If you add up all the capacities of the cells in parallel, that's your mAh of storage (divide by 1000 to get Ah). Then multiply by the nominal voltage of the string (this is dependent on your voltage settings from fully charged to discharged, then the median of those. If you use between 3.2 and 4.0V, then the nominal voltage is 3.6V; take that times 14 = 50.6V) and this will give you the kWh of storage.