A battery helps you keep the electricity to yourself, and might well work financially. In terms of greenness it doesn't add any electrons to the world, if you didn't have the battery then someone else would be getting your electrons and a powerstation would be working less hard. It isn't a terrible idea, and lots of them with grid signalling could add stability and smoothing, allowing the grid to have less peak power. The iBoost immersion thing is actually rather anti-social. It is a big "screw you, not sharing my electrons", all your spare power goes into making hot water whether you need it or not, rather than going to the grid. Again it might pay for itself financially, but it isn't a green thing at all.