Yes, the Tesla Powerwall now comes with a back-up gateway. The gateway sits on the incoming electricity supply to the house, and if it detects a problem on the grid, a relay trips and your house is physically disconnected from the grid. Once you are physically disconnected like this, your battery can discharge to run the loads in your home, without causing a safety risk to workmen fixing the power cut on the grid. It provides you with extra security, knowing you'll always have power in your home.