Read voltage and capacity together
Nominal energy in watt-hours is nominal voltage multiplied by amp-hour capacity. This is a starting figure under nominal conditions, not the energy guaranteed at an AC socket.
Explain the operating conditions
Allowed depth of discharge, temperature, discharge rate, battery condition and system losses affect useful energy. Battery chemistry and manufacturer limits matter. Ask for the operating assumptions behind a runtime estimate.
For an illustrative 24 V, 100 Ah bank, nominal energy is 2.4 kWh. Confirm the actual series/parallel arrangement before calculating bank capacity. Neither a unit count nor a nominal figure establishes usable energy or guaranteed runtime.