0PSU LogoPSU Efficiency: worth the upgrade? part 3

This is the third install­ment in a series of art­icles address­ing PSU effi­ciency.  Art­icle one focused on a the­or­et­ic­al new serv­er, art­icle 2 on a typ­ic­al desktop PC.  This art­icle will focus on a cur­rent real serv­er, and intro­duces a new PSU — an 80+ plat­in­um mod­el.


The sys­tem spends a lot of time semi-idling (serving net­work and disk requests but not doing too much CPU work).  I estim­ated that it would pull around 150W on aver­age, and then cal­cu­lated it below.  It really did come out exactly like that!
Base sys­tem Sem­pron LE-1100 (1.9GHz single-core) on nForce 630i / 7050 mother­board.  2Gb DDR-II35W
2x Sam­sung F4 2Tb HDDs, 7x 1Tb Green HDDs inc. WD Green 1Tb, Seag­ate 1Tb and Sam­sung 1Tb, 1x WD 750Gb 7200rpm drive, 1x Max­tor 300Gb 7200rpm drive, 1x WD 200Gb 7200rpm IDE drive108W
Areca 1220 RAID Card5W
2x 120mm Case Fan2W
TOTAL150W

Out of interest, assum­ing a cost of £0.10 per kWh this sys­tem would cost a total of £131.50 a year to run even with a per­fectly effi­cient PSU!

The ATX spe­cific­a­tion requires 70% effi­ciency.  Below is a com­par­is­on between a bare min­im­um ATX PSU and a few highly effi­cient PSU’s.

The fig­ure for the gen­er­ic PSU is based on the min­im­um require­ment and is there­fore a worst case.  I have cal­cu­lated a best-case yearly sav­ing, based on a kWh charge of 10p and assum­ing 24.7 usage.

Make / Mod­elEffi­ciency at 150WEst. A.C. power drawSav­ing
ATX v2.2 250w ATX70% (at 150W)215W
Season­ic X‑40088.8%169W£40.39
Cor­sair AX85087.4%172W£38.02
Ener­max Modu87+ 500w88.5%169W£39.89
Cor­sair CX400W82.1%183W£28.31
King­win Lazer Plat­in­um 550W93.3%161W£47.54
HuntKey Jump­er R90 300W 80+ Gold90%167W£41.74

Con­clu­sion: This is the first scen­ario where new high­er-effi­ciency PSUs are reas­on­ably price com­pet­it­ive based only on power usage.  For such a sys­tem, a high effi­ciency PSU will pay for itself with­in approx­im­ately 3 years.  If you plan to keep the sys­tem for more than 3 years a high effi­ciency PSU is a worth­while invest­ment as its TCO will be lower.  As energy prices con­tin­ue to rise this TCO advant­age will widen.

Update (25-Dec-2011)

Added the HuntKey Jump­er R90.  This PSU changes the con­clu­sion con­clus­ively.  It pro­duces sav­ings of over £40, which is at least the price of the PSU!  A pay­back peri­od of 1 year, with an expect­a­tion that elec­tri­city prices will con­tin­ue to rise is well worth the invest­ment.

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