Have you ever used a Kill-A-Watt meter? Not sure if it would help your situation.... Measuring the CPU power use will be difficult.
http://www.amazon.com/P3-International-P4400-Electricity-Monitor/dp/B00009MDBU
It gives AC Watts/Amps/Volts at the plug. It is a handy device to have around the home.
EDIT: Just realized you're in NZ... Likely use 220V down there... I am uncertain if they make one for balanced power.
peace,
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