2 cpu g5, imac, sometimes an old pc or macbook, don't use electricity for much else besides light, varies but often 80-100. in berkeley. i wonder what's causing

2 cpu g5, imac, sometimes an old pc or macbook, don't use electricity for much else besides light, varies but often 80-100. in berkeley. i wonder what's causing the large difference. edit: i'm buying this http://www.amazon.com/P3-International-Kill-Electricity-Monitor/dp/B00009MDBU/ http://www.amazon.com/P3-International-Kill-Electricity-Moni...