We're used to cars and their petrol engines.  Rubber tyres gripping the road
surface and pushing the car along.  Boats use oars to push them through the
water or propellors much as aircraft have.  Rockets work like a deflating
party balloon.  Energy.  Potential energy released through a propulsion
mechanism.

How heavy is a flying saucer?

We just don't know.  How much energy do you need say, in petrol terms to
lift a one tonne mass? And how much to perform accelleration?  An
electrogravitic mechanism needs energy too - the same amount of energy that
petrol would need to lift it.  But we wouldn't understand if flying saucers
use large block V8s for propulsion.  We need a power supply, that's all.