The answer to this depends on a variety of factors, including how much energy the property consumes in a given day, the battery capacity power rating, what loads the battery will help support, the quantity of batteries and more. A battery or bank of batteries should be sized to keep your home running until the grid comes back online. When it comes to Solar + Battery for business, if your operations are energy intensive, you may need to pick and choose which systems are vital to be kept running when the grid goes down.