I recently began a series of experiments on my desktop PC to see what benefits do I get from underclocking the CPU (and the penalties for overclocking). I want this CPU to consume as little power as possible because since we got this DSL connection, it has been on for almost the whole day.
I used 7z to force the CPU to it's consistent maximum load and used CPUID's CPU-Z monitoring software to check the clock speed and core voltage.
My CPU is an Intel E5200 on stock cooling.
Trial 1:
Bus Speed = 200
Multiplier = 12.5
Core Speed = 2500Ghz
Core Voltage = 1.312 V
This is the default configuration of this CPU.
Trial 2:
Bus Speed = 200
Multiplier = 6
Core Speed = 1200Ghz
Core Voltage = 1.320 V
Next, I tried the lowest possible multiplier for this CPU. As you can see, it reduced the performance but consumed a bit more power than the default configuration.
For comparison's sake, I also did the tests on an Intel Atom N550.
Core Speed = 997Mhz
Core Voltage = 0.950 V
Core Speed = 1496.58Mhz
Core Voltage = 1.113 V
