It depends on usage patterns. CPU power consumption tend to be non-linear as speed ramps up, consuming more as it approaches higher speeds. What that means is that a single CPU running at 2Ghz will usually use more power than 2 identical CPUs running at 1Ghz each. That's assuming the CPU was originally designed to run at 2Ghz.
Originally Posted by kelvinlin
Let's say you have an app, and it can complete a task in 5 seconds on a 2Ghz CPU. The same app, if designed to take advantage of multi-CPU processing, can theoretically complete the same task on two 1Ghz CPUs in the same 5 seconds. The advantage being, it would use less power because power use isn't linear.
Now, in the real world, there are a bunch of other factors that will make it impossible for the same task to be completed in the same amount of time. Things like badly written app, missed cache hits, memory bandwidth, out-of-order execution misses, and a bunch of others. Even with all that, the power savings is enough to make laptop makers use a dual-core 1,3Ghz CPU over a single core 2.4Ghz one.
Now, that's when the CPU is in use. The issue lies in when the CPU package is idle. Obviously a dual core CPU at idle consumes a lot more power than a single core one. Not exactly twice as much, since there are other things in a CPU package such as cache memory, and for mobile phone SoCs, GPUs.
That's not the end of story though. The more cores you have, the smoother your device will run. The is because if a core is busy with a task, and there isn't another one available, it has to wait until that task is completed. Multi-core computers tend to have less hiccups and the experience tends to be smoother overall.
Basically, the more you use a multi-core CPU package, the more advantageous it is from a power stand point to have it be used, over a CPU package with few cores.. Although double the cores can theoretically double performance, it usually doesn't happen. It's closer to 50-70% increase rather than 100%, although some tasks do show 100% increases.. Power usage can potentially be less on 4 cores than on 2, but it depends on usage patterns. If your usage pattern has it so that 2 cores are constantly running at close to top speeds, it's probably better from a power stand point to run 4 to keep the speed ramp up contained because CPU power consumption is not linear with speed.
That's the basics on effect on performance and power usage for multi-core computers. There's a lot I didn't get into, including the reasons why power consumption isn't linear, but that would turn this into a 10 page post.
Addendum: It is very easy to write code to shut down any number of cores. The LG Optimus G has the option to shut down 2 of it's 4. So basically, there's really no downside to using 4 cores over 2 if you use this option, with the obvious exceptions being additional cost, and additional space requirement in the device. Of course if you're Apple, you won't do it yet, because you figure that option is too complicated for normal people to understand.