First, I measured my refrigerator, a Sears Frigidaire which came with the house. As it's energy use varies depending on whether its compressor is running, I used the Kill-A-Watt to monitor its energy usage for an hour. After an hour, it had used .110 kilowatt hours, so it used an average of 110 Watts. Assuming this is representative of a month's usage and at 15 cents per kilowatt, in a 30 day Month I pay (0.11*24*30*0.15 )$12 keeping myself in cold raspberry ice tea.
I also measured my TiVo (32W/$3.46), and satellite receiver (28W/$2.92) as they are also on all the time. My TV and speaker system are not on all the time (really, I swear), but assuming they were on 1/3 of the time then I am spending 0.33*(.090kW+0.037kW)*24*30*0.15=$4.50 per month for the energy used by my speaker and TV systems, for a grand total of TV room energy usage of around $11, very close to my refrigeration costs.
Which brings me to the MythTV in the basement. It's a Dell 2GHz Pentium 4. Just sitting there doing nothing it draws 90 Watts. If MythTV's preview panel is showing a thumbnail preview of 720p content, it draws 120W (the same as if it were actually showing 720p content), if it's 1080i content it draws 133W. If I turn on "CPU Friendly" preview, it draws 110 Watts. Yes, that's right, whether or not the monitor is on or not, whether or not the blank screen-saver is on, the preview is using a minimum of 20W. Assuming my MythTV spends most of the month doing "CPU friendly" previewing, it's costing me $2.16/month extra just for the previewing. Therefore, I turn off previewing. Previewing or not, the computer is still drawing 90 Watts just sitting there, costing $9.72/month.
My MythTV is attached to two monitors, a 15" Sony LCD, and a 20" Dell Widescreen LCD. While on, the Dell draws 40 Watts, and the Sony draws 20. KDE had been set to use the "blank" screen saver after 2 hours inactivity. You would think, this would cause the monitors to use less electricity; not according to the Kill-A-Watt. Assuming I neglected to turn off the monitors half the time, I was spending $3.25/month powering unwatched monitors. I opened the display control panel and told Linux to power down the monitors after 2 hours which will eliminate most of this expense. Also, I often neglect to turn off my speakers (24W on/12W "off"), indicating I'm spending $2/month powering unheard speakers.
Adding this all up, and my MythTV had been costing me $17 a month in electrical costs. Turning off previewing and having the system power down the monitors should bring this down to $12 which is precisely the same amount of money I spend on my refrigerator, and coincidentally is the same amount of power used by the upstairs TV room. Now if I could only figure out where the other 11 kilowatt hours a day is going.
[Update: My MythTV box has two internal hard drives. Drive /dev/hda is used when I boot into Windows or when I want to share files with Windows. Drive /dev/hdb is used when I'm running Linux. It occurred to me I could put the unused drive into standby mode (not sleep mode) using the Linux hdparm utility, doing so saved an additional 7W. So now my MythTV computer is drawing 83W instead of 90W; I hope it's just a bit quieter now.
The following line tells my unused drive to go into standby mode after 30 minutes. I'll have to figure out which command file to put this to make it automatic.
#sudo /sbin/hdparm -S 241 /dev/hda