The genny doesn't use that much petrol for most jobs take an hour or so to do, also if you are using a powerful vac around 3-4 kw there is always the chance you are going to trip the custies electic.
We have a small clark vac that does most domestic properties, but on bigger jobs we use Kevins machine which holds a lot more waste.
The biggest problem we are having at the moment is the carbon fibre tubes sticking together . . . I have had three lengths stuck together on the roof rack