View Full Version : PSUs and calculating power requirements

27-09-2012, 07:54 PM
Question 1: Can I get by with a c.400 watt PSU?
Question 2: Can anyone recommend a c.400 watt PSU that is, in order of priority, (a) exceedingly quiet, (b) efficient to at least 80+ standards (pref Bronze and up), (c) is available in the UK at pref <60 mark. edit: preferably modular or semi modular cabling and a sub 170mm length too
Question 3: Have I forgotten any significant power draws below?

Partial upgrade incoming, replacing an elderly but still suprisingly capable (but oh so power hungry and noisy) ATI HD4870 with a new shiny HD7770 (hopeful result being modest performance boost, huge power and noise savings) and a system SSD drive (Samsung 830 128gb, now they're cheap). No-name PSU is also due a refresh due to being very noisy, both accoustically and electrically.

Anyhow, end system and TDPs will end up something like this:
i3-530 CPU - 75w
HD7770 GPU - 80w
3 * 120mm system fans @ 1.5w each = 4.5w
Samsung 830 SSD = 0.5w
2 * elderly WD Raptor 10k 74gb HDDs @ 15w each (striped as app drive) = 30w
DVD drive = 20w
Asus mATX m/board (h55/lga 1156 chipset) = 40w

Adding all those up we get a theoretical max in the region of 250 watts.

There are some significant guesses in there, the old Western Digital raptors will likely draw 50% more or so at spin up (but at that time other components won't be maxed, so no worries) - similarly, they may be drawing about 10w less between them during full-sparkle-shiny gaming time as everything's loaded into memory).
Motherboard power draw (independent of other components) is a complete guess based on limited googling, it'll be running onboard sound/networking and other motherboardy things.

Rough guess at a typical absolute maximum during gaming is 220w - basically the same as the calculation above but subtracting the DVD drive and reducing the app drive HDDs down to 10w each.

This makes me think a 400w PSU will be fine, putting likely max draw smack bang in the middle of a PSU's most efficient loading (50% of rated). The old HDDs will likely be replaced at some near future point for a single more efficient app drive (maybe a Seagate Momentus XT hybrid - http://www.anandtech.com/show/5160/seagate-2nd-generation-momentus-xt-750gb-hybrid-hdd-review) which should reduce power consumption by another 20 watts or so.

27-09-2012, 07:58 PM
Maybe try this: -


27-09-2012, 08:09 PM

That is good. With 20% capacitor aging that calculator comes out with a recommend PSU of 403watts. Pretty much as per my guesses masquerading as calculations.

That is very good. Thankee :o)

28-09-2012, 12:22 AM
Hmm, modular is a bit difficult. Cheapest quality one I could find at Amazon.co.uk was the OCZ 500W ModXStream Pro (http://www.amazon.co.uk/OCZ-500W-ModXStream-Power-Supply/dp/B001GLFD4U/ref=sr_1_148?s=computers&ie=UTF8&qid=1348786875&sr=1-148).

A little cheaper, but not modular, there are the Antec Neo Eco 400C (http://www.amazon.co.uk/Antec-Neo-400C-Power-Supply/dp/B005K3C5L4/ref=sr_1_91?s=computers&ie=UTF8&qid=1348785550&sr=1-91), the Antec Earthwatts 380W (http://www.amazon.co.uk/Antec-EA-380GREEN-380W-EarthWatts-Green/dp/B002UOR17Y/ref=sr_1_95?s=computers&ie=UTF8&qid=1348785550&sr=1-95), and the Corsair CX-430 (http://www.amazon.co.uk/Corsair-CMPSU-430CX-Builder-Series-CX430/dp/B003Y3LYAU/ref=sr_1_109?s=computers&ie=UTF8&qid=1348787011&sr=1-109). The number of amps on the 12V rail(s) - the one(s) that matter(s) - are 30, 27, and 28 respectively. For comparison, the OCZ has 36.

