A conservative estimate is that 9.4% of total US electric consumption is from computers. This includes data servers and PCs, but not printers, scanners, faxes and the endless rechargeable devices that access the Net.
Too many devices continue to draw power even when turned off. The best way to ensure that the power is really off is by having the power cords plugged into a power strip, then turn off the power strip.
Or get a smart power strip, and let it turn things off for you. Until devices get smarter and don’t draw unneeded power, that is.