Small correction, Most home batteries don't help you in a powercut, the Tesla Powerwall does have backup mode if you have the 2nd Gen gateway, which is pretty much all the ones installed in the last 12 mths. It even has something called storm watch where if they see a storm coming and therefore likelyhood of powercuts it will charge the battery to 100%, more use in the USA with their proper storms we very rarely get over here. Depending on how you house is wired you can choose to have specfic circuits protected by the battery in a power failure, so perhaps keep the lights and sockets on but don't support the oven and electric shower. A single powerwall can supply a 5kW load.