This article goes into more detail than most people will find necessary, myself included. However, I was surprised to see that the actual power efficiencies of the three test systems were [over 80% at both idle and peak load]. I was expecting at least one end of the spectrum to have poor efficiency.
Ultimately, though, if you want to save energy costs, *turn off your devices!* If your devices are plugged into a surge protector, then pull it out of the wall socket after you’ve powered everything down. I plugged my surge protector into a [Kill A Watt] and was shocked to see how much energy was still being drawn by my ostensibly off computers and peripherals.
Kill A Watt Electricity Usage Monitor (Amazon)