I did some measurements on two of my devices (no display, no keyboard). The power supply on each device is a Thermaltake 850 Watts XT of the 80plus series which guarantees a high electrical efficiency.

One device is running an i7 920 at 3.5 Ghz (the standard WCG crunching speed) with 6Gb DDR3 RAM clocked at 2000 Mhz.
I measured the power consumption at the wall socket (WSM) with a precise instrument made for that purpose and got 246 Watts or 2'154 kw/hr /year.
Overclocking is the culprit for such a high consumption. A 920 is rated at 2.66 Ghz so at 3.5 it is overclocked by 31.5%. Pushing it to 3.7 Ghz has uneconomic power consumption figures and high core temps.

The other device is running an i7 950 with 6Gb of DDR3 RAM clocked at 1'600 Mhz. I did two measurements:
at 3.2 Ghz, WSM was 160 Watts or 1'401 kw/hr / year
at 3.5 Ghz, WSM was 200 Watts or 1'752 kw/hr / year
at 3.7 Ghz (WCG standard crunching speed) WSM was 222 Watts or 1'944 kw/hr / year.

An increase in frequency from 3.2 to 3.7 is of 15.6% has resulted in an increase in power consumption of 38.7%. The last Mhz become expensive. The stock speed of the 950 is 3.0Ghz. So 3.7 is an increase of 23.3%.

This is a costly matter. If we now multiply this for many devices and running 24/7 then overclocking means cash burning .

On that basis it is more economical to run a 950 at 3.7 then a 920 even at 3.3-3.4. But is it really?
The cost of a 950 is nearly the double of a 920. On a continuous consumption let see what is the difference.

In Switzerland (allways more expensive then in the US) I have the following figures:
Retail price for 920 is 350 US$ and for 950 is 680 US$. Difference is 330 US$.
A difference of 46 watts means 402 kw/hr / year which boils down to 74 US$/year. My energy costs are at 0.186 US$ the kw/hr. It would take 4.45 years to payback the difference. That is too long in this business.
But if we take the energy cost that Sek mentioned (0.55 US$ / kw/hr) then payback is in 1.5 year which is a very acceptable figure for a powerful CPU.

So 950 is surely the best solution for crunchers in places where energy costs are high.