0PSU LogoPSU Efficiency 2 — A typical gaming PC

Fol­low­ing on from the slight sur­prise that a new PSU is a poor invest­ment for a low power PC I decided I’d reas­sure myself that an effi­cient PSU is a good invest­ment for a typ­ic­al “enthu­si­ast” PC.

To do this depends on the con­fig­ur­a­tion, how much time the PC spends on, and how much time it spends being heav­ily util­ised. I’m basing this on a fairly-high power single-GPU sys­tem with a Core i7 CPU and a Radeon 5870. I’m also going to assume the sys­tem is oper­ated from 3pm — 11pm week­days and 9am — 1am week­ends, mak­ing for a total of 72 hours a week. I’ll also assume the sys­tem spends 50% time idle and 50% full-load.

The sys­tem in ques­tion pulls ~80w idle and ~340w under full load.  Find­ing effi­cien­cies for this is tricky, so the value will be roun­ded up to 400w (the PSU’s in ques­tion don’t vary greatly between 300w and 400w)

All of these estim­ates are delib­er­ately on the high side. I expect a typ­ic­al PC will spend more time off, more time idle, and wont be loaded as intensely as the example. Most sys­tems also use lower power CPU’s and GPU’s too.

The fig­ure for the gen­er­ic PSU is based on the min­im­um require­ment and is there­fore a worst case. Even with a very poor effi­ciency PSU the AC draw from the wall won’t increase as the DC draw decreases. I have cal­cu­lated a best-case yearly sav­ing, based on a kWh charge of 10p.

Make / ModelEffi­ciency at idle (80W)Effi­ciency at load (340W)Est. A.C. power draw at idleEst. A.C. power draw at loadSav­ing
ATX v2.2 400w65% (at 80w)70% (at 400w)123W486W
Season­ic X‑40085.4% (at 90.4w)89.9% (at 400w)94W378W£25.63
Cor­sair AX85083.5% (at 90w)89% (at 400w)96W382W£24.52
Ener­max Modu87+ 500w85.9% (at 88.5w)87.6% (at 400w)93W388W£23.87
Cor­sair CX400W81.2% (at 90.2w)81.5% (at 400w)99W417W£17.43
King­win Lazer Plat­in­um 550W92.1% (at 91.2w)92.9% (at 400w)87W366W£29.19
HuntKey Jump­er R90 300W 80+ Gold88% (at 60w)86% (at 330w)87W366W£23.17

A new PSU in this scen­ario is prob­ably not advis­able on a fin­an­cial only basis.  Even with quite a lot of heavy use, and com­par­ing the most effi­cient PSU’s with the worst a budget ATX PSU can per­form the sav­ing will take around 5 years to recoup. A real world upgrade is unlikely to net this much saving.

If you are con­sid­er­ing repla­cing an exist­ing PSU my advice would be not to both­er, unless you have addi­tion­al reas­ons. If you are buy­ing new then it is worth pay­ing more for a bet­ter PSU, with­in reas­on. Of course if you want the effi­ciency for oth­er reas­ons (low noise, low heat) then those factors may make the value dif­fer­ence irrelevant.

Update (25-Dec-2011)

Added the HuntKey Jump­er R90.  Whilst it is sold as a 300W PSU, it does work slightly above spec, espe­cially if well cooled.  This PSU does make a bit of dif­fer­ent to the con­clu­sions.  It pro­duces sav­ings of over £20, which is a sig­ni­fic­ant pro­por­tion of the price of the PSU!  A pay­back peri­od of 2 year, with an expect­a­tion that elec­tri­city prices will con­tin­ue to rise would cer­tainly be worth think­ing about.  How­ever, the HuntKey isn’t spe­cified high enough to really provide for the con­fig­ur­a­tion spe­cified in this scen­ario, and as such we could­n’t recom­mend run­ning it bey­ond its rating.

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