Sizable Energy has raised $8 million in new funding to commercialise its ocean-based energy storage system, designed to deliver long-duration power using gravity. The investment round was led by Playground Global, with participation from Exa Ventures, Verve Ventures, Satgana, EDEN/IAG, and Unruly Capital.
The company’s patented offshore pumped hydro system stores energy by pumping saturated brine from seabed reservoirs to surface tanks, exploiting the ocean’s depth to create gravitational potential energy. Once released, the brine flows back to the seabed through turbines, generating electricity. According to Sizable Energy, the approach combines the scalability of traditional pumped hydro with the flexibility of offshore deployment — avoiding the geographic and environmental constraints of onshore sites.
Recent trials at the Maritime Research Institute Netherlands (MARIN) confirmed the system’s ability to operate under harsh marine conditions, clearing a key technical hurdle. The company has now begun sea trials off Reggio Calabria, Italy, to validate full-scale floating components and deployment processes. A multi-megawatt demonstration plant in the Mediterranean is planned to follow, ahead of commercial projects expected from 2026.
“Our ocean-based system stores gigawatt-scale power affordably, making the grid more stable, resilient, and ready for the future,” said Dr Manuele Aufiero, CEO and Co-Founder of Sizable Energy.
Bruce Leak, general partner at Playground Global and a board member of Sizable Energy, added: “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.”
The company claims its design offers a lower levelised cost of storage (LCOS) than any current or forecast lithium-ion solution, while remaining modular and scalable — from one to hundreds of gigawatt-hours. It also uses widely available materials and existing maritime infrastructure, allowing deployment at depths exceeding 500 metres.
The need for long-duration energy storage (LDES) is growing rapidly as grids integrate variable renewable power. Analysts estimate global demand could reach 120 terawatt-hours by 2040 — more than ten times current installed capacity. Traditional pumped hydro provides most existing storage but remains slow to build and constrained by terrain. Ocean-based systems such as Sizable Energy’s offer a potential alternative route to scale, provided they can demonstrate durability, cost control, and regulatory acceptance.
With its Mediterranean pilot under way, Sizable Energy aims to establish partnerships with manufacturers, energy providers, and local governments to begin commercial project development within the next 18 months.
