PSUs are most efficient at 50% load....so a 1200 watt PSU is most efficient at 600 watts.
So yes, the 1200 watter will be very efficient when loaded at 600 watts but it will be grossly inefficient at idle.
To give you an idea a highly overclocked Haswell w/ twin overclocked 780s will draw up to 730 watts from the wall....that's about 650 watts of output power.
Now if ya buying a large PSU to save money, it's never gonna happen unless you live in an area with extremely high electric rates.
Seasonic X-1250 Gold $230
Seasonic X-850 Gold $155
On 625 watt load .....
X-1250 = (625 watts / 90%) x 36 hours / week x $0.10 per kwhr / 1000 watts per kw = $2.50 a week
X-850 = (625 watts / 88.5%) x 36 hours / week x $0.10 per kwhr / 1000 watts per kw = $2.54 a week
To make up for the $75 ya spent on the bigger PSU, would take 1,875 weeks or 36 years for you to break even.
looking at it from a bronze to Platinum standpoint.
Platinum 850 = (625 watts / 90.5%) x 36 hours / week x $0.10 per kwhr / 1000 watts per kw = $2.49 a week
Bronze 850 = (625 watts / 83.5%) x 36 hours / week x $0.10 per kwhr / 1000 watts per kw = $2.69 a week
Seasonic Platinum is $190
Seasonic Bronze is $110
At 20 cents a week .... will take 7.7 years to get ya money back and those are some pretty heavy usages. Precludes peeps with jobs, wives, kids, GFs etc
Yes, my PC is on many more hours that that but that's pretty much the max load most see.... right now while tying this the system pulling 118.7 watts from the wall (Kil-o-Meter reading) ... with 20% of that the water pump.
So electricity cost savings while a pleasant byproduct is not going to prove out as a reason to get a high "precious metal" rating PSU. What ya should focus on is that to get that rating requires high quality components which will have better voltage regulation, produce less noise, have greater durability, produce less heat .... and due to all of the above, can actually help you get higher oCs.