Power supplies are labeled according to the maximum power they can deliver – at least in theory. The problem is that a lot of power supplies can’t deliver their labeled power, usually because the manufacturer:

• Labeled the power supply with peak wattage, which can only be achieved during some seconds and, in some cases, in less than one second.
• Measured the power supply maximum wattage with an unrealistic room temperature, normally 25º C (77º F), while thetemperature inside the PC will always be higher than that – at least 35º C (95º F). Semiconductors and inductors have a physical effect calling de-rating where they lose their ability to deliver current (and thus power) with temperature (see Figure 28). So a maximum power measured at a lower temperature may not be achieved when temperature is increased.
• Simply lied. This is probably the case with “generic” units.

Just to illustrate the effect that temperature makes on the ability of a power supply to deliver current, consider the de-rating curve presented on Figure 28, which belongs to a transistor called FQA24N50. As you can see, this transistor can deliver up to 24 A when working at 25º C (77º F), but as soon as temperature increases (x axis) the maximum supported current (y axis) decreases. At 100º C (212º F) the maximum current this device can deliver is 15 A, a 37.5% decrease. Power, which is measured in watts, is a factor between current and voltage (P = V x I). If this transistor were operating at 12 V we would see a decrease on the maximum power from 288 W (12 V x 24 A) to 180 W (12 V x 15 A).

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Figure 28: De-rating curve of a transistor.

Knowing this situation good manufacturers started to disclosure at what temperature their power supplies were labeled. You can find some power supplies on the market where the manufacturer guarantees that they can deliver their labeled power at 40º C, 45º C or even at 50º C. In other words, the manufacturer guarantees that they can deliver their labeled power under a real-world scenario and not only at the manufacturer lab. This is a good parameter when deciding on which power supply to buy.

You may think that the maximum amount of power a power supply can deliver is simply the sum of the maximum amount of power each output can deliver. But in fact the math isn’t that simple because of the way PC power supplies work internally: the main positive outputs (+12 V, +5 V and +3.3 V) share some components and thus even though each output has an individual maximum output, this maximum can only be reached when no power is being pulled from the other outputs.

The most common case is the +5 V and +3.3 V outputs. Even though they have individual maximum current and power limits, these maximum values can only be pulled when no power is being pulled from the other output: together they have a combined maximum power, which is lower than the simply addition of the maximum capacity from +5 V and +3.3 V outputs.

For a practical example, consider the power supply on Figure 29. Its label says that the +5 V output can deliver up to 24 A (which equals to 120 W, 5 V x 24 A) and the +3.3 V output can also deliver up to 24 A (which equals to 79.2 W, 3.3 V x 24 A). The maximum combined power printed on the label is 155 W, which is less than the simply addition of the maximum power each output can deliver individually (which would be 199.2 W, 120 W + 79.2 W).

The same idea goes to the +12 V outputs. On the power supply from Figure 29 each +12 V rail can deliver up to 16 A (192 W, 12 V x 16 A), but the maximum combined power for the +12 V outputs is 504 W, and not 768 W (192 W x 4).

And finally we have a combined power for the +12 V, +5 V and +3.3 V at the same time, which isn’t a simply addition of the maximum combined power for the +5 V/+3.3 V outputs with the maximum combined power for the +12 V outputs. On the power supply from our example the maximum combined power for these outputs is 581.5 W and not 659 W (155 W + 504 W).

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Figure 29: A typical power supply label.

Finally we have power distribution, which is something very few users are aware of. Two power supplies with the same maximum power can have a completely different power distribution.

Nowadays a typical PC pulls more power from the +12 V outputs. This happens because the two most power-hunger components from the PC – the CPU and the video card – are connected to the + 12 V outputs (thru the ATX12V/EPS12V connector and thru the PEG connector, respectively).

Take another look on the power supply label from Figure 29. From this label you can clearly see that this power supply uses an updated project, where the power supply is capable of delivering more power from the +12 V outputs (504 W) than from the +3.3 V/+5 V outputs (155 W).

Now consider the power supply from Figure 30. This unit can deliver more power/current from its +5 V/+3.3 V outputs than from its +12 V outputs, meaning that this power supply uses an outdated design. Believe it or not, this power supply is still being sold and there are several power supplies with outdated designs around.

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Figure 30: Label of a power supply with an outdated design.

In summary, buy power supplies where the maximum capacity is on the +12 V outputs and not on the +5 V/+ 3.3 V lines.

Finally you will need to know how much power your PC will really consume before picking a power supply. There are several calculators on the internet that can help you out with this; we recommend this one. We also recommend you to choose a power supply that will be working between 40% and 60% of its maximum capacity. There are two reasons for that. First, efficiency, subject that we will explain next. Second, you will have headroom for future upgrades. So get the result obtained from the calculator and multiply it by 2. This is the power supply wattage we recommend you to buy (you will be surprised that most systems will require a power supply with less than 450 W, even with our adjustment).