What you really want to know about is PUE.... a measurement of power/cooling use vs Server power use.
Power to Useful Equipment (PUE)
PUE used to rate data center designs, and for traditional hardware you get get approximately 2.6
which means that for every watt of power going to your server, 1.6 watts get used on plant and cooling.
Now.... taking some really typical stuff, a Dell PowerEdge R710 ( their green model) will use 90-120 watts at idle. At 100% CPU its more like 230 watts. It costs $1299 US.
Now at 0% CPU that will total up as 100W from the Dell, and another 160W from plant.
So you spend 260W, 24x7 for no computing.
assuming power at 10c per kw/hr ( more pricey in say, california, more like 15c there)
that is $4.36 per week; $227 per year; And that's at idle!
Now at 100% cpu, the total load is about 680W. Most of that load is inefficiency in the infrastructure and air cooling.
That cost you $11.42 per week, or $594.00 per year.
So in two just over two years, at 100% CPU the server would cost more in electricity than it cost to purchase.
At which point a new server (same $) would have more than twice the processing power for the same power consumption
It's a really good idea to replace equipment more often than you might think, to get back to more efficient hardware.
Where I work, we do offer an energy consultancy to data centers.
Free advice: virtual machines cost less. Compress idle-ish machines into VM servers,
Get a VM server that will automatically turn on and turn off physical servers and migrate VM's around as load changes.
This can save a fortune!
People spec system much better than PUE of 2.6. A PUE of 1.2 is quite achievable, but is not a plain old industrial building with racks of servers and big air conditioning units.
The really picky people start looking at tPUE, where you consider power lost in the PC PSU , CPU voltage regulators and cooling fans as "wasted" instead of useful.
James Hamilton is a big blogger in this space, he has a great article over at