Rigorous tests confirm offshore pumped hydro system can operate reliably in harsh ocean environments ahead of pilot in the Mediterranean Sea

Sizable Energy, a pioneer in long-duration ocean energy storage,  announced it has raised $8 million to accelerate its path to commercial deployment. The round was led by Playground Global. With successful testing completed at the offshore wave basin at Maritime Research Institute Netherlands (MARIN) and new sea trials launching in Reggio Calabria, Italy, Sizable is now advancing its offshore pumped hydro system to deliver low-cost, long-duration energy storage using gravity

“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.”

This investment round will accelerate the development and deployment of Sizable Energy’s innovative offshore pumped hydro system, designed to provide economical and reliable long duration energy storage (LDES).

The world needs up to 120 terawatt-hours of long-duration energy storage by 2040, ten times more than we have today. Most of today’s storage installed capacity comes from onshore pumped hydro, but it’s slow to build, expensive to scale, and limited by geography and environmental concerns. Sizable Energy offers a faster, more flexible solution: offshore pumped hydro that’s modular, cost-effective, and ready to deploy at scale.

The New Gravity Battery: Sizable Energy to Build Offshore Pumped Hydro

Sizable Energy’s patented offshore pumped hydro system stores energy by pumping saturated sea salt brine (heavier than seawater) from the seabed to a surface reservoir, leveraging the depth of the ocean for efficient energy storage.

Invisible from the shore, Sizable Energy’s ocean energy storage solution:

• Offers the lowest cost Levelized Cost of Storage (LCOS) for LDES even when considering the most optimistic projections for Li-ion cost decreases.
• Adopts a modular and scalable design capable of delivering from one to hundreds of gigawatt hours of storage.
• Combines readily available materials that can be manufactured, assembled, and installed at depths of 500 meters or more, using existing maritime infrastructure.

“As a society, we’ve proven that wind and solar can deliver the cheapest electrons. Now we urgently need a long duration energy storage solution that can be economically deployed at massive scale,” said Bruce Leak, general partner at Playground Global and board member of Sizable Energy. “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. Their gravity-based approach is the clear path to enabling energy abundance.”

Rigorous Tests Leading to Megawatt Demonstration Plant

Sizable Energy has validated its core technology at both wave basin labs and in-ocean demonstrations, proving it can withstand real marine conditions and scale up. In September, the team tested the system at MARIN, one of the world’s leading maritime research institutes. These tests confirmed that the system can operate reliably in harsh ocean environments, clearing a critical hurdle as Sizable prepares to deploy its megawatt-scale pilot system.

Sizable Energy has also begun deployment of its next sea trial off the coast of Reggio Calabria, Italy. This pilot will validate all key floating components, demonstrate the full-scale assembly and deployment process, setting the stage for a multi-MWh demonstration plant in the Mediterranean Sea off the coast of Italy.

With this milestone approaching, the company is preparing for commercial project development in 2026 across multiple global sites, in partnership with local manufacturers, governments, and energy providers.

Additional investors include Exa Ventures, Verve Ventures, Satgana, EDEN/IAG, and Unruly Capital which led a previous round.