A battery in a phone lives and dies by weight and volume; a battery bolted to the electrical grid answers to a different master entirely, which is why the lithium cell that conquered our pockets is the wrong tool for the job. The knock against renewable power was never that it was too expensive. It was that it arrived on its own schedule, a flood of electrons at noon and nothing at dusk. Storing that surplus for a few hours was solvable; storing it for days, cheaply, at the scale of a city, was not. The batteries now being built for that task give up everything portability demanded. Iron-air cells that quite literally rust and un-rust on command, sodium-ion packs made from some of the commonest elements on Earth: they are heavy, slow and unbothered by it. Sitting still is the whole point: made stationary, storage stops being a luxury and becomes a schedule.