# Coolbook, does 2128MHz vs 798MHz at same voltage use same power?

Discussion in 'MacBook Air' started by jamone13, Jun 20, 2011.

1. ### jamone13 macrumors regular

Joined:
Apr 20, 2010
#1
I have a 2010 13" Air with the 2.13 GHz CPU, I run coolbook with all frequencies set to the minimum 0.9250 Voltage. Since it runs stable at that minimum voltage, do I save power enabling 798 MHz frequency and other low frequencies vs just leaving 2128 MHz as the only option? Is there any way I can test the amperage or wattage currently being drawn to test this? IE would it be best to only lock the frequency at 2128 MHz?

2. ### Muncher macrumors 65816

Joined:
Apr 19, 2007
Location:
California
#2
Power consumed is dictated by the formula P = C*V^2*f (where voltage is V, f is frequency). It is directly proportional to the frequency. Whether your air will be more stable at standard or highly underclocked settings I cannot say. I recommend staying at the factory stock frequency just to be safe.

3. ### Phantom Gremlin macrumors regular

Joined:
Feb 10, 2010
Location:
Tualatin, Oregon
#3
As the equation shows, V is a squared term, while f is linear.

So if you increase f by 10%, you will increase power consumption by 10%.

However, look what happens with V. Let's say you start at 1.0 V and increase to 1.10 V. That's a 10% voltage increase, but 1.1 * 1.1 = 1.21 or a 21% increase in power consumption.

Is there any wonder why a few decades ago ICs ran at 5.0 V and now run at 1.0 V?

4. ### jamone13 thread starter macrumors regular

Joined:
Apr 20, 2010
#4
So when I lock my CPU down at 768MHz is that really underclocking it? Wouldn't it scale down to that speed when not in heavy use by default? Or is it locked at 21xxMHz, and does some other scaling not apparent to a user?

5. ### amarcus macrumors 6502

Joined:
Feb 26, 2008
Location:
London, UK
#5
That's somewhat the point. Coolbook overrides the default down-clocking of the CPU with it's custom clocks/voltage settings. So you would have to set a lower frequency within the program for it to be used.