The switch over time is around 30 seconds. When your power is restored from the grid, it switched back seamlessly.
Fuel is a great question. I have never looked at the consumption. Mine runs off the natural gas line, so I have no tank to fill. With this setup I feel I am in good shape for short and extended outages.
This has always felt risky to me. Sure, your setup will work for a garden-variety power outage, but not when it really matters like an earthquake or other natural disaster.
My uncle lives in a place that loses power often. His system is propane. When the power goes out you might notice the lights flicker. I don't know how much fuel it consumes.
Fuel is a great question. I have never looked at the consumption. Mine runs off the natural gas line, so I have no tank to fill. With this setup I feel I am in good shape for short and extended outages.