Munjon...You can buy AC generators or DC generators. If all you need is to charge your batteries, a DC generator might be a good choice. AC generators let you run things like airconditioners, TV's etc. just like you were plugged into dockside. Assuming you have a battery charger that charges when you plug into shore power, that is exactly what your AC generator will use to charge your batteries. We have a 3 position switch that lets us choose dockside, generator or OFF for the AC needs of the boat. Our 8kw westerbeke uses about a quart an hour on average but it all depends on what you have running...
As to brands...I have good friends who are most dissatisfied with Panda after-service. Happy with my Westerbeke. Hear good things about Kubota and know nothing about Mase. Having previously had wind/solar...would not now trade my generator for anything! Unlimited ice and hot water...Spoiled rotten!!