Disabling cores does not save power. In fact, it is likely to use more power.
OS X supports SpeedStep, which automatically scales frequency and voltage to support the operation being performed. So if a core is not being used, it will not use (much) power.
But there is more:
Suppose you have a task which can be run in parallel, such as encoding video. If you have two cores, you can run it at least twice as fast with one. This means that the computer can stop using energy for other tasks needed for the process - processor overhead, RAM allocation, hard drive I/O, etc much sooner.
Also, while that long process is running, it has to keep switching all the other tasks the computer normally does. This wastefull switching itself is going to use power and resources.
See this StackOverflow thread for a longer explanation.
(Here is the Apple page on the Processor preference pane and how to disable cores from the command line.)