Sizable Energy secures funding for ocean-based long-duration storage technology commercialization

• Italy’s Sizable Energy raised USD 8 million to commercialize long-duration ocean energy storage, targeting affordable, safe multi-hour to multi-day capacity.

Italian startup Sizable Energy has closed an USD 8 million round led by Playground Global to bring its ocean-based long-duration energy storage technology to market. The concept: use the ocean as an “energy reservoir,” enabling multi-hour to multi-day storage at coastal nodes where land is constrained but renewables potential—and grid variability—is high.

While the company hasn’t published all engineering details, ocean storage concepts generally rely on pressure differentials or buoyancy: store energy by pumping water or compressing air into subsea vessels during periods of excess generation, then recover it through turbines when demand and prices rise. The appeal is durability and safety—non-flammable working fluids, robust marine-grade materials—and the ability to scale energy (duration) independently of power (turbine/convertor size).

Why it matters now: grids worldwide are discovering that short, one- to two-hour batteries alone won’t cover extended weather events, evening peaks that stretch, or multi-day lulls in wind. Long-duration technologies can firm renewables, reduce peaker dependence and provide black-start and inertia-like services if coupled with grid-forming power electronics.

The fresh capital will likely fund three tracks. First, prototypes and sea trials to validate round-trip efficiency, corrosion resistance, and maintenance regimes under real conditions. Second, permits and environmental assessments—marine projects must meet rigorous standards on fisheries, seabed impact and navigation. Third, commercial structuring: power-purchase and capacity agreements that value duration, not just energy, plus insurance frameworks tuned to maritime assets.

Challenges remain—marine logistics, weather windows for installation, and ensuring serviceability without costly vessel time. But if Sizable Energy can demonstrate reliable, bankable performance, coastal grids and island systems could gain a scalable alternative where land-based storage struggles to fit.

The broader takeaway: long-duration storage is diversifying fast. Ocean-based systems add a new tool to a kit that includes flow batteries, thermal storage and green hydrogen, each suited to different duration and siting constraints.