When Should I Turn Off My Computer?

Occasionally, people ask how long they have to be away from their machines to make turning it off worthwhile. From the perspective of energy savings, if you are going to be away from your computer from more than 8-15 minutes, you should turn it off. However, the energy concerns are only part of the equation.

To answer this question, we have to find the point at which the extra power consumption required to shut down and restart a machine is offset by the power saved. If the machine is off for any period of time longer than that, we've saved energy.

Modern computers use much more energy when they are doing work than when they are idle. So, to find the breakeven point, I started my typical set of applications: Firefox with a couple of tabs, Microsoft Word, a media player, and Visual Studio. Then, I measured the power consumption using a wattmeter while I shutdown the machine, waited for a little while, then turned the computer back on and restarted all the applications.

Here's a graph of the energy usage on "both sides" of a shutdown and restart:

What you see in the graph is pretty typical, starting at the left of the graph, you see some idle time, followed by some activity caused by the shutdown processing, then some off time. Then you see the startup processing and finally some idle time.

Note that, even though the machine did not return to the idle power consumption for 7 minutes, the computer was usable long before that.I ran this test on a computer that's an energy hog. The hog takes about 30 seconds to shutdown and has peak wattage of 183 watts. It also takes 7 minutes (60*7 = 420 seconds) from startup to idle with peak wattage of 230 watts. The idle wattage is 115 watts. Let's just estimate by taking the shutdown cost as (0.125 hours)*(230 - 115) watts = 14.375 watt hours. The off wattage is 0.7. Therefore, the breakeven point comes when the amount of saved energy is equal to the shutdown energy. In numbers, when 14.375 = 114.3 * h, where h is the time the machine is shutdown in hours. For the energy hog, this works out to be about 7.5 minutes.

Therefore, for this machine, anytime you are going to be away from the machine for more than 7.5 minutes, you can save energy by shutting the machine down.

Some of the most efficient machines available use much less energy but it's safe to assume that, if you're going to be away from your computer for more than 15 minutes, you're probably better off, from an energy perspective, in shutting it down.

So, if you are managing large groups of machines you might consider using software that saves energy and mitigates the productivity losses by actively managing machine power states.