Development [LWN.net]

Profiling the Power Usage of a Desktop PC

By Forrest Cook
December 17, 2008

Reducing the power usage of a desktop computer can bring about a number of benefits. Whether your goal is to save money on your power bill, reduce your carbon footprint or eliminate unwanted heat and noise from your office, a bit of effort can produce a more power-efficient computer. Effort spent reducing power can have an even larger effect on servers and other machines that run 24 hours a day compared to machines that are only on during work hours. This work was done on a nearly ten year old PC, but the process still applies to more modern hardware.

The test setup consisted of an opened-up desktop PC, a P3 International Kill-a-watt meter and a collection of peripheral cards and disk drives. The Kill-a-watt has a 1W resolution, if a reading alternated between 2 values such as 8 and 9 Watts, the estimated value was called 8.5 Watts. Some of the measurements made were small enough that they were "in the noise". Other variables included devices with inconsistent power usage and inconsistent line voltage. The resulting measurements were actual power used by the power supply, this may vary from the DC power used by the tested components. Lastly, the Kill-a-watt meter also shows power factor; a fairly consistent value of 0.67 was read.

The tests were performed on the machine while it was in a number of different software states. Many of the tests were done while at the BIOS prompt, disk drive and network adapter tests were done while the machine was running Linux (Ubuntu 8.10). Power consumed by external devices such as the LCD video monitor and amplified speakers was not taken into account. When a peripheral such as a disk drive was removed for a test, the drive was disconnected from power and the interface cable was removed to eliminate possible power consumption by bus termination resistors.

The tested computer used a fairly old, but still adequate Asus A7V333 motherboard with an AMD Athlon 1700 processor clocked at 1466 Mhz. The RAID option was not present on the motherboard. A pair of 256MB PC2700 DIMMs were used for the memory. The power supply was a 300W Antec PP-303X. Initially, the machine was loaded down with two hard drives, both CDR and DVD-RW drives, a floppy drive, an AGP video card with an ATI Radeon 8500 GPU, and both wired and wireless 802.11 networking cards.

The machine was shut down, all of the PCI and AGP cards were removed and the disks were disconnected. The first power test involved the PC2700 memory DIMMs. With no memory, power consumption was 72 Watts. Adding one DIMM caused the power to drop to 67 Watts. Your author guesses that with no memory, the CPU runs in some kind of power-consuming loop. Interestingly, the two DIMMs had significantly different power usage. The Kensington Value Ram with Hynix chips caused the machine to use 73 Watts versus 67 Watts with the generic Chinese RAM with unbranded chips. With both DIMMS installed, power consumption as 75 Watts. We can deduce that the Kensington RAM used 8 Watts while the Chinese RAM used 2 Watts. Sufficient RAM is critical for good system performance, the brand seems to be significant in the area of power usage. Tests with additional brands of memory seem to be in order.

Fans consume a fair amount of power. A quick unplugging of the noisy CPU fan caused the power to go from 75 Watts to 72 Watts, the CPU would melt down without this 3 Watt component, so it was left in place. It may be possible to find a more efficient CPU fan. The case had a front-mounted "push fan". This consumed around 2 Watts of power. The power supply's built-in fan provides plenty of air circulation so the front fan was disconnected. This also made the machine a bit quieter.

The floppy drive is virtually useless now that 4GB USB memory sticks can be purchased for under $10. The floppy drive consumes about one half Watt of power, so the savings are small. But big savings can come from many small cuts, so the device was left unplugged. The Asus CD-S500/A CDR drive was tested, it consumed about 1 Watt of power. The Sony CRX320E DVD-RW drive was tested, it consumed about 2 Watts of power. Most people can get by with a single removable media drive, or none at all. The DVD-RW drive would be the obvious choice for a single-drive system. If one can put up with the occasional inconvenience of rebooting, it should be possible to put a DPDT power switch on the back of the machine to allow shutting off the +5V and +12V lines to the removable media drive. All together, the floppy and two optical drives consumed around 3.5W when idle.

The Radeon 8200 video card was somewhat of a power hog, it consumed around 8 Watts of power with no built-in fan. A lower performance ATI-S3 AGP video card consumed 4 Watts. If high performance video operation is not critical, example: running Google Earth, the S3 card should be sufficient. As with sufficient memory, this sacrifice may not be worth the power savings.

The next part of the power test involved the fixed disk drives. The main boot device was a Western Digital WD600 60GB PATA disk. It consumed about 7 Watts of power at the BIOS prompt, power went up by about 5 Watts when the system was running Linux and the drive was active. Some of this power is likely being consumed by the CPU and memory and some is used to power the disk's head actuator motor. An auxiliary Western Digital WD2500 250GB SATA drive and associated SATA PCI adapter card consumed around 9 Watts of power when idle and also about 5 watts more when active. Interestingly, as the machine was more heavily loaded with drives and peripherals, system usage became less of a variable to overall power consumption. Hard drives are one of the more power hungry devices in a system, putting all of your data on a single drive is a good way to save power.

A generic-brand 10/100 Ethernet controller with an Intel chip consumed about 1 Watt of power at the BIOS level. Running Linux and moving a lot of data across the card caused the power consumption to jump by about 8 Watts, as with the disk drive test, a lot of that increase is likely caused by CPU and memory use. A Hawking Technology HWP54G 802.11 wireless Ethernet card also consumed about 1 Watt when idle and a few watts more when busy.

The fully loaded system with 512MB of RAM, two hard drives, two optical drives, two network adapters, the Radeon video the floppy disk drive and the front fan consumed about 108 Watts of power when idle and a similar amount when busy. When the machine was stripped down to one hard drive, no optical or floppy drives, the lower performance S3 video card and no front fan, its power dropped to 80 Watts idle and 88 Watts when busy, or between 74 and 81 percent of the original power consumption. This is enough of a reduction in power usage to justify the effort of testing.

Don't forget that even when it is completely powered down, the computer may still act as a phantom load, this system consumed a full 3 Watts when it was off. An easy remedy to that problem is to route the power plugs for the CPU, video monitor and speaker through a switched power strip.

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