To convert the capacity of a battery from ampere-hours (Ah) to kilowatt-hours (kWh), you need to multiply the capacity in ampere-hours by the conversion voltage (which is usually the nominal voltage of the battery) and then divide by 1000. The formula is:
kWh = (Ah * voltage) / 1000
Where the capacity of the battery is 6 * 220 Ah = 1320 Ah (ampere-hours), and the typical voltage for this type of battery is around 12V.