Depends on the tractor, but in general, PTO generators are not as efficient with regards to fuel as a stand alone genny. This is pretty obvious when you consider that the tractor engine involved may be 25-70 HP, while a typical genny is less than 25 HP (often a lot less).

The PTO genny gives you easy portability and they tend to crank out a larger amount of power than the smaller units, so they do have some uses. Just make sure you have lots of fuel.

Greg