Electrical costs to run a home web server

[techlists at phpcoderusa.com]

Hi,

I just read this quote about the electrical costs to run a web server 
from home:

Cost: While it may sound cheaper to use that computer lying around doing 
nothing when creating your web server, when you factor in the cost of 
powering an old computer 24 hours a day, it can get very expensive. A 
250W desktop computer running 24 hours per day at 12 cents per KW/h is a 
whopping $262.00 per year!

---
I think their math is wrong.

The average residential electricity rate in Chandler is 10.85¢/kWh.

I'm thinking a low traffic PHP web server running on an old Dell with a 
400 watt power supply is not using but maybe 100 watts on average.  I've 
read that the computer should use no more than half the power supply 
capacity.  Is this correct?

If my home web server is using 100 watts an hour that mean 100 watts * 
30 days * 24 hours or 72K watts.

I'm thinking 72 * .1085 = $7.81 a month.

Any thoughts are much appreciated.