Due to geography, I can't get DSL at the house. So, I struck a deal with my sister (who lives much closer to the center of town) to put a couple of PCs in her basement; alpha and beta are Dell PIII PCs running Linux as web servers. I thought that the electricity cost should be minimal, until my sister mentioned that she'd seen a good-sized jump in her light bill.
I did the math (using the load statistics from their UPS units) and discovered that these two PCs + 8-port switch + DSL modem cost $13.76 a month to run. Annualized that's $168. Ouch. This is where I got out my check book and started writing.
Alpha and Beta are equivalent to plain-Jane desktop PCs, so you can use that $84 a year per PC as a base number, but read on for the kicker...
The two servers I run here used to cost $260 a year to run. Over the past two weeks I upgraded both to new PCs with fewer but higher-capacity hard drives. Going from running a total of twelve disks to six knocked over $75 off the annualized cost.
In all of those examples the monitors are routinely turned off except when I'm standing right in front of the PCs. If you have a CRT monitor, that's probably the single most power-hungry thing you run. LCDs are a lot more power efficient: 2/3 less power and heat (which saves you twice if you run air conditioning during the summer). Going into Windows' power settings (hidden on the control panel for the screen saver) and setting it to put your monitor into power-save after 30 min, and spin down your hard drive after 30 or 60 minutes of inactivity is a nearly pain-free way to save some bucks on your personal PC. Nota bene: don't configure a server for disk spin-down. Bad bad bad.
Common sense should have clued me into the fact that what my mother taught me in the '70s about turning off lights applies just as much to PCs!