As I was going to demonstrate the Zonbu Linux PC at the the ACS Green ICT group, I thought I should check its actual power consumption. My first attempt was almost fatal, the second, showed is was much as expected and about one tenth that of a regular desktop PC.
Do Not Try This at Home
I set my digital multimeter to measure Amperes and prepared to attach it to the power terminals. There is then a simple calculation to convert Amps x Volts into Watts. But exactly what was the voltage? I thought I should check that works, so I connected the meter across the power terminals, neglecting to reset the meter first to measure voltage. The meter created a short circuit, there was a loud "BANG", a flash and a cloud of acrid smoke as the multimeter absorbed 10 Amps and saved me from serious injury or death.
Electricity Usage Monitor
I then purchased a purpose designed Electricity Usage Monitor this has a plug to fit into a standard wall socket and the appliance to be tested plugs into the meter. I obtained one for AU$39.90 with an Australian plug. The unit has a small LCD screen to show power measured and a few buttons. The specifications state it is accurate to +-10% or +- 10 Watts. Given that the device I am testing draws only a few tens of Watts, the measurements could be suspect.
To test the meter I plugged in some 60 Watt incandescent light bulbs, these were measured at between 61 and 63 Watts, suggesting the meter was within the 10% stated accuracy.
Connected my the Zonbu computer the power measured was between 14 and 19 Watts. The power supply is rated at 20 Watts, so this is a reasonable reading.
For comparison I measured the power for my Twinhead subnotebook computer. To make a fair comparison, I removed the battery. This gave a wider range of readings than for the Zonbu:
Booting: 34 W
Screen off: 12W
Screen on: 17W
Installing the battery increased the power consumption by 5 Watts. This was when the batter was charged, indiciating the charging circuit was drawi9ng a significant amount of power.
I will test a desktop computer when I have one to hand.