C2D Power consumption and temprature

[twinclouds]

I have an interesting observation:
I tried to build a mini-ITX box using Zotak motherboard and Rosewill box from NewEgg. At first, I tried using E7300 @2667 MHz, and everything are fine except it does not support Intel Virtualizeation technology. Then I tried using E8400 @3000MHz. It has all of the features that I need but consume a little more power than E7300 and the junction temperature got hotter. I put a fan inside the box and the temperature is acceptable now. From the Kill a Watt meter, the box with E7300 consumes about 43 Watts (@55-60 degree c) and E8400 consumes 53 Watts (@45-50 degree c) when idle. All of these make sense. Then I tried another processor E5200 that I have. I though E5200 should consume less power but what I observe is that E5200 consumes almost the same amount of power as E8400.
In all three cases, the junction temperature are all proportional to the power consumed.
Can someone give me an explanation for this? The main question is why E5200 consumes more power than E7300 and hotter. Is this just due to the particular lot of the chip or it is in general true? For the same clock speed, does E7xxx series the most power efficient one?
Hope someone can give me an explanation.
(I am new here so I don't know if this question has been discussed before. Please let me know if it has and point me to the references.)

 

[Kari]

Have you checked the Vcore values for the different cpus? It seems there are slight variations with the default Vcore between the individual cpus even within the same model. Higher Vcore will cause the chip to use more power.

It would be interesting to test how low can you go while keeping the clocks at stock values and how it affects the power usage.