The two Antec PSUs are probably the highest quality of these four, followed by the OCZ and with Corsair bringing up the rear with pretty much the lowest-end PSU I would ever recommend anyone use. Since it's not the cheapest one at the moment, I'd go with either one of the Antecs or the OCZ if modular is your thing. You get to pay for it, but then you also end up with room for some upgrades or even a new build later on.

Oh, one thing; I think the Earthwatts PSU doesn't come with a power cable, ostensibly to save the environment by letting people reuse their old power cable. Which is fine for most computer veterans who end up with a pile of them, but could be an annoying surprise to some.

28-09-2012, 06:35 AM
80W GPU.

Goddamn 28nm. Now I feel like I'm living in the past again.

28-09-2012, 07:18 AM
Thanks Sakkura, yep that OCZ 500w ModXStream Pro keeps going in and out of my Dabs basket (46 atm there - http://www.dabs.com/products/ocz-technology-psu--modxstream-pro--500w---uk-5D14.html?refs=50121&src=3). Only thing that keeps me doubting is that it appears to be quite an old Sirtec/Highpower design and I can find little verification of it's "quiet" billing.

Considering a look at beQuiet's range which Scan & Aria are carrying over here, 50 for a modular 430w 80+Bronze certified (54 for 530w) and a bit too expensive 78 for 480w 80+Gold certified. Seem to be heavily modified FSP designs but tend to get decent writeups at sites that appear to know their stuff (jonnyguru etc).

Oh and love the idea of not shipping kettle leads with power supplies. Noticed that HTC are looking to do the same thing with phone chargers - http://www.guardian.co.uk/environment/2012/sep/26/htc-phone-charger-waste - now that everyone has more or less standardised on micro USB (apart from those polar bear killing bastards at Apple).

28-09-2012, 08:03 PM
Well, the ModXStream Pro 500W does have a large fan, which is likely to result in less noise than with the smaller fans on some other PSUs. The two Antecs I mentioned have 120mm and 80mm fans respectively, and the Corsair has a 120mm fan as well.

Overclock3d (http://www.overclock3d.net/reviews/power_supply/ocz_modxstream_pro_500w/5) does say that the ModXStream is pretty quiet, but that's still only an opinion, and not from one of the top review sites for PSUs (Hardwaresecrets, Johnnyguru and such).
Hardwaresecrets confirm that it is a Highpower unit by the way.

28-09-2012, 09:33 PM
One thing to note is that the voltage of a PSU can be missleading. Many cheap nasty PSU's deliver amps over rails that have a very small voltage. This isn't stable enough. Ideally you want to get a PSU that delivers all the power you require over 12 volt rails. Most well made PSU's (Corsair) only use 12v rails. Don't fall into the trap of buying a really cheap PSU that delivers a large voltage for a very low price.

28-09-2012, 10:56 PM
I've gone through several PSUs over the years from Enermax, Antec and Corsair. Settled on a Zalman 850w PSU, by far the quietest power supply I have ever come across. The 140mm fan and heatpipe design helps keep it at an incredibly low noise level.

28-09-2012, 11:05 PM
Regarding the OCZ 500W ModXstream, I used it and loved it.
It's very quiet, the cable managment ist awesome (you get a nice little bag to put the unused cables in).
And I never had any troubles with it.

It powered an Athlon x4 640 @3 Ghz with a Geforce 460 GTX and later an Intel i5 2500k @ 4 Ghz with the same GPU without any problems.

29-09-2012, 04:32 PM
I did a ridiculous amount of research before building my current system a year ago, and one of the things I learned is that PSU's are a branding and quality minefield. Some companies are the real OEM's who build stuff, some only make designs and contract out the builds, and lots just rebrand units. This article has a huge list of who makes what: Who's Who In Power Supplies, 2011: Brands Vs. Manufacturers (http://www.tomshardware.com/reviews/power-supply-oem-manufacturer,2913.html).

