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, typically because the manufacturer:

  • Labeled the power supply with peak wattage, which can only be achieved for a few seconds and, in some cases, in less than one second.
  • Measured the power supply maximum wattage with an unrealistic room temperature, normally 25° C (77° F), while the temperature inside the PC will always be higher than that – at least 35° C (95° F). Semiconductors and inductors have a physical effect called de-rating where they lose their ability to deliver current (and thus power) with an increase in 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.

To illustrate the effect that temperature makes on the ability of a power supply to deliver current, consider the de-rating curve presented in Figure 28, which belongs to a transistor called FQA24N50. As you can see, this transistor can deliver up to 24 A when working at 25° C (77° F), but as soon as temperature increases (x axis), the maximum supported current (y axis) decreases. At 100° C (212° 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 in the maximum power from 288 W (12 V x 24 A) to 180 W (12 V x 15 A).

De-rating curveFigure 28: De-rating curve of a transistor.

Knowing this situation, reputable manufacturers started to disclose 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 40° C, 45° C or even at 50° C. In other words, the manufacturer guarantees that they can deliver their labeled power in a real-world scenario not only at the manufacturer’s lab. This is a reliable parameter when deciding 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. In truth, 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, so 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 simple addition of the maximum capacity from +5 V and +3.3 V outputs.

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

The same idea holds true for the +12 V outputs. On the power supply from Figure 29, each +12 V rail can deliver up to 16 A (192 W, or 12 V x 16 A), but the maximum combined power for the +12 V outputs is 504 W, 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).

Power supply labelFigure 29: A typical power supply label.

Finally, we have power distribution, something about which very few users are aware. 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 occurs because the two most power-hungry components from the PC – the CPU and the video card – are connected to the + 12 V outputs (through the ATX12V/EPS12V connector and through the PEG connector, respectively).

Take another look at 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.

Power Supply LabelFigure 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 that you choose a power supply that will be working between 40% and 60% of its maximum capacity. There are two reasons for this. The first is efficiency, a subject that we will explain next. Second, you will have headroom for future upgrades. Get the result obtained from the calculator and multiply it by 2. This is the power supply wattage we recommend that you buy. (You will be surprised that most systems will require a power supply with less than 450 W, even with our adjustment.)

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.