How can I find out the estimated power use of my desktop computer?

At that point, I would think about using a power monitor to measure the load on the computer at any time.

You could hook up your computer or surge protector to something like Tweet-a-Watt and then keep track of the metrics from there on a per day/week/month basis.

I imagine you could use ACPI/APM to monitor some aspects (and mayhaps power, as well) of the different components of a computer.

And if you're knowledgable about electronics, you could always home-grow your own solution by attaching voltmeters and ammeters to different components to see i.e. graphics card load, hard drive load, etc.

some UPS devices have displays that track power consumption, too, and there may be ways with the USB connectivity and a program like apcupsd to poll that kind of information for graphing with something like Cacti.

otherwise you're looking at a wall outlet with a display, like a Belkin Conserve Insight F7C005q Energy Use Monitor.

Btw, powertop will work with desktops and show power consumption, as long as you have a relatively modern Intel(tm) system (Intel as in Not AMD, not Intel as in Not Sparc).