GE Ventures announced in June that it had bought a stake in Germany’s Sonnen for an undisclosed sum to boost its presence in the fast-growing market for battery systems built to store solar power.

“This is an extraordinary endorsement of Sonnen,” said one industry commentator. “It’s not just about an investor covering its bases so much as not to miss out on an opportunity that may happen, but one taking a stake in what it reckons will be a major player in the sector.”

GE’s unit is paying a “mid double-digit million-euro” sum for a minority stake in the company, according to a statement by the Bavaria-based Sonnen and reported in the press.

The company said other investors are considering taking stakes in Sonnen, whose ownership includes founder Christoph Ostermann and four private equity funds. Ostermann recently announced the firm was seeking an investment round of up to €50 million.

Sonnen claims to be Europe’s biggest maker of lithium-battery systems for solar storage. It competes with Tesla Motors Powerwall home battery and Bosch Power Tec. This year it claimed that it had shipped more sonnenBatterie than Tesla’s Powerwall.

Ostermann later was quoted in the press as saying: “With GE Ventures, we have not only partnered with one of the world’s largest power and water technology companies but gained a supporter for our vision of a sustainable and affordable energy future for all.”

“Sonnen is helping to reshape the energy industry,” said Jonathan Pulitzer, managing director at GE Ventures. “We believe in Sonnen’s vision. that’s why we are excited to be a partner to provide clean and affordable energy to all.”