If PC is idle its average CPU usage wouldn't be much about 5% but that can vary, it depends on what background tasks it decides to run, defrag can chew disk and ram while PC is idle, windows update can run the malicious software check (but only once a month), and obviously install updates but generally my PC (even with chrome running) is pretty much at 4% CPU usage or less, even if I am using it. Just moving mouse makes CPU usage go up.