Step #1 : Turn On EIST, C1E Or Cool'n'Quiet

The easiest way to reduce power consumption whenever an Intel processor is idling is by turning on EIST (Enhanced Intel SpeedStep Technology), or C1E if you have a newer Intel processor. AMD processors have a similar technology, called Cool'n'Quiet. All these features have one thing in common - they reduce power consumption and heat output by reducing the processor's clock speed and voltage whenever it is idling (not doing anything).

Both EIST and Cool'n'Quiet though require operating system support to benefit from this power saving mode. The good news is both Windows XP and Windows Vista support EIST. You do not need to install any driver for it to work. Cool'n'Quiet might require additional drivers to work in Windows XP, but I won't be covering here since I'm using an Intel processor. Sorry, AMD users.

Unlike EIST or Cool'n'Quiet, the C1E halt state does not require any operating system support to work. Whenever the operating system sends halt commands to the processor, the processor will trigger C1E and both voltage and multiplier will drop just like in EIST. The advantage is that it works independently of the operating system. You do not need to enable any power saving mode or make sure your operating system supports it.

Although all Intel Core 2 processors supports C1E or EIST, some models will not work as efficiently as others purely because of the CPU multipliers they support. Core 2 processors have a minimum clock multiplier of 6x, so that's the lowest that C1E or EIST can drop to.

If you have a processor with high default multiplier like the Core 2 Duo E6850 (9X multiplier), you will see a large drop in processor clock speed from 3 GHz to just 2 GHz (with the multiplier at 6X) when EIST or C1E is enabled.

On the other hand, if you have a Core 2 Duo E6550 (with a 7X multiplier) instead, the speed reduction will be much smaller when EIST or C1E is activated. In fact, the processor will only drop 333 MHz in clock speed, from 2.33 GHz to 2 GHz. That results in a much less significant drop in power consumption and thermal output.

The table above shows the difference in clock speeds when C1E or EIST kicks in for the two processor examples. If you want to read more about the different C-states, you can read the excellent article by charge-n-go here.

Although Windows will send halt commands to the CPU whenever there's no work to be done, you can further reduce the power usage with EIST or C1E. Take a look at the table on the right.

With an Intel Core 2 Duo processor running at 3.9 GHz, I can save a whooping 36 W just by turning on EIST. That's almost 20 % lower power consumption and thermal output when the processor idles.

Even when I tested the Core 2 Duo E6850 processor at stock voltage and a lower overclocked speed of 3.4 GHz, I can already save at least 18 W of power. That is an improvement in power efficiency of 11.5 %. Not bad at all.

For those who are worried about the spiralling cost of electricity, this tip alone can save you quite a bit of money. Using my overclocked PC as an example, enabling EIST or C1E would save me about 13 kilowatt-hours of power a month if I just run the processor at 3.4 GHz without overvolting. When running at full 3.9 GHz, I would save about 26 kilowatt-hours a month. Sweet!

Do take note that it is possible that these power saving features may reduce the stability of your overclocked PC. So, it would be good practice to re-certify your overclocked PC using your favourite stress-testing program, e.g. Prime 95. Personally, I have not encountered any problems with EIST or C1E.