How is battery power calculated?

Watts = amps × volts. Watt-hours = volts × amp-hours. A 12 V 9 Ah battery stores 108 Wh. A 24 V 500 Ah bank stores 12,000 Wh, or 12 kWh. A 48 V (51.2 V nominal) 100 Ah lithium module stores approximately 5.1 kWh. UPS loads are usually specified in watts or kVA, so converting the load to watt-hours before sizing is the necessary first step.