One startup’s quest to store electricity in the ocean

Summary

When Manuele Aufiero was a child, his parents would take him hiking along a reservoir in northern Italy. It wasn’t a typical reservoir, though. This one drained and refilled constantly, with pumps raising the water level when electricity was cheap. When nearby cities needed electricity, the pumps would reverse, turning into generators as the water drained out of the reservoir. The technology, known as pumped-storage hydropower, or pumped hydro for short, has been around for over a century. Such facilities are some of the biggest “batteries” humans have ever built. Globally, pumped hydro reservoirs store 8,500 gigawatt-hours of electricity, according to the International Energy Agency. Pumped hydro can generate electricity for hours on end, and the power plants have grown in importance as intermittent energy sources like wind and solar have become more widespread. But there are only so many places on Earth with suitable topography to host a pumped hydro reservoir. “I’m in love with pumped hydro,” Aufiero told TechCrunch. “It’s just not enough to keep up with renewables.” So Aufiero decided to solve that problem by moving the technology to the sea. He co-founded a startup, Sizable Energy, to turn his idea into reality. Sizable recently raised $8 million in a funding round led by Playground Global with participation from EDEN/IAG, Exa Ventures, Satgana, Unruly Capital, and Verve Ventures, the company exclusively told TechCrunch. The startup’s power plant looks something like an hourglass. Sizable’s concept specifies two sealed, flexible reservoirs, one that floats at the top and another that sits at the bottom on the seabed. They’re connected by a plastic tube and some turbines. Techcrunch event San Francisco | October 27-29, 2025 When power is cheap, the turbines will pump super salty water from the bottom reservoir to the top. When the grid needs energy, Sizable will open a valve, and because the water in the reservoir contains more salt than the surrounding seawater...