Powersupply requirements make no since??

Discussion in 'General Hardware' started by KoncreteAngel, Jun 28, 2008.

  1. KoncreteAngel

    KoncreteAngel Master Guru

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    1800xt 512mb
    How come when I see in a review, "we measured peak system usage at 259 watts, so you must have a 450w power supply at the minimum."

    This makes absoutely no since whatsoever, cause even after you add the efficiency 450x75%=338w you still have a minimum of 75 more watts to play with. So you should only really need to have a 350w at the minimum.

    Then I see a review that says "our 1000w ocz powerstream wasnt enough for the 500w usage in our gtx280 sli, so we had to upgrade to 1200w." This makes me LOL because you should still have AT LEAST 250 more watts to play with. 1000x75%=750w

    I mean seriously what since does any of that make? are the reviewers crazy?
  2. Ali_NZ

    Ali_NZ Master Guru

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    Strix GTX970
    PSUs are not really designed to be stressed to the max. It's always good to leave some headroom imo. My guess is Nvidia & ATi claim the minimum to be higher to cover their asses for people who use terrible; over-rated PSUs.
  3. PhonyEye

    PhonyEye Maha Guru

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    Sapphire HD7970
    the problem is not the Watt i think, it's the amper. you don't see a 350W PSU with 36A on two 12v rails do you?
    350W PSUs have about 25A and only 15A or so on one rail. so it's fine for office use only.
    you have to get the 600W for decent juice for today's graphic cards. in the end less than 600W is used, but that is not bad news...
  4. Makalu

    Makalu Ancient Guru

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    EVGA 8800 Ultra
    hmm well I haven't noticed reviewers or retailers or manufacturers saying "you must have" such and such really. I think they all tend to say "recommended", which isn't the same as "required". So like Ali-NZ said they are covering their asses and also leaving some room for overhead and what PhonyEye mentioned about the 12V wattage capacity (normally expressed in amps) is true too.

    On a side note...you're confused about how energy efficiency ratings work. It doesn't lower the amount that the PSU can output, it raises the amount of AC input wattage that the PSU requires for it's DC output. For instance a 1kw PSU that's 75% efficient at full load would need 1334 watts of AC power to output 1kw DC...600W AC would be needed to output 450W DC at 75% efficiency.

    On the other hand all of these reviewers are measuring and reporting AC watts input...so when you see a peak system usage of 259W then figure that the DC load on the PSU is about 200W only (depending on the energy efficiency of the PSU being used at that load level and also depending on the AC voltage in the area where the reviewer is located).

    I do agree that reviewers and manufacturers and especially retailers tend to go overkill with their recommendations but they have their reasons like variations in system configurations and variations in PSU quality, age and rating truthfulness. The generally accepted recommendation is that PSU loads should be kept below 75-80% of total capacity. The unit will last longer, be more efficient, be cooler/quieter and have optimal outputs (voltages closer to nominal and not fluctuating) at that load level or less.

    The OCZ EliteXStream 1000 that failed to power Anandtechs test system in two different GPU reviews is a special case. I think the unit they have is either faulty or being a new model has some incompatibility issues with the new cards...probably just faulty though.

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