0PSU LogoLow power PC — new or old PSU

I have a serv­er that oper­ates 24.7. It occurred to me that I may be able to save some money on elec­tri­city bills if I could reduce its power con­sump­tion. It already has a low power AMD Sem­pron LE-1100 CPU so the power draw of the sys­tem should be quite low, but the PSU is quite old and not espe­cially effi­cient. This got me think­ing about Power sup­plies designed for low power sys­tems. There are plenty of 80+ cer­ti­fied PSU’s out there with 500+ watt rat­ings, but are these actu­ally worth the invest­ment for a low power system?

My sys­tem spends most of its time idle so I decided I’d work out the power draw would be if I were to build a new sys­tem with cur­rent com­pon­ents to do it’s job..
Base sys­tem (e.g. atom or Core i5)18W
4x 2Tb (e.g. Sam­sung Eco­Green F3)19W
RAID Card (e.g. Areca 1220)5W
1x 120mm Case / HDD Fan (e.g. Nex­us Real Silent 120mm)1W

Using the above fig­ures, I estim­ate that a new sys­tem with a sim­il­ar role and cap­ab­il­ity to what I have will con­sume just under 45watts at idle.

For a typ­ic­al low spec (450w) PSU, this load is only 10%. At this low a load a typ­ic­al PSU can be quite inef­fi­cient. The ATX spe­cific­a­tion requires 70% effi­ciency at 20% of rated power, but a 43W draw is below 20% rated power for all PSU’s rated for 250W or above (in effect all PSU’s). 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. 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 and assum­ing 24.7 usage.

Make / ModelEffi­ciency at 43WEst. Total power draw (idle)Sav­ing
ATX v2.2 250w ATX70% (at 50W)61W
Season­ic X‑40080.4% (at 42.6W)53W£6.97
Cor­sair AX85077.3% (at 43W)55W£5.09
Ener­max Modu87+ 500w77.5% (at 43.4W)56W£5.21
Cor­sair CX400W73.2% (at 43.2W)59W£2.35
Sparkle Power SPI220LE Flex­ATX 220W80.5% (at 42.4w)53W£7.02
HuntKey Jump­er R90 300W 80+ Gold85% (at 30w)51W£9.50

Con­clu­sion: For a low power sys­tem it is cheap­er, and prob­ably more envir­on­ment­ally sound to keep (or re-use) an exist­ing power sup­ply than to replace it with a high power mod­el, even if the new mod­el is very efficient.

Update (25-Dec-2011)

Cor­rec­ted a cal­cu­la­tion error and added the HuntKey Jump­er R90.  Even a very good value, highly effi­cient PSU can­’t change the con­clu­sion much in this scen­ario — the sav­ings are just far too small to jus­ti­fy the cost (and envir­on­ment­al cost) of a new PSU.  The HuntKey would take in excess of 3 years to pay for itself, and this is in an ideal scen­ario, com­pared to a a worth case scen­ario PSU!  For an even lower power sys­tem (e.g. a AMD E350) the sav­ings would be lower still.  Even a “per­fect” PSU would only net sav­ings of around £16 a year!

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