The Computer and the Dynamo
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.
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.