generator just to charge batteries

if you have a quality converter that will output 40+ amps, you'll likely find that a 1000-1500W inverter will overload when the batteries are deeply discharged. Converters draw lots more current than one would first guess. As an example, a PD 65A converter draws close to 13A @120V. My advice buy a honda or yamaha 2000W. cost isn't much more than the 1000W, noise level is virtually the same, and you won't risk overloading the generator.
If your converter is in the 45A range then a 1500W would probably work.