After looking at some of the draws of the typical desktop computer system I am confused by power supply ratings which seem to be excessive.

The big item in a desktop computer, the CPU typically has a peak load of around 50W. Heavy duty CD-ROM burners can be 30W and the typical HDD or other device is around 10-15W max. All told, if everything in a computer is going full blast it is sucking down maybe 250W - 300W at the outside.

Nevertheless, the computer power supplies typically sold are touted as 500W, 600W, even 800W devices. This does not seem to make much sense unless these are the input wattage of the power supply. Since there is a lot of heat loss, to deliver 250W of regulated power, you might need as much as 500W inbound.

So, is that the way these power supplies are marketed? By reporting the input power consumption, not the output wattage?

If so, that would seem to be very deceptive because the input power is irrelevant. What is important is how much regulated power the supply generates. After all, who wants a 2000-Watt power supply if it can only generate 100W of regulated power?

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  • Your estimates are off. The low-end Intel Celeron G3900 has a TDP of 51 W. High-end Intel Core i7-6900K's TDP is 140 W. GeForce GTX Titan Z GPU can draw up to 375 W alone (75 W from PCI-E slot + 2x150 W from 8-pin connectors) and you can have two of these in SLI setup. – gronostaj Jan 8 '17 at 23:45
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    You forgot to include the hotplate and coffee maker you power off the USB ports. – fixer1234 Jan 9 '17 at 1:30

The labelled power on a PC power supply is the Total power output of the device. It is NOT the input power specification. The input power requirement is impacted by the efficiency and the power factor of the supply and will always be higher than the label on the power supply.
Read up from here on power supply standards.

Here's the Technical data for an 860W Corsair power supply (very reputable company with well specified product).

enter image description here

Notice above that the individual supplies add up to more than 860W. This has nothing to do with input power vs output power but is what's called the output "Cross Regulation" limit.

You will find Cross regulation explained on Formfactors.org and you will see it in detailed power supply product datasheets. Below is a clip from a 220W power supply where the power outputs for (+5 & +3.3) and +12 V are interrelated (Cross regulated). In the graph below you have to keep the maximum output power for those three voltages within the Cross Regulation limits. Going outside the limits will result in one or more of the voltages being out of specification.

enter image description here

Virtually all power supplies are now 80-90% efficient, so to get the input power required you sum your output loads and divide by 0.8 to get an indication of input power at 0.9 power factor (which most supplies guarantee).

Cheap power supplies may be significantly less that 80% efficiency at low power levels, but within their major usable power range window will typically be 80% or better. It's difficult with cheap supplies to get details for Cross Regulation, but for most good quality supplies it will be at least mentioned, though may not be as well documented as the Intel Power supplies (formfactors.org) were. However the work Intel did here became the gold standard for ATX specifications.


It's because while a typical computer may only need 300 Watts, this is just a baseline model. Many towers can be upgraded for multiple disc and hard drives. Most can support multiple accessory cards, like PCI or AGP. Some video cards are extremely power hungry. You have to include full draw on multiple USB ports, with USB 3.0 getting into 10 or more Watts per port. People often need 800 Watt supplies because they have computers that need 800 Watts.

That said, a modern cpu like a Core i7 has a typical wattage rating of 130W, not 50W.

A secondary reason that many pcs have higher rated supplies than needed is because of the mass production of computers have lead to these supplies being cheap to produce en mass. Instead of stocking a few of multiple wattage supplies, a OEM uses a bunch of a single rating. Bull discounts.


It is always the Output Power which is not the sum of each Voltage power limit, rather an internal temp rise.

Input power is implied by suggested breaker current allocation. [A]

For example 8 core GPU's with high speed DDR5 VRAM are the biggest power load not the basic MOBO.

enter image description here

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