# Thread: How Power Supplies Are Rated

1. ## How Power Supplies Are Rated

What is the "wattage" number (i.e. 650W, 800W, etc.) actually telling me?

When you see the wattage rating of a power supply, you’re seeing the total maximum output capability of that particular power supply, but a computer has multiple voltage needs, and newer computers require more of the power supply’s capability to be on the +12V DC output rail. CPU’s and GPU’s regulate their power off of the +12V DC rail. Also, all of the computer’s motors run off of +12V DC: hard drive and optical drive motors, fan motors, pumps for water-cooling, etc. It wasn't too long ago that graphics cards did not require auxiliary +12V power and CPU's use to regulate their voltage from the +5V rail. An older power supply may have a lower percentage of it's power on the +12V than a more current unit.

What is the difference between "continuous" and "peak" ratings?

Some power supply units are rated for continuous output while others are rated at peak. "Continuous" means that the power supply is rated to run at it's maximum capability for no pre-determined period of time, while "Peak" indicates that the power supply will only run at the specified wattage for a brief period of time, possibly only a few seconds or up to a minute. This number is typically about 100W more than the power supply's actual continuous rating.

How does the temperature inside of my case affect the performance of my power supply?

Power supplies can perform differently depending on the temperature at which they are operating at. When a power supply is rated for it's total output wattage, it is rated to do so at a particular temperature. Anything beyond this temperature may take away from the power supply's capability. A power supply that is rated to put out 550W at 25°C or 30°C (room temperature) may only be able to put out 75% of that at 40°C or 50°C (actual operating temperature). This difference is called the "de-rating curve". A normal operating temperature for a power supply is 40°C.

Is the temperature at which MTBF is measured at an indication of what temperature the power supply's output rating is measured at?

Unfortunately, no. It's a tough race out there and there are a lot of guys rating their PSU's MTBF at room temperature, even if they rate their PSU at operating temperature. Fact of the matter is, MTBF can be a significantly, often exponentially, lower number when going from 25°C to 40°C. For example, one unit with an MTBF at 100,000 hours @ 25°C can have an MTBF of 20,000 hours at 40°C. That's a pretty big difference! So it's not unusual for a manufacturer to use the higher MTBF number at the lower temperature and, in most cases, not tell us at what temperature that MTBF is derived at. But even when they do tell us the MTBF temperature, this doesn't mean the PSU is rated at this. A PSU's output capability may not be seriously compromised by heat. If the PSU does 700W continuous @ 25°C and only 600W @ 40°C, the difference may not be significant enough to the manufacturer to increase their continuous wattage claims, so although they may measure MTBF at 25°C., they may very well be rating the PSU at 25, 40 or even 50°C. Unfortunately, it all comes down to marketing. It's easier to market a PSU that runs at what it's rated at 40°C then it is to market a significantly lower MTBF at the same temperature.
Last edited by Jon Gerow; 04-24-2008 at 08:00 AM.

When a power supply is not rated for it's total output wattage at a particular temperature (or not mentioned), the power supply is normally considered to be rated at 25°C.

3. micro ATX User
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Does it mean a powerful fan is always better? My old Antec run pretty quietly and not really blowing.

4. If the PSU is inefficient, it produces more heat. Hopefully the fan is spinning up to evacuate that heat.

5. Am new so I had to look for what MTBF meant but in any case are we assuming that PSU's have regular or more than one "failure"??
If so what is a failure??

6. Flux Capacitor User
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MTBF is completely irrelevant for end customer.

Keyword is "mean". As in average. IE. you could meet MTBF of 100k with 50% of units being able to work 200k hours until failure and 50% of units not working right away, and you have no way to tell, which group the sample you get belongs to.

Extreme example, but certainly valid under definition of MTBF.

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Very interesting to see how the temps actually affect things, and with this in mind, i would like to know what MTBF ...thanks in advance Jonny, you know your shit man! sorry for lang but very nice threads in your category

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