IMO this question is simply unanswerable in the general case.
On my desk I have one of these new-fangled Core i7-980x 6-core TurboBoost enabled CPUs. If you load up a single core on this machine, it will boost the clock speed of that core, thereby increasing the power consumption of the CPU, so that will make for a huge difference. This CPU as well as other, more modern CPUs can partially de-power inactive cores, increasing the power savings. That's not as true on older CPU models.
Keep in mind that the work that you have to do with one core will necessarily be different than the work you do with multiple cores. If it's the case that you can spread the load out onto multiple cores, you'll find that a lot of extra time will be spent on the single core performing expensive context switches, and your performance will suffer.
Anyway, the problem has too many variables to really answer. One could go on and on about the different types of loads, and anyone who tries to benchmark to get an answer will end up (probably inadvertently) making apples-to-oranges comparisons which don't apply in other use cases.
Oh, and one more thing. Most people will probably quote efficiency in terms of power. You don't want to fall into that trap. Since the time to process the loads will necessarily be different depending on how the loads are balanced, you must integrate the result over time to find the net amount of energy used to complete the jobs in order to arrive at a credible result.
+1 Really interesting question! Here are some links http://www.tomshardware.com/reviews/truth-pc-power-consumption,1707.html http://www.tomshardware.com/reviews/intel-cpu-power-consumption,1750.html however they don't really address your question. I have C2D T8100 and one of these days I'm going to do some testing to see which type of load makes more heat.
– AndrejaKo – 2010-08-03T13:18:28.9704 cores at 25% is definitely going to spread the heat out better than 1 core at 100%. – Brian Knoblauch – 2010-08-03T13:41:43.283
If the amount of work is constant across the use-cases, piling up all the load on the same core is going to make it perform lower -- effectively taking more time and thrashing. You might loose the instantaneous energy savings when the same work requires longer time to complete. (+1, thought provoking question) – nik – 2010-08-03T14:01:09.183
@Brian agreed that using more cores spreads the heat out, but does it generate the same amount of heat in total? – Kevin Panko – 2010-08-03T14:11:01.503
AMD has announced CPUs that will be able to power down unused cores, allowin to run the active cores at higher clock speed – b0fh – 2010-08-03T21:27:32.123
Even if there is some power saving utility, I think you can't turn off the unused cores completely. So, if you still have some power consumption for idle cores, it somehow would seem logical to assume that 4 cores at 25% power consumption would be more efficient than 1 core at 100% and 3 at 1%, but I'm unsure if 25% load on CPU cores translates to 25% power usage. – private_meta – 2010-08-04T23:11:17.193
I am a bit confused regarding the heat. Theoretically, wouldn't a CPU basically run at the same temperature regardless of the number of cores doing th work? Also, what are the real advantages here? Are we talking savings at the consumer level, or only at a level where savings can scale up such as in a data center. – Jonathan – 2010-08-05T16:35:26.440
In terms of heat, a CPU dissipates almost all of its energy as heat. (Also, heat and temperature are different things, but that's a different discussion). Basically, the more power a CPU uses, the more heat it generates and the higher its temperature. New, more efficient processors use less energy and therefore produce less heat because they can partially depower unused cores. – David Markle – 2010-08-07T15:29:18.537