PSUs and efficency help

Laytonoid

Honorable
Dec 22, 2013
214
0
10,710
So, I just built a computer that will have 300w power usage. I haven't put a psu in yet. I have a 650w seasonic lying around that is bronze. Would this be inefficient to have 2x the PSU needed? At what percent of the PSU is a PSU running most efficiently? Also, there is a 400w platinum on sale I could get too. Don't know Much about the efficency stuff.

Specs..

Intel i7 4770 non k
EVGA GTX 750ti FTW ACX
12gb RAM
1 HDD and 1 SSD
ASUS Z87m plus motherboard
Sound Blaster Z card