We measured the power consumption of our entire test systems, except for the monitor, at the wall outlet using a Watts Up PRO watt meter. The test rigs were all equipped with OCZ PowerStream 520W power supply units. The idle results were measured at the Windows desktop, and we used 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 Managerso four for the Pentium XE 840, two for the Athlon 64 X2, and so on.
The graphs below have results for "power management" and "no power management." That deserves some explanation. By "power management," we mean SpeedStep, PowerNow!, or Cool'n'Quiet. In the case of the Pentium XE 840 CPU, the C1E halt state is always active, even in the "no power management" tests. The Extreme Edition 955 and the P4 Extreme Edition 3.73GHz don't support the C1E halt state or SpeedStep. We have omitted the Pentium D 930 and 950 processors here because we don't have actual samples of these individual chips; our "simulated" versions with an underclocked Extreme Edition 955 are fine for performance testing, but not for power consumption.
Generally, systems based on recent AMD processors have consumed quite a bit less power under load than the competing Pentium boxes. That's certainly the case again here. Despite its lower clock speed, the Opteron 165 requires more power under load than the Athlon 64 X2 3800+, perhaps due to its larger onboard cache eating up more power.