Just to give a real-world example of how I use my kill-a-watt P4400:

The 10+ year old 3com 10/100 network switch I used at home was getting noisy, and I realized that it was probably not a bad time to upgrade it.   Power wasn’t specifically on my mind when I went to do the upgrade, though it did occur to me mid-process when I noted that many of the home network switches were very low on power draw. I’ve ended up replacing  10ish year old 3com 24 port 10/100 switch that was built for enterprise use with a 5 port 10/100/1000 switch that was built for home use.   How this has played out power-wise with measurements from my kill-a-watt:

Now, 41 watts may not seem like much of a difference, but it can add up to real money.   When all is said and done, I pay about 18 cents a kilowatt-hour, including, generation, transmission, and all the assorted fees that go with it.

The basic math breaks down like this:
45 watts (3com) - 4 watts (Trendnet) = a power savings of 41 watts.

41 (watts) * 24 (hours in a day) = 984 watt-hours

984 (watt-hours) * 365 (days in a year) = 359160 watt-hours per year or 359 kilowatt-hours per year.

359 kilowatt-hours * .18 cents per kilowatt-hour = $64.62/year

or, to refer to the above chart: $70.92 - $6.30 = $64.62/year.

So, by investing $19.99 in a newer, better, quieter, switch, I’ll be saving $64.62 a year.   I find myself doing these sorts of calculations on many things around the house.    I’m a big fan of old and reliable, but there is a place for new and cost-effective too.     I’m in middle of a slow and gradual process of replacing all of my old CRT monitors (which can often draw 100-150 watts) with newer, smaller LCD monitors ( the 19″ LCD on my desk draws 30 watts).   If you run the calculations above, you can see that it can add up to real money fairly quickly.  To be fair, monitors are a little trickier, since they may not be on all the time.   My network switch is though, so $64.62 is real savings to me, and the switch itself will be paid off fairly quickly.