries,” said Bruce Leak, general partner at Playground Global and board member. The company targets €20 per kilowatt-hour—roughly one-tenth the cost of grid-scale batteries.

Sizable plans to begin commercial project development in 2026 at multiple global sites. The technology pairs naturally with offshore wind farms, sharing electrical connections to shore and reducing infrastructure costs. For coastal regions with deep waters, this could unlock gigawatt-scale storage without sacrificing land or fresh water.