The main factor in determining power needs is your CPU. Some CPUs draw lots of power, namely P4 and Athlon64 units. The CPU is the biggest variable in systems, as far as power consumption.

Look up your CPU on Chris Hare's PROCESSOR ELECTRICAL SPECIFICATIONS page. He lists the electrical specs of just about every CPU produced in the past ten years or so. Look at the Max Power Dissipation column. That's the peak power usage of the CPU under heavy load. Using this figure in your calculations will allow power overhead for the CPU to perform any tasks you throw at it without choking off your power supply.

Now that you know the max power draw of your CPU, you need to know how much power the PSUs provide on the 12v rail. The 12v rail is the one that will provide power to the CPU. It's stepped down to 1-2v (typically) with more amperage so the CPU doesn't burn up. There's circuitry on the motherboard for this. Typically, the PSU will tell you how many amps it will output on the individual rails. To calculate wattage output, you multiply amps by volts (A x v = w). If the wattage output of the PSU on the 12v rail is less than the max dissipation for your CPU, it won't work.

The other thing you need to consider is other 12v devices. There aren't that many. 3.5" HDDs, full-sized optical drives, case fans, external audio like a Creative SB Audigy NX, possibly an LCD screen, etc. There are other power calculators that will give you power usage estimates for these devices. These need to be added to the total wattage load on the 12v rail to determine if the PSU will suit your needs.

EXAMPLE: Let's say you want to use an Opus 150w PSU with a P4 2.8C CPU. If you look up that CPU on Chris Hare's page, you can see that the max power usage is either 68.4 or 69.7w, depending on flavor. Let's call it 70w for simple math. ALWAYS ROUND UP!!!! So you look up the specs for the Opus 150w and it says that the output on the 12v rail is 5A. 12v x 5A = 60w. You'd get 60w out of the Opus PSU and the CPU requires 70w. It's not going to work, so there's no point in calculating how much power the other components are going to draw from the 12v rail, as the CPU itself uses more than the PSU can supply.

The ratings of all the major DC-DC power supplies are listed above, under their respective manufacturer product listings.
[edit] Power Calculators

These make things easy

* For Via Epia and other mini-itx systems
* Extreme Calculator