The OptiPlex 755 we tested was relatively energy efficient: It used a frugal 2 watts when off and 3W under S3 sleep, so as long as you let the 755 enter sleep mode when idle, you'll save a lot of power. S3 sleep takes only a few seconds to wake up from, so unless your workers are financial trader types, they won't be too put off by having to wake the system up. While running in idle, with the desktop on the screen and no other apps open, the OptiPlex 755 used about 50W of power.
That's pretty good compared with mainstream desktops that expend about 70W to 80W in idle, but it's a little more than some other energy-efficient desktops that idle at 30W to 37W. This number is important, because PCs are essentially in the idle state when they are being used for Office tasks like word processing and e-mail. In the power-use scenario, when the OptiPlex is performing 3D rendering, the system consumes an average of only about 70W. That's about as much as an incandescent light bulb, so even under load the OptiPlex 755 is an energy-efficient desktop.