Eric Drexler, a great scientist and engineer who also recently started blogging, noticed something interesting about the average electricity rate in the US ($0.115 per kilowatt-hour): One watt for one year costs one dollar.
Why is this interesting? Because most of us aren't really good at estimating our energy usage and what part of our electricity bill comes from what. Even if you buy or borrow an energy monitor (like the Kill-A-Watt), all you'll end up with is a number of watts. It can certainly be useful; for example, you could find out that your computer is using more power than you thought and that it would be a good idea to put it in 'sleep' mode when you're not using it, or that certain electronics are using power even when they're in 'standby' mode (the oh-so-scary phantom power).
But once you know the wattage of everything around your house, what do you do with that information? Your toaster might use a lot more power than a lightbulb, but the light is on for hours while the toaster is only active for a few minutes each day. Will you need a computerized 'smart home' to keep track of everything? Start a spreadsheet and keep track of everything?
No, at least not if you only want to have a general idea of your energy consumption patterns. To do that, you can use Eric Drexler's simple rule of thumb: One watt, one year, one dollar.
So if you have four 100-watt lightbulbs that are on for about 6 hours each day (1/4 of the time), that's about $100/year using the average rate in the US (you can get a more accurate number by substituting your local rate). So by switching to 20-watt CFLs, you could save $80 a year on just those four lights.
If your desktop computer plus your monitor and accessories (printer, modem, speakers, etc) use on average 250-watt and you leave them on all the time, it's costing you approximately $250 per year. In a few short years, you'll have paid enough just to let your computer idle to buy a whole new one. I'm sure that, especially in this economy, this money could be better used elsewhere. So better learn about the 'sleep' feature (all modern operating systems offer it).
The beauty of One Watt, One Year, One Dollar isn't that it's so precise, it's that it's easy to use and it can give you a ballpark estimate of energy usage in your home. This is good because generally when people become aware of their consumption, they become more careful and try to reduce it. This works with hybrid cars that have big LCDs showing MPG, and I hope that someday all houses will have a way (on a website?) to see real-time energy usage and costs, but in the meantime, the easiest way to get an idea of where your electricity bill is coming from is with this rule of thumb.