Buyers Guide - Power Supply Units (PSUs)

Discussion in 'Articles / Guides' started by Graeme*Kustom*, Aug 18, 2005.

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  1. Graeme*Kustom*

    Graeme*Kustom* Administrator

    Apr 23, 2001
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    by Mikael Strand


    The aim of this guide is:

    • To explain the basic functions of a power supply
    • To explain the difference between the ATX 1.3 and the ATX 2.01/2.2 PSU standards.
    • To explain different PSU parameters and how these will make a difference to your system.
    • To help you choose the right size of PSU for your system and what to consider when purchasing a power supply.

    What does a PSU do?

    The main purpose of a power supply is to convert the 230V AC (110V in the US) electric supply that comes out of the wall socket into 3.3V, 5V and 12V DC that the computer uses.

    A secondary purpose is to provide air flow in the case. The heat output of computer components is increasing. To cope with this case fans have become the primary exhaust in most systems. This has lessened the importance of the PSU as a provider of airflow in the case.

    What are the ATX 1.3 and the ATX 2.01/2.2 PSU standards?

    The ATX PSU standard is a standard set by Intel.

    The ATX 1.3 standard was released in April 2003. This standard was the first PSU standard to include SATA connectors. At the moment this is the most common standard for PSUs.

    ATX 2.01 was released in June 2004. This was a major update of the ATX PSU standard, making the PSUs more suited to the modern PC with its increasing load on the 12V line of the PSU. Graphics cards and the CPU(s) are the main consumers on the 12V line.
    The latest version of this standard, ATX 2.2, was released in March 2005.

    What are the main differences between the ATX 1.3 and ATX 2.01/2.2 standard?

    The ATX connector

    The ATX connector has changed from having 20 connectors to 24 connectors. The added connectors are 12V1, 5V, 3.3V and COM.

    Second 12V line added

    The purpose of the addition of a second 12V line on the power supply was to eliminate power surges on the 12V line when the CPU suddenly changes from idle to full load.

    SATA connectors

    The SATA connectors have changed from being optional in the 1.3 standard to being compulsory. The standard does not state how many SATA connectors the PSU should have.


    Efficiency has been changed from just being recommendations to being a requirement. The required minimum efficiency is 70% for full load and typical load, and 60% for low load.
    There is still recommended efficiencies in ATX 2.01/2.2, these are 78% for full load, 80% for typical load and 75% for low loads.
    Full load is 100%, typical load is stated as 50% of overall output and low load as 20% of overall output.
  2. Graeme*Kustom*

    Graeme*Kustom* Administrator

    Apr 23, 2001
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    PSU parameters and the difference they make to your system?


    Assume you have a modern system that consumes 200W from the PSU. A PSU with 80% efficiency would then draw 250W from the wall. The 50W difference between the power drawn by the system and the 250W that the PSU draws is a loss that ends up as heat in the PSU.

    If the same system uses a PSU with 60% efficiency, it would draw 333W from the wall, a loss of 133W. This leads to a couple of drawbacks compared to the more efficient PSU. Instead of getting rid of 50W of heat, this PSU needs to get rid of 133W. This in turn leads to a niosier power supply because the fans need to work harder.

    If you look at the long-term cost of the increased power consumption, this might be more than the difference in price for a more efficient PSU in the first place. Assuming you are using the system 3h a day on average and your power costs 10p / kWh, the increased power consumption would cost you £9 a year.

    (0.333kW - 0.250kW) * 3h * 365days * £0.10 = £9

    If you always have your system on, running something like folding@home, this extra loss would cost you £73 a year.

    0.083kW * 24h * 365days * £0.10 = £72.70

    A problem when comparing PSUs is that manufactures might not state the efficiency of their PSUs. If it follows the ATX 2.01 standard or above, it will have a minimum efficiency of 60-70% depending on load.

