Sure, go ahead.  You will definitely save a little energy.  The key word is that sentence is “little”.

First of all, computers aren’t the energy hogs that many people think they are.  The laptop I use at work has a 90 watt AC adapter, and the Mac Mini I have at home has a 100 watt AC adapter.  It’s important to note that these are maximum wattages.  Most computer usage doesn’t come anywhere close to the max, and when it does, it’s for short bursts.

Electricity usage is measured as a kilowatt hour (kWh).  The means that a device that has constant usage of 1000 watts would use 1 kWh every 60 minutes.  The cost per kWh varies, but is generally in the ballpark of 10 cents.  This means my Mac Mini costs about 1 cent per hour to operate when it is at its maximum consumption rate (100 watts = 1/10 kW).

But that’s not even what we’re talking about. I wouldn’t save the entire 0.1 kWh by powering off the computer.  Like most users, I’m going to put my computer into sleep mode at night.  Sleep mode typically uses between 1 and 5 watts.  In the case of my Mac Mini, 1.39 watts.  I wasn’t able to find this information on my exact model of laptop, but I suspect that its usage is similarly low, especially in “hibernate” mode.

Is it worth the effort to shut down your computer to save a few watts per hour?  Let’s assume that your computer uses a relatively high 5 watts when in sleep mode.  Let’s also assume that it is idle 75% of the time.  That’s 126 idle hours per week, or 6552 hours per year.  Multiply this by a usage of 5 watt per hour, and you get a savings of 32,670 watt hours, or 33 kilowatt hours.  Congratulations – you just saved $3.30.  Have a Mac Mini like mine, and the savings is just $1.25 per year.

This makes even less financial sense for businesses, which are paying their employees to shut down the computer at night and restart them in the morning.  If we assume 1 minute of lost productivity per day for 250 work days, this is more than 4 hours of lost productivity!  Unless you’re paying your employees less than 90 cents per hour, you lost money on this exchange (assuming the $3.30 in energy savings).

Of course, you could argue that every little bit of saved energy adds up, in terms of environmental impact.  This is definitely true – and if you want to conserve every little bit of energy, then go ahead and shut down the computer.  However, there are a lot of other changes that give you a lot more bang for the buck.  If you have a 60 watt incandescent bulb in a lamp you use an average of two hours per day and switch it out with an equivalent compact fluorescent bulb (using 13 watts), you save 47 watts per hour of use – or 34 kWh during the course of a year ((34 X 2 X 365)/1000).  Yep, taking a minute to switch out a bulb saves as much energy as shutting down and restarting your computer 365 times!

I’m not suggesting that we waste energy.  I’m not even suggesting that we shouldn’t power off computers and other unused electronics.  I’m simply suggesting that if you’re willing to put the effort into this very small energy savings, you might also focus some energy on things that save even more energy.