As suggested, use a Kill-O-Watt to determine exactlly how much power the PC will pull from the wall, at a typical load that it will be under.
Then do the math from there.
One other thing, if saving electricity is the main goal you may want to invest in a Gold rated power supply.
Without knowing the exact specs, it sounds like the system will be a fairly low power system, hich will limit your options a bit.
Seasonic has some low power Gold units starting at 400W and up.
Corsair also has good Gold units, but they start at 650W and up I believe, which is just too big, and way overkill.
The Seasonic for example is approx 88-90% effecient.
Which means it will use 88-90% of the power it pulls from the wall, resulting in less electricty used. It will essentially need to pull less from the wall to meet the PCs power demand.
This will be an investment now, but will save money over the long term.
It may or may not not be worth it to you.
For such a low power draw setup anyway, I would think it may not be worth it .. but it really depends on how well the one have is doing as it is.