Generator is producer of Voltage and Current, we can 
connect load as such draining full load current or half 
load, even noload,but voltage remain moreover same.So it 
willl rated in terms of product of V*I, secondly it gives a 
electrical output for any Load(may Capacitor,Inductor & 
resistor),so it is consider as Procuct of Voltage & Current 
excluding powerfactor.
Whereas motor is machines coverts electrical Energy to 
Mechanical energy, so it is rated in KW