Tom's test shows that Core i5 750 + LGA1156 draws 60W less then Core i7 920 + LGA1366 at full load. Since the test is done during CPU full load, then I guess that the i7 is using all of it's virtual 8 cores during the test, while i5 is using only 4 cores.
This looks like a major power difference for pieces of hardware that do about the same performance in most cases. However, the big difference in performance is on applications that uses the i7 hyper-threading, in which case the 920 may be 20% faster then the 750.
Now, I've found some results that shows that an i7 920 based system uses 70W less power when running 4 threads comparing to 8 threads.
This means, that Core i5 750 + LGA1156 draws about the same power as Core i7 920 + LGA1366 and has about the same performance when running 4 or less threads. The 920 is using more power when running more then 4 threads, but then it delivers much better performance.
My conclusion is that power efficiency is not one of the reasons to choose i5 over i7. What do you think ?
I do agree, the power consumption comparison should be between i7 920 and 860. However, those two processors are in different price scale. Most of the user will probably wonder between the 750 and 920.
OK, but comparing 860 to 920 will give you "clean" results of 95w 2.8GHz vs 130w 2.66GHz; both with HT, and the 860 with Turbo Mode. Thins way, you get to know if the 860 really is much more efficient (35w less, 200MHz more, higher Turbo modes).
Anyway, the i5 should consume less than an i7 920 with HT off, yet the 920 might have tighter power requirements than i5, so that the 920 without HT consumes less power than i5 which has more "loose" TDP requirements.