Windfall Energy Ltd

It plugs into a standard wall socket, charges when time-of-use electricity is cheapest and the grid is greenest, then feeds that energy back into the home's wiring at peak times so appliances draw on it before the grid. Setup is through an app and needs no installer.

Windfall Energy Ltd is based in London. It estimates that an average flat or small home will save around £250 a year, and its first batch ships only to UK customers.