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My current processor is a 110W AMD X2 4600+. For fun, I was looking at a 35W Core i3 2100T. Looking around I couldn't find a direct performance comparison anywhere between the two, but it looks like the i3 would actually bring a (very) small performance bump vs the 4600+. The upgrade would also involve a move from 2GB DD2 to 8GB DDR3, so as long as I don't lose too much this will be a win, performance-wise. The price (cpu,mb,and ram) would come to about $310.

I really don't have any budget for upgrades right now, but I was curious: if I were to go ahead and get this, how long would it take for the energy savings to pay for the upgrade? Could I justify the purchase this year on energy savings alone?

I suspect not, and I know it depends on usage and local rates. But I'm hoping to be able to put a ballpark figure together for how much I could save in a year. As a reference, local rates are $.0933/kWhr

I'll start with a basic calculation that uses a few bad assumptions, and we'll see if we can refine it any:

Assuming both systems are running full-capacity, that means I save 75W/hr. I also have to dissipate that energy, so I really save 150W/hr, or .15kWhrs. .15 * .0933 = $.013995 save per hour I run the new system. .013995 * 24hrs/day * 7days/wk * 52wks/year results in $122.26 saved per year. So I'd need to run this for about 2 1/2 years to pay back the investment. And again, that's assuming 100% run all the time and an additional 100% cost for heat, neither of which are very good. Can we get even more accurate numbers?

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get yourself a kill-a-watt and actually make these measurements. google.com/products/… –  madmaze Jun 12 '11 at 20:58
    
I made interesting discoveries, my window fan on high uses more power than my AC on high fan and the AC moves more air than the window fan. –  madmaze Jun 12 '11 at 21:01
    
You'll spend more for heating your house :) –  oneat Jun 12 '11 at 21:36
    
Do you live in the tropics and never have cool or cold days? Otherwise you are inflating your energy cost by adding "100% cost for heat". –  sawdust Jun 13 '11 at 1:21
    
I'm not sure if there are any more accurate ways. –  Sathya Jun 13 '11 at 5:41
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