Transformer Bank Loads

if the transformer bank(wye delta) has a connected loads of (1 phase and 3 phase buildings ) how i can get the reserve capacity of transformer bank??

the method i made is ,i get the KW rating of all electrical facilities(lights, acu, etc) and i assume a unity power factor,(kw=KVA),,,and add all the KVA off all building loads that connected to the transformer banks.

formula: Reserve Capacity = Transformer bank KVA rating – (KVA off all building loads)

is this possible even the building has a 1phase and 3phase loads????