You'll want an Inverter type generator. Take the peak amp draw of your on-board charger and multiply by 120. I have a 4 bank that draws 14amps @ 120 VAC which equates to 1680 watts.
Therefore, I use a Yamaha EF2000 and there's still a bit left for USB chargers (Cell Phone, Tablet, USB Speaker, etc) or even a laptop.