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Power consumption
We took our power readings at the wall outlet using an Extech 380803 power meter. Only the PC was plugged into the watt meter; the system's monitor and speakers, for instance, were not. The "idle" readings were taken at the Windows desktop, while the "load" readings were taken using SMPOV and the 64-bit version of the POV-Ray renderer to load up the CPUs. In all cases, we asked SMPOV to use the same number of threads as there were CPU front ends in Task Manager—so four for the Extreme Edition 965, two for the Core 2 and Athlon 64 X2 processors. The test rigs were all equipped with OCZ GameXStream 700W power supply units.

The graph below for idle power use has results with and without "power management." By "power management," we mean the dynamic clock speed and voltage throttling technologies from Intel and AMD, known as SpeedStep and Cool'n'Quiet, respectively. The Intel processors also have an enhanced halt state known as C1E. A processor's halt state is invoked by the OS whenever the system is able to sit idle for a moment. The C1E halt state in the Intel processors ramps down the CPU clock speed and voltage in order to save power, so even without SpeedStep, the CPU's idle power use is reduced. Keep that in mind when considering the "No power management" results for the Intel processors at idle.

Interestingly, we found that the Core 2's C1E state doesn't lower CPU voltage. The CPU multiplier drops to 6.0, bringing the clock speed down to 1.6GHz, but voltage appears to remain unchanged. Turning on SpeedStep, however, drops the CPU's core voltage, allowing for even lower idle power use.

Another tricky part about power consumption testing is getting good numbers for our "simulated" CPU speed grades. In order to make it work, you have to set the proper CPU core voltage, not just the right clock speeds. I made an attempt at simulating the Athlon 64 X2 models 4800+, 4600+, and 4200+ and the Pentium D 950/960 by setting the CPU voltages manually, but I've put an asterisk next to those CPUs in our results as a reminder that they're simulated. I didn't even bother including some simulated CPU models because of the difficulty involved and a few questionable results.

For the Athlon 64 X2 4800+, I set the voltage at 1.35V. The X2 4600+ and 4200+ were set to 1.3V. The "power management" idle scores were simply taken from chips with the same cache size (the FX-62 and 5000+, respectively), because all of these processors share the same 1 GHz/1.1V idle with Cool'n'Quiet.

The Pentium D 950 and 960 were trickier, since each Pentium D's voltage needs are programmed at the factory. In this case, I stuck with the default of 1.312V for both speed grades. On an 800MHz bus, the Pentium D 950 and 950 both clocked down to 2.4 GHz at idle via the C1E halt mechanism. The Extreme Edition 965 clocked down to 3.2 GHz at idle.

Our Core 2 Extreme QX6700 system uses about as much power, and thus generates about as much heat, as our Pentium Extreme Edition 965 and Athlon 64 FX-62 rigs. So it's not the Toyota Prius of the processor world, but it is remarkable that Intel was able to shoehorn two copies of the Core 2 Duo into the same basic thermal envelope as an Extreme Edition 965. If you want to talk about performance per watt—that fake shorthand metric for energy efficiency—then the QX6700 looks pretty good, provided you are using all four of its cores effectively. That's because of the performance side of that ratio; peak performance is much higher than the Extreme Edition 965 or the FX-62.