Whereas batteries will normally only power computers and comms equipment, the generator must be sized to cope with all ancilliaries.
A Rotary UPS is a combined generator and UPS, utilising a flywheel, electo-magnetic clutch and stato-alternator. The power to the computers is then totally isolated from the mains, thus eliminating spikes, harmonics and "brown-outs" and ensuring a "no-switch" change-over in the event of a mains failure.