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When people talk about a power supply's output, they usually say it produces a certain number of watts. While a convenient shorthand, it is unfortunately both vague and imprecise. Buying a power supply solely on the basis of its wattage rating would be like buying a house solely on the basis of its square footage, or a car by how many horsepower its engine produces. In both cases the parameter is an important one, but it doesn't come remotely close to telling the whole story.
Let's start by taking a look at what the output rating means. For purposes of this section, let's consider a "300 W" power supply. What does that number really mean? It is the nominal, total maximum output power of all the voltages that the power supply provides. As described in the power basics section on power, for DC electricity the computation of power is as simple as multiplying its current in Amps, and its voltage in Volts. Of course, power supplies produce several different voltages. That's the first reason why just knowing the total number of watts is insufficient!
If you check the output specifications for the power supply, you will normally see listed all the different voltages that the power supply provides, and the amount of current it can supply for each. (You will also likely see listings for peak output and minimum load.) This is sometimes called the power distribution for the supply. Each voltage produced by the PC is used for a different purpose, which is why it is crucial to check out the individual current ratings for each voltage, and not just use the total wattage of the supply. We can also use the power distribution to calculate the actual total output rating of the supply, and compare it to the marketing numbers--you might be surprised how they differ. How we do this depends a bit on the form factor of the power supply, and in particular, whether or not the supply provides +3.3 V power.
For the PC/XT, AT, Baby AT and LPX form factors, which do not supply +3.3 V power, multiplying each voltage and maximum current value and adding them together should yield the approximate total output power of the supply. Note that the "negative voltages" are added to the total, not subtracted from it. Here's a sample (actual) 300 W AT form factor power supply's distribution. You'll see that the total is fairly close to the rated specification of the power supply:
For the ATX/NLX, SFX and WTX form factors, which provide +3.3 V power (as well as +5 V Standby power and potentially others), there is an added complication: there is a maximum rating for each of the +3.3 V and +5 V currents, but also a combined "+3.3 V / +5 V" rating. The power supply will provide up to the combined total on these two voltages, in any combination as long as the individual current ratings are not exceeded. Here's a sample (actual) 300 W ATX form factor power supply's distribution:
You'll notice a few things here:
Incidentally, it's important to remember that these are maximum figures, again, just like the horsepower specification for your car. The power supply will only provide as much current as is needed by the PC. A 300 W power supply isn't always putting out 300 W of power. Most PCs uses significantly less than the maximum rating of their power supply. This is important to remember for UPS sizing purposes.
Some power supply form factor specifications provide guidelines showing typical power distributions for various output rating totals. You can use these to help you get a better idea of the sort of distribution is typical for the form factor at a given size. They can be found in the appropriate form factor's specification.