Leaving computers on overnight = $2.8 billion a year

Admittedly I don't think much about it at all. I leave my laptop running overnight because I know it'll take five minutes or more to get things going in the morning -- not just booting up, but launching the various apps I start the day with, downloading my overnight email, filtering out the spam, and otherwise "getting settled."

But all the power wasted while computers are sitting idle overnight adds up, and one study has finally tried to measure it. The tally: An estimated $2.8 billion wasted on excess energy costs each year in the U.S. alone.

On a CO2 basis, that's 20 million tons of carbon dioxide, about the amount produced by 4 million cars on the road.

The full report is available for download here (scroll down to "PC Energy Report US 2009").

But big numbers like that become almost meaningless in an era of trillion-dollar bailouts, so to put the wasted energy in perspective, the study provides the data in terms you can better understand: If you run a company with 1,000 PCs left on overnight, you can save about $28,000 a year if they are turned off after hours. That's not chump change.

Of course, it's also a fact that your PC will function better if you restart it regularly, and nightly shutdowns can help you avoid having to suddenly reboot in the middle of the day when you'd otherwise be productive. So even though this little laptop, by my math, eats up only about a quarter's worth of power overnight, maybe it's a smart idea -- and ultimately a time-saver, too -- to shut it down after hours after all.