I have an 850W powering 2 5970s, 3 out of the 4 cores are overclocked, ones just too hot.
If my electric is billed at $0.117 KW/Hour I can find my monthly electric cost by .85 X .117 X 24 X 30 = 71.6
My KW, X electric cost, X hours in a day, X days in a month.
But if I move up to a 1250PSU I can theoretically OC more and get more use from everything, currently my computer freezes at an OC or 900/1000, but are stable at 800/1000.
I am not sure if any additional OC I can do with a 1250W is worth paying $105.3 a month.
Can an increase to a 1250 PSU from a 850 PSU generate enough OC ability to make up for $35/mo in electric?
Only if your OC limit is due to insufficient power manifesting as voltage instability or drops that causes the overclock to fail.
If the issue is with heat or just a lousy batch of GPU, upgrading your PSU won't help.
Lastly, a 850W PSU does not use 850W of power, neither does a 1250W PSU use 1250W of power. They just use whatever is needed by your system / efficiency of their design. So if they are equally efficient, they will use almost the same amount of electricity, with the 1250W actually using a bit LESS.