You can leave for now 1300W and other customers will probably measure how much need for 2-3 K|NGP|N cards.
I read somewhere guy with same 1300 G2 didn't had space with 2xGTX780Ti K|NGP|N under water and overclocking with
Classified Voltage Tool.
But he probably try to pull less MHz from card how much water-cooling allow.
Anyway you will not have problem to turn and use cards and overclock them and than you can consult someone who will check on wall how much need exactly.