I have the fast charger plus the big 4 AH battery and use a 1000 watt inverter, which I think us a lot bigger than required plus I always charge with the engine running.
4 AH at 56 Volts = 215 watt hours
In a half hour charge that's 430 watts. Allow for 20% losses and you get about 600 watts. If you have the small battery and slow charger 250 watts might work fine.
Hope this makes sense.