While most energy storage firms hunt for large plots of land near strong points on the grid, Base Power went after something far less romantic: ordinary people's yards. The startup has raised another one billion dollars (around 900 million euros), less than a year after a previous round of the same size, and is now worth 13 billion dollars post-investment.

The numbers behind that are more interesting than the valuation. The company installs around 100 batteries a day and hopes to double that by the end of the year - that is nearly 8 megawatt-hours of new capacity every single day, requiring neither a new transmission line nor an industrial zoning permit. So far it has installed a total of over 500 megawatt-hours.

The model is subscription, not purchase, and that is where the real trick sits. In the Houston area Base Power charges 695 dollars for installation, then 19 dollars a month and 13.1 cents per kilowatt-hour - roughly the ordinary regional rate. The battery remains the company's property, and the company then sells the stored electricity back to the grid when demand spikes. The customer gets a day or more of backup power during an outage; the company gets a distributed virtual power plant paid for by the subscriber.

The new unit, called Base Core, is made at a factory in Austin, Texas, and stores 39.2 kilowatt-hours, considerably more than competitors offer. Customers can choose one or two batteries. The company currently operates only in Texas and Illinois.

Why is anyone throwing a billion at home batteries now? Because the American grid is growing again for the first time after decades of stagnation - and immediately choked. Demand from electrification and the race to build AI data centres has squeezed the system especially in PJM, the grid operator covering the largest number of data centres and parts of Illinois. When the grid cannot expand fast enough, somebody will sell the solution that requires no expansion.

The round was led by Ribbit, Addition, Valor Equity Partners and the strategic investment group at JPMorganChase, with participation from a16z, Lightspeed, Coatue, CapitalG, Thrive Capital and a whole row of other funds. The co-founder and chief executive is Zak Dell, and his father, Dell chief executive Michael Dell, did not take part in the round - a detail someone felt worth clarifying.