Our monitoring goal is to develop a technique that will allow us to monitor power consumption of a desktop computer's mainboard and drives, as well as the actual AC energy draw. This will allow us to separate power consumption actually caused by system components from consumption due to inefficiency in the computer's switching power supply.
Initially, we used an AC plugmeter which wirelessly delivers power statistics to a PC. However, this meter does not return data in any sensible units. It was useful for initial verification of our hypothesis (that a desktop PC will use noticeably more power under moderate or heavy load than its idle consumption), but we needed a better solution to collect meaningful data.
To obtain more accurate data, we used a Hewlett-Packard model 34401A digital multimeter connected directly to the desktop's AC power cable. As the 34401A can measure AC currents up to 3A, and the PC's current draw never exceeds this amount, a direct measurement is possible, yielding more accurate results than inductive current measurement or other techniques. To automate the logging of results from this multimeter, we wrote a simple Linux shell script that communicates with the meter over RS-232. A copy of this script is available for download.
Actual DC power monitoring of a modern desktop is more difficult than the simple AC monitoring strategies based on monitoring current draw from the wall outlet. Modern power supplies supply 4 different voltages (+5V, +12V, -12V, and +3.3V) along with control signals over a 24 pin connector to the PC's motherboard, as well as an auxillary 4-pin connector for additional +12V and ground supply, and multiple connectors supplying power to internal components such as disk drives.
To accurately measure the power consumption on these 4 DC power rails, we are using several Hewlett-Packard 34401A digital multimeters. These are very accurate and high-speed multimeters which provide RS-232 and GPIB I/O. These interfaces can be used to remotely control the multimeter and obtain readings. We will use another PC to read data from the multimeters, using a software application currently in development. When this application is complete, it will be made available on this website. A detailed description of the interface protocol used by these multimeters is available in the 34401A User's Guide.