Energy cost is the same. A 65w PSU that is pegged will draw 65 watts regardless of what it drives. If the hardware is not being heavily tasked and the platform is lean, then there is only a difference on responsiveness but not on energy.
I can't believe I'm saying this, and I'm aware of the potential for innuendo here, but I am going to have to ask what you mean by "pegging" a PSU.
A 65W PSU can supply at most 65W, but it doesn't "draw 65 watts regardless". It will draw less than max power if it is using less.
This can be easily verified by using a power meter on the outlet. I have machines with 90W and 25W PSUs drawing only 9W and 4W respectively at idle.