RoyB is right. Generators are not very efficient charging devices. You can run a microwave on a generator (has to be at least 1500W then), but getting batteries to really full 100% is a royal pain. Doesn't matter whether it's 1000W or 3000W, when battery is nearing 90%, its charge acceptance rate decreases so it draws a lower current. So it takes long hours, with same noise and fumes as if it were running full power.
Power estimates made earlier are correct - if this is only for charging, 1000W would suffice.