Power consumption was measured using a Voltcraft Plus Digital Multimeter VC-940, obtained from Conrad.com. With the optional USB adapter and recording software, the data stream from the multimeter can be captured on a PC and saved in Excel format for further analysis.
The Voltcraft VC-940 digital multimeter, which we used to measure power consumption. Here, it's measuring the power drawn by an OLPC XO (with 100 percent screen brightness and with the battery charging) and delivering a data stream over USB to recording software on a Toshiba notebook.
Typical workload test Our main benchmark was a typical workload test — 'typical' in this context meaning relatively low-end tasks using mainstream productivity applications that are likely to be undertaken routinely by shoolchildren in developing countries.
To this end, the test involved (1) typing a short (187-word) document into a word processor, (2) entering three columns of figures into a spreadsheet and creating a graph, (3) opening a browser and using Google to find the Computer Aid web site and then open it, and (4) going to YouTube and playing a video showing Nelson Mandela talking about the meaning of the word Ubuntu. From the resulting data stream, we extracted the following measurements: power draw when idle (at the OS desktop); average power draw during the workload test; peak power; time taken to complete the workload test; and total power consumed during the workload test (in watt-hours).
Output from the Voltcraft VC-940 digital multimeter for the Inveneo Computing Station running our workload test, under Windows XP (blue line) and Linux (red line).
The workload test was performed by a human rather than run from an automated script, so the time taken to complete the test is not purely a measure of system performance. However, we used the same human operator (CM), and on the systems where we performed repeat tests, we got a variability of around 7 percent on average.
Other measurements we recorded separately were standby power (with the computer plugged in but turned off), boot-up time (to the OS desktop) and browser launch time (to Google, set as the home page). Finally, for the three notebooks in the test, we recorded battery rundown and recharge times.
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