There is no processor on earth that you can buy that will give you similar performance and justify it's cost in power savings by the time the processor is no longer useful. The cost to purchase the CPU is just too high, and electricity is just too cheap. The math does not work out.
This is especially true with you living in a colder climate. Every watt that your CPU uses becomes heat. That waste heat heats your home. If you also heat your home with electric baseboard or forced-air heaters, this discussion becomes even sillier. It doesn't matter where a watt of electricity becomes heat, it converts and costs exactly the same.
Doing some rough back of the napkin math, where power costs $0.20/kwh, you are pumping the heat directly outside because you don't want it, and you save an unbelievably optimistic 30w per hour running the machine every hour of every day, you break even on a $200 CPU at about 3.5 years in to it's life. A) A 3.5 year old CPU is reaching the end of it's life anyway, B) these numbers are super-generous in favor of proving out positive power savings. Back in the real world, where you probably use the machine a maximum of 6 hours each day, and actually want the majority of that waste heat, you'd see a cost savings in like 15 years at the earliest, but probably closer to 60 years living in a cold area of the world. Go ahead and pick up the 45w Core-i7, then make sure to come back in 2072 when you do your next upgrade to let us know how it worked out for you.
So, no. Do not change CPUs purely for power savings. That is not a smart idea. Instead, the next time you upgrade for performance reasons take power efficiency in to concideration.
Math. It's what's for dinner.