killerclick :
Well yeah $3 a month would be only $36 a year but why is it different from a CPU that performs the same and costs $36 more? Would you recommend one CPU over another in that case?
I pulled $3 a month out of my butt as an example of how futile the difference is, but if it seems significant to you let's look at the actual numbers:
The highest TDP of any of the CPUs is the Phenom II X4 940 with 125 W, the lowest TDP of any of the CPUs are the Clarkdales with a 73W max. That's a theoretical difference of 52W under full load.
Electricity is about 15 cents/killowatt hour in the most expensive parts of the US. that means each hour it costs 15 cents for 1000 watts of usage. So in the worst case scenario, under full load, the per hour cost difference between a Core i3 540 and a Phenom II X4 940 is 0.78 cents ((52w/1000w=0.052)x15 cents=0.78 cents/hour). Not even a penny.
Assuming someone games four hours a day, putting the CPU to full load, every day, for a 31-day month. The cost difference under full load is 0.78 cents x 4 hours, or 3.12 cents per day times 31 days, which equals 96.72 cents.
Almost a dollar. That's almost a dollar at the maximum theoretical limits, worst case scenario, gaming for four hours a day, every day. no days off, always full CPU load. Not even 12 dollars a year. And that's using the most expensive electrical rates in the country.
In real life, no CPU is pushed to it's max limits for four hours a day, even if the user games for four hours a day. With normal desktop usage past experience has shown us that we would expect to see a 10-20 watt difference because all of these CPUs will throttle down and sip the juice unless they're pushed. And it all becomes irrelevant when the PC is put to sleep.
And there are other factors. Which chipset do I use? What peripherals? What power supply? You can't easily apply a calculation to a CPU vs. CPU and say "this one costs less!".
All of this is why I have a hard time being concerned about power usage differences with CPUs, especially sub-$150 models.