My ccomputer uses 15 watts of power. You'll save a lot more energy by addressing your heating, cooling, and lighting use rather than obsessing over your computer. For most people, their computers' energy use is not a significant portion of their total use, even if they use their computers a lot. Of course, you should absolutely make sure your computer is set to sleep automatically when you're not using it, because it's silly to waste energy, but your computer likely isn't even close to being the biggest energy-waster in your home.
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