A friend asked me how much it costs to have his office computers running 24/7. So, I used my current measurement interface and multimeter to measure how much current one of the systems drew in various states of use. Knowing the current draw I could figure out how much wattage the system was using in those various states of use.
volts x amps = watts
I found the Oregon commercial electric rate of ¢7.63 per kilowatthour from here and used the following formula to figure out the cost to run one system 24/7:
watts x 24 hours x 30.5 days/month = watt hours (Wh)
Wh / 1000 = kilowatthour (kWh)
kWh x ¢7.63 = cost per month to run computer 24/7 for month
What’s most interesting is that when the system and the monitor were powered off they were still drawing 9.6 watts! That’s ¢54 a month just for the privilege of having the device plugged in!
Wikipedia – Vampire Power