The computer you're reading this on may not seem like a huge energy waster, but the power consumption adds up when joined by the other PCs worldwide (Stamford, Conn., research firm Gartner estimates there are more than one billion). A study released last week puts a finer point on this assertion, reporting that U.S. workers waste $2.8 billion annually in energy costs by failing to shut off their PCs at the end of the work day. What's more, machines left on during off hours may emit up to 20 million tons of carbon dioxide (CO2) this year alone, roughly the equivalent impact of four million cars.

The 2009 PC Energy Report, (conducted by Harris Interactive and commissioned by London-based energy-management software maker 1E Ltd and the Alliance to Save Energy, a Washington, D.C., coalition of business, government and environmental organizations) says that nearly half of U.S. workers leave their PCs running overnight. Among reasons cited, according to the report: it takes too long to shut them down, people forget to turn them off or they deliberately leave them on so they can receive software updates overnight.

Harris collected online responses from 4,743 U.S. computer users over the age of 18, about half of whom were employed. The company also polled computer users in the U.K. and Germany about PC energy usage there and found that turning off PCs could save $1.2 billion in Germany and about $400 million in the U.K. this year.

Computers, data centers and communications networks are big offenders when it comes to climate-change-causing CO2 emissions, producing 2 percent of the world's overall emissions, the same as the world's air traffic produces, according to Gartner.

The debate over the potential benefits of shutting down PCs is ongoing. Alliance to Save Energy president Kateri Callahan said in a statement that powering down PCs not in use "can provide a simple yet effective way for businesses to reduce overhead costs and environmental impact." Others point out that today's PCs are highly energy efficient already and that placing them in sleep/standby or hibernate modes (rather than turning them off completely) cuts energy usage sufficiently while allowing the computers to come to life more quickly when workers return in the morning. (ScientificAmerican.com has covered this debate in the past.)

Desktop PCs (those with separate monitors) meeting the U.S. Environmental Protection Agency's (EPA) Energy Star qualifications use two watts or less of electricity in hibernate mode and four watts or less in sleep mode. Laptops qualify for Energy Star ratings if they use one watt or less in hibernate and 1.7 watts or less in sleep mode.

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