You could find a lot of detailed design information about your components' power requirements and add them up, but you'd still only have an estimate based on the manufacturers' wishful thinking. The thing to do is to measure how much power a unit actually draws. The meter in the next module will do that, and probably pay for itself in reduced utility bills as you compulsively measure the power consumption of all the appliances around the house or office. Besides, we'll be using this again.

You'd expect a 100-Watt lightbulb to draw 100 Watts, and you'd be right, but a 200-Watt power supply is a different matter. In the case of the power supply, 200 Watts is the rated capacity -- the maximum the unit can be expected to deliver under "full load."

How much power the PS will actually draw depends upon the equipment connected to it. Like the light bulb, the computer components are the load, and determine the amount of power consumed.

Your power supply needs to be big enough for your computer's peak load -- the amount of power it consumes in it's most power-hungry mode -- whether that means recalculating spreadsheets, online gaming, or just sending e-mail.

The physical dimensions or form factor of ATX power supplies are standardized for most common ATX case designs (about 3.5" x 6" x 5.5"). The size we are considering here is in terms of capacity, measured in Watts.

From the foregoing discussion of load vs. capacity it should be clear that "bigger is better." A higher-capacity power supply will not run up your electric bill or burn out any of your components.

That being said, cost becomes the main determining factor. At this time, the "sweet spot" seems to be around 350-Watt capacity power supplies. This should be more than adequate for general purpose machines.

Specialized file servers or state-of-the art gaming platforms may require more power, but such systems present a variety of unusual design requirements beyond the scope of this article.

Our electric power utilities are used in a variety of high-load applications such as electric lighting, HVAC, and medium to large electric motors. Between these wildly fluctuating loads, occasional lightning strikes, and kamikaze squirrels attacking transformers there is little wonder that typical AC power is a little "ragged" for digital electronics

Of course, part of the function of a good power supply is to "clean up" these inconsistencies, but by now most people are probably aware that a good surge & spike suppressor is cheap insurance for expensive equipment. Fewer realize that low-voltage sags and brown-outs can be just as destructive to data, and even to hardware as overvoltage, and that a battery back-up is a far more satisfactory solution.

You may recall that electric power (P) equals current (I) times voltage (E or V), and you might logically conclude that an uninterruptable power supply's rated capacity (in Volt Amps) should equal the attatched load (in Watts).

Unfortunately, this is only true for DC circuits. The AC systems we're considering require about 50% more capacity. In other words, the 350W PS above would require a UPS supplying about 525 Volt Amps if it were fully loaded.

A small network might require two or three times that capacity. The UPSs listed here are very handy and popular sizes with nice price points. I know I've spent a lot more for a lot less UPS.