A whole-house UPS is a fairly non-trivial affair. As you mention, there is a generator involved, but also batteries to feed the inverter which would carry the load whilst the generator got up to speed.
Another, often overlooked aspect, is the speed of the reaction necessary by the equipment switching the electrical load from the grid feed to the inverter feed. In many cases, it is too slow to be truly uninterruptable. Ideally, below 10ms, which is outside the capability of Tesla's Powerwall, Generac's offering, or Sonnen's gear.
