The peak volt-amps may be higher than the wattage. A kill-a-watt won't tell you that, though.
Usually no need to worry about that running a charger.
The volt-amps rating is because 100 volts AC at five amps can equal exactly ZERO watts if the load is only a capacitor. IOW, big load to the generator as zero watts converted to heat in the capacitor. IOW, all the power gets reflected back to the generator as a big load with a capacitor using zero watts.
It's called "apparent power" which is really no power at all, but can still be big load to an AC source. Such as a transformer, generator, etc. That's why AC stuff is often rated in volt-amps instead of watts. The 100 volts I mentioned above will still measure as 500 watts when we measure 100 volts at 50 amps, regardless if the load is reactive (zero watts) or resistive (500 watts).
BTW, some cheap generators have a very distorted waveform. Best to check on a scope. I use my 4KW Onan RV generator in my RV to charge my DS as the sine-wave is reasonably clean (still not as good as household current, but close enough):
A very distorted waveform could damage some sensitive equipment or not output as many USABLE watts (unless a pure resistive load).
-Don- Reno, NV