Keep in mind the power supply is a measure of the maximum DC power it can deliver to the components inside the PC. A 300 watt power supply that's 65% efficient would actually draw 460 watts A/C from the plug at 100% max load. Having said that, most folks don't realize just how little power most PC's use. Although mine has a 200 watt power supply, it only draws a fraction of that amount at the plug, which means the power supply isn't even breaking a sweat. I'd guess your linux systems probably are no more than 60-100 watts depending on how busy you keep them loaded up, CPU-wise.