If you really want to find out how much power you're using you need one of these:
Plug-In Mains Power & Energy Monitor (L61AQ)
http://www.maplin.co.uk/Module.aspx?...rce=1&doy=1m12
I bought one a few months ago. My system (tower, modem, router, monitor etc) uses 750-800 watts.
DW