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COMPUTING SCIENCE

The Computer and the Dynamo

Brian Hayes

Meanwhile, the Home Fires Burn

After reading so many disparate claims about the wattage of PCs, I became curious about my own computers' contribution to the energy budget. If national statistics are hard to pin down, maybe I could at least figure out what proportion of my own household electric bill feeds my digital habits. To make a first crude estimate, I went around the house with a flashlight, crawling under desks to read the power ratings on nameplates. I knew that the result of this exercise would be an overestimate; in fact it proved useless even as an upper bound.

Figure 2. Measurements of computer energy use . . .Click to Enlarge Image

Adding up all the nameplate data suggested that the computer equipment in my home could draw as much as 2,400 watts. This is a fair amount of power; it would fully load a 20-ampere circuit. That the actual consumption can't be anywhere close to this level became apparent when I dug out the old utility bills. For the past two years the average electricity demand for the whole house was 868 watts—roughly a third of the nameplate rating for the computer equipment alone.

Obviously, I needed a better measurement technology. To this end I was aided by Ethan Brand, of Brand Electronics in Whitefield, Maine, who lent me a digital power meter that measures both demand in watts and cumulative energy consumption in watt-hours. After crawling back under the desk to plug in this instrument, I soon had a clearer picture. A computer rated at 400 watts actually draws about 50 watts in active use; in its standby or "sleep" mode the power consumption falls to 3 watts. The color monitor attached to this computer uses more power than the CPU—97 watts in active mode, 6 watts when sleeping.

Adding up figures for all of the monitored equipment, I found that the most I could manage to consume was about 700 watts—and I could get near that level only by having three computers simultaneously spit out pages from three printers, while at the same time I scanned a photograph and burned a CD-ROM. The typical wattage reading, at times when I was working, ranged from 150 to 170 watts. Note that this is right on target according to Koomey.

But that's not the end of the story. I left the monitor attached to the big bundle of power cords under the desk, measuring total energy consumption over 10 days. In 240 hours, some 18,540 watt-hours of electricity flowed through the meter. That works out to an average demand of 77 watts, which implies that computer equipment is responsible for some 9 percent of my electricity consumption.

Of course it would be foolish to extrapolate from my home office to the entire national economy. I have no reason to believe my experience is representative; on the contrary, as someone who writes about computers, I surely spend more time at the keyboard than most people do. But, the fact remains, the proportion of my electric bill that goes to bit-shuffling is far greater than I ever would have guessed. I had assumed that my power consumption would be so dominated by air conditioning, refrigeration, water heating and lighting that the computing load would barely be detectable. Now I know otherwise.








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