Sizable Energy Secures $8 Million to Launch Ocean-Based Long-Duration Energy Storage Technology
Sizable Energy, the Milan‑based clean‑energy startup developing ocean‑based long‑duration energy storage, has raised $8 million in funding in a round aimed at accelerating the commercial deployment of its innovative offshore pumped hydro system. The financing, announced in October 2025, comes as the company looks to bring its patented gravity‑based storage technology from successful lab tests into larger sea trials and eventual grid‑scale applications.
The funding round was led by Playground Global, a venture capital firm known for backing deep technology startups, with participation from a syndicate of specialized clean‑tech and early‑stage investors. Participants in the investment include Exa Ventures, Verve Ventures, Satgana, EDEN/IAG, and Unruly Capital, the latter of which had supported Sizable in an earlier round.
Sizable Energy’s technology repurposes the well‑understood concept of pumped hydro — traditionally used in onshore reservoirs — for deployment in the deep ocean. By pumping dense saturated sea‑salt brine from the seabed to a surface reservoir, the system stores energy using the gravitational potential of the brine column, and then generates power as it flows back down through turbines when electricity is needed. This approach is designed to overcome the geographical and environmental constraints that limit onshore hydro storage and to offer a modular, scalable, and cost‑effective solution for long‑duration energy storage.
In statements accompanying the financing announcement, Sizable Energy’s CEO and co‑founder Dr. Manuele Aufiero emphasized the critical role of long‑duration storage in enabling the energy transition, noting that without economically viable solutions to store energy over extended periods, grids powered increasingly by intermittent renewables could struggle to maintain stability and reliability.
Playground Global’s general partner Bruce Leak, who also joined Sizable Energy’s board as part of the financing, highlighted the strategic importance of ocean depth as an almost unlimited resource for energy storage. According to Leak, leveraging the ocean’s natural vertical space could allow Sizable to deliver gigawatt‑scale storage at costs well below those of battery‑based systems, positioning the company to play a major role in future clean‑energy infrastructures.
The fresh capital will support Sizable Energy’s ongoing testing and demonstration efforts, including the company’s next sea trial off the coast of Reggio Calabria, Italy, in the Mediterranean Sea. Prior to the funding, the startup successfully validated its system in a controlled offshore wave basin at the Maritime Research Institute Netherlands (MARIN), confirming that its components can withstand harsh marine environments — a key milestone on the path to commercial readiness.
The upcoming trial will not only test the floating assembly and deployment process at sea but is expected to inform the design of a multi‑megawatt‑hour demonstration plant scheduled to follow. If these demonstrations are successful, Sizable plans to pursue commercial project development in 2026 across global sites, collaborating with local manufacturers, energy providers, and government partners to scale its long‑duration storage solutions.
Industry analysts estimate that the world could require up to 120 terawatt‑hours of long‑duration energy storage by 2040 to support the widespread adoption of renewables and ensure grid reliability. Traditional pumped hydro accounts for most of today’s bulk storage capacity but faces long build times, high costs, and limited suitable geography. By contrast, Sizable’s ocean‑based model aims to be invisible from shore, rapidly deployable, and adaptable to a wide range of coastal locations.
With this latest funding round, Sizable Energy is positioning itself at the forefront of next‑generation energy storage technologies, offering a potentially transformative path to balance variable renewable generation and decarbonize electricity systems around the world.