    Max Power output

    It is a common mistake to evaluate different PSUs by comparing their maximum total power output. This is not a very accurate way of comparison. It’s a bit like comparing different cars by how high their speedometer has been marked. For a modern system the 12V will be the most important load. PSUs are labelled with max power output for the different lines. Here are two examples of ATX 1.3 PSUs:


    Qtec dual fan gold 550W
    +3.3V 20A
    +5V 35A
    +12V 14A

    Nexus NX3000 300W
    +3.3V 15A
    +5V 30A
    +12V 15A

    As you can see above, the Nexus gives a higher output on the 12V line, even though it is only rated for a maximum power output of 300W compared with 550W for the Qtec.

    Comparison of ATX 2.01/2.2 PSUs can be a bit more complicated. The problem is to find out the maximum 12V output when having more than one 12V line. Unfortunately it is not always correct to just add up the maximum output of the different 12V lines. A good example of this is the two Enermax PSUs:


    Enermax EG375AX-VE
    +3.3V 27A
    +5V 27A
    +12V1 18A
    +12V2 18A
    12V max 27A

    Enermax EG495AX-VE
    +3.3V 27A
    +5V 27A
    +12V1 18A
    +12V2 18A
    12V max 32A

    Both of them state a max output on the individual 12V lines of 18A, but a 12V max of 27A and 32A. This means that if there is a draw of 18A on one of the 12V line, the other line could only have a load of 9A and 14A.

    So what do we do if the manufacturer does not state a 12 max output? We start with the Seasonic S12.


    Seasonic S12-600 PSU
    +3.3V 30A
    +5V 30A
    Combined 3.3V & 5V 180W
    +12V1 18A
    +12V2 18A

    We know the PSU has a max output of 600W. They also state a combined max output of 180W of the 3.3V and the 5V line. We can use this to calculate the 12V max output.

    600W - 180W = 420W max power on the 12V lines
    420W / 12V = 35A max current on the 12V lines

    So what do we do if the manufacturer does not state either a 12V max or a max combined 3.3V and 5V? We can use the TT PurePower 680W for this example:


    Thermaltake Silent PurePower 680W
    +3.3V 28A
    +5V 50A
    +12V1 15A
    +12V2 15A
    +12V3 8A

    Lets start by calculating the max power of the individual lines.

    3.3V * 28A = 92.4 W
    5V * 50A = 250W
    12V1,2 * 15A = 180W
    12V3 * 8A = 96 W

    Then add the lines together for comparison with the maximum power stated for the PSU.
    92.4W + 250W + 180W + 180W + 96W = 798.4 W

    This is much more that the 680W max stated by TT. We then have to assume a worst case scenario for the 12V power.

    680W total - 92.4W - 250W = 337.6W max power on the 12V lines
    337.6W / 12V = 28A max current on the 12V lines
  3. Graeme*Kustom*

    Graeme*Kustom* Administrator

    Apr 23, 2001
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    Other things to consider when buying a power supply

    Fan types and fan positioning

    Will the PSU fan positioning work well with the case it will be in?
    Check if your case is designed with a PSU fan position in mind. Most cases are design to work with any type of PSU. But some cases work best with a PSU with front or back mounted 80mm fans and some are designed for a 120mm PSU fan mounted at the bottom of the PSU. If you are considering a fan-less PSU, make sure that the case will have enough airflow without airflow through the PSU and that it will cope with the extra heat generated by the PSU. This might be a problem with some HTPC cases.


    The noise level of the PSU depends on a combination of three factors: PSU fan (or lack of fan), efficiency of the PSU and case airflow.
    The better efficiency of the PSU, the less heat it will generate. The less heat, the less airflow will be needed to get rid of it. The less airflow, the less noise from the fan.
    The cooler the air is going into the PSU, the less airflow is needed to get rid of the heat from the PSU. If the PSU is the primary exhaust of hot air, then the PSU fan will have to work much harder to handle both the heat from the other components and the heat from the PSU.

    Length and type of connectors

    Make sure the PSU will have all the connections your system needs. Make sure that these connections are long enough for your system. This can be more of an issue if you use a case where the PSU is mounted in a separate compartment as in the Antec P180 and the Lian Li V-series.

    Total power output

    It might not be a case of the more power the better. Make sure that the PSU has enough power for your current system and future upgrades. If you use a PSU with too much power, it will be in the less efficient low output (20% of max) band of the PSU. Check the efficiencies for the PSUs you are considering and compare this to the power band of your system.