Last week, Shell became the latest supermajor to make a big bet on battery storage with the announced acquisition of German energy storage startup Sonnen.

Sonnen focuses specifically on household energy storage, a nascent market but one with a huge potential if ambitious renewable power plans by governments around the world work out.

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Household energy storage has not garnered as much attention as EV batteries, but it has a bright future as rooftop solar installations become increasingly popular in places like Europe, Australia, and North America and especially as the prices for such household storage installations decline. Yet, as Quartz’ Akshat Rathi notes in a report on Shell’s acquisition, the upfront costs of such installations are still high: the Tesla Powerwall, for example, costs around US$7,800 and that’s excluding installation costs, which can range from US$2,000 to US$8,000.

Despite the drawbacks, however, Shell’s announcement is the latest sign Big Oil is getting increasingly interested in batteries as part of its preparation for a more renewable energy future. Total, another European supermajor, has been expanding steadily in batteries. In 2016, it bought French Saft, a leading developer of batteries for various industries including energy storage, but also transportation and telecoms.

Read more: Oil Price