Picking The Right Power Supply: What You Should Know

How To Spot An Efficient PSU?

Guidelines, Rules, And Regulations

One of the most important indicators of a PSU’s efficiency is whether it complies with the Energy Star 5.0 guidelines and the 80 PLUS standard. The latter applies primarily to computer power supplies and is recognized worldwide. Additionally, if you’re located in a European country, CE conformity and compliance with ErP guidelines are also important.

80 PLUS PSUs Are More Efficient

The specifications, norms, and guidelines we just mentioned all call for high efficiency, as well as improved power quality. Power supplies that conform to these demanding and very strictly-interpreted rules by passing a defined set of tests may then be marked with the 80 PLUS badge appropriate to its efficiency level. While the load/stress tests may not correspond to those defined by the ATX specification, that’s acceptable in this case. Here’s some good news for our European readers: since the tests are conducted using the lower US voltages, these power supplies achieve even higher efficiency levels in the European 220 V grid.

80 PLUS: Platinum, Gold, Silver, Bronze

The original concept of the 80 PLUS certification has been revised, adding new, more strictly-defined efficiency levels. The bronze, silver, gold, and platinum certifications each come with their own requirements. Thus, a PSU that is certified “80 PLUS Gold” or “80 PLUS Platinum” is more efficient than a normal unit. The downside is that the more complex circuitry needed to hit those levels generally results in a higher price tag, too.

Below you’ll find a table that shows what efficiency levels a PSU has to achieve at a given load to make the grade for a specific certification level.

 Efficiency at 20% Load
Efficiency at 50% LoadEfficiency at 100% Load
80 PLUS80%80%
80%
80 PLUS Bronze82%85% 82%
80 PLUS Silver85%88%85%
80 PLUS Gold87%
90%87%
80 PLUS Platinum90%92%89%


When Off Isn't Really Off: A Few Words On Standby Power Consumption

When you shut down your computer, the PSU doesn’t really switch off completely. This is necessary for features such as Wake-on-LAN to work. The point is that the power supply keeps drawing some power, even when the computer is off. Newer PSUs, especially ones sold in Europe and certified to be ErP/EuP-compliant, draw less than 1 W in this standby mode. If you’re serious about conserving power, go for a newer model with ErP support.

Which Power Rails Are Important?

That brings us to one of the most crucial points of modern power supplies: namely, the power they are able to supply at various voltages. Nowadays, PCs draw the majority of their power from the +12 V rail. By comparison, the other two voltages, 3.3 and 5 V, play a far less important role. That’s why you can use the following as a rule of thumb: if a PSU’s 12 V rail can supply all of the required power with room to spare, then the lower voltages are sufficient as well.

However, the opposite is not necessarily the case. Let’s compare the spec stickers of two PSU models:

The difference is quite obvious. Although the second model is billed as a 550 W unit, its +12 V rails only add up to 380 W, and even that only holds true if the other rails aren’t being stressed simultaneously! Nobody needs 315 W on the 3.3 and 5 V rails. In practice, this power supply would probably reach its limit at a load of 350 W on the 12 V rail.

Ironically, even a good 425 W PSU could push more power than this model at 12 V. Don’t fall for this sort of trickery. 

Initial Cost Vs. Energy Savings

Quality products cost more initially, but that doesn’t necessarily always translate into lower cost in the long run. That’s why we’ll take a look at a few specific components and their prices in a moment to determine the type of PSU makes the most sense in a given environment, and what kind of savings you can achieve, if any. Some of the results may surprise you!

It’s not enough to focus solely on the financial aspect, though, because we also have to consider durability, reliability, and safety. We go into more detail on these points on the next page.

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  • 13thmonkey
    Great article, since you mentioned power draw at 'off' could you please include it? and perhaps go to the next stage as below and work out the power drawn per day/year and the cost of that? I've included an example of what I mean below, assuming off=10% of idle. There's £9/year spread from least to most efficient, the Be Quiet seems to be the be value if you are going to keep the PSU (not the PC) for 3.5 years or more, I'm not sure about Be-Quiet as a brand as I've not seen reviews and such for them.

    The addition below would turn this from a great article into an incredible article
    -------------BQ---------RA-----------SF-------------LC------------HM-----------Hrs
    Idle----------53.2---------56.7---------60.8---------68.2---------72.1---------4
    Normal-----91.6---------90.4---------92.2---------98.9---------102.5-------4
    Load--------125.6-------124.8-------122.2-------131.7-------136.4-------1
    Off------------5.32---------5.67---------6.08---------6.82---------7.21---------15
    Watt-hrs-per-day
    ----------------213----------227----------243----------273----------288
    ----------------366----------362----------369----------396----------410
    ----------------126----------125----------122----------132----------136
    ----------------80------------85------------91------------102----------108
    Units-per-day
    ----------------0.78---------0.80---------0.83---------0.90---------0.94
    Units-per-year
    ----------------286----------291----------301----------329--------344
    Cost-@-15p/unit
    --------------£42.96-----£43.70-----£45.19-----£49.41-----£51.63


    Apologies for the formatting - WSIWIG please please pleaseits 2011 for crying out loud, just spent 5mins putting spaces instead of tabs and its still looses formatting
  • bobbyp86
    Which model is that Corsair case at the end? Looks nice!!
  • bobbyp86
    aha it's the Obsidian 800D, better get saving :)
  • flaminggerbil
    Yet to have a PSU go pop on me (touch wood), great article though, very interesting.
    Wonder how long that 450w superflower could have kept running on full load.

    But glad I've always stuck with Corsair & Enermax.
  • silver565
    Corsair have been good to me. My 550vx is still going strong
    I only use the cheaper cooler master ones(something like this http://tinyurl.com/3wxcs3t) for my file server. No high drain, just a few HDDs.
  • Solitaire
    flaminggerbilYet to have a PSU go pop on me (touch wood), great article though, very interesting.Wonder how long that 450w superflower could have kept running on full load.But glad I've always stuck with Corsair & Enermax.


    A long time, although its overall lifetime would likely have been shortened running it at 110% load for extended periods. Their last two generations have been fantastic, just wish you could actually get them that cheap in Europe! :p

    And saying all current-gen quads use the same amount of power is just silly Toms! The top-end Phenoms can creep past 130W at full speed, but the i5-2500k uses barely half as much, not 125W!
  • shanky887614
    i had a 600watt winpower supply

    it ran fine for 4 years

    then there was an electronic whine coming from it under load it frove me crazy

    it was a 5 or 600 watt i beleive i know a core 2 duo and hd 4670 cant draw that much power

    could get a decent brand one at a local shop so had to go for a gigabyte 550 (450 watt continuos power supply)

    im not kidding ive had quiter hair dryers!!!!