Johnny GURU (http://www.jonnyguru.com/) is by far the best site I found for PSU reviews and info. They are incredibly thorough, and actually disassemble the products to examine components and build quality.

I went with a Seasonic x750 because they design and build their own excellently reviewed units and manufacture many others.

29-09-2012, 09:34 PM
^ The above is true. However, the reliable brands basically never source their products from an unreliable OEM, so it doesn't really matter. You can pretty much buy any PSU from Antec, Corsair, Seasonic, or XFX and be confident that it's not a lemon. OCZ usually works well too, but a few of their cheaper models can be risky business. Cooler Master and Thermaltake make some decent stuff from time to time, but also famously churn out lots of downright bad PSUs - so only buy their stuff if you've checked up on it beforehand (with a review from Johnnyguru, Hardwaresecrets, HardOCP or perhaps one or two other reliable PSU reviewers).

Also, the whole OEM vs. brand thing goes for other components too. RAM and SSDs come to mind.

30-09-2012, 05:17 PM
Also, the whole OEM vs. brand thing goes for other components too. RAM and SSDs come to mind.

Yes, but it's arguably more critical for a PSU than other components because the health of the entire system depends on it. PSU's may be the most skimped-on or overlooked or misunderstood component, and big numbers and a brandname can do more harm here than elsewhere in misleading the average builder.

An unreliable component doesn't typically threaten the lifespan of the rest of the system. Bad PSU's lead to horror stories.

30-09-2012, 10:59 PM
Yes, but it's arguably more critical for a PSU than other components because the health of the entire system depends on it. PSU's may be the most skimped-on or overlooked or misunderstood component, and big numbers and a brandname can do more harm here than elsewhere in misleading the average builder.

An unreliable component doesn't typically threaten the lifespan of the rest of the system. Bad PSU's lead to horror stories.
Big numbers, definitely. Brand names, not so much. Antec, Corsair, Seasonic, XFX... I can't think of any seriously flawed PSU any of them have released in the last many years. Regardless of who the original OEM is (well, it's pretty obvious when it comes to Seasonic).

30-09-2012, 11:36 PM
I didn't read much of the above, but whatever the above: just buy Seasonic. They are the ones producing the re-branded PSUs worth a damn in any case (some of the stuff from Corsair, for instance). There isn't much real competition in PSUs these days.

The X-series is basically all you'll ever need. No fan up until 460W and heat-activated fan past that (which in most cases means silent operation).

01-10-2012, 12:51 AM
Dont forget that many of the known brands dont use their own components, they use the base from a P&C for example or some lower end material from another manufacturer.

My suggestion since Sakkura already said everything important, would be to read alot of review before deciding. Seems boring, but you learn while being sure that you buy something that is worth :)

02-10-2012, 11:25 AM
Finally went for.... beQuiet E9 480 CM - http://www.bequiet.com/en/powersupply/283

Once electrical quality was assured (80+ Gold certified and jonnyguru reviews of the 680w model at http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=269 and the previous generation, non modular, 400w model at http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=247) it came down to accoustics.

I'm extremely obsessive about noise. Frankly, "quiet" is insufficient for my tastes, I'm looking for effectively "silent", as in inaudible to my 30-something ears.* It's going in a system hand picked for accoustics, from damped case, to 80w GPU and controlled chassis fans. Thing about noise is that it doesn't matter if one component is running noisy or five, whatever is the loudest thing in your case will be the subjective noise you experience, and my concern was that even a "quiet" PSU could end up being the loudest thing in there.

All reviews I could find on the subject were unfortunately purely subjective (albeit positive), but looking at the specs it at least has all the right ingredients for "silence" - temp controlled 135mm fan rated at max RPM of just 920, fan mechanically decoupled from the PSU chassis for vibration damping, and whilst manufacturer stated decibel levels are never to be believed, their distribution against load suggest that the fan doesn't kick in above tickover until 50% load (stated 13.7db at 20%, 13.7db at 50%, 18.8db at 100%).

Seasonic were a close choice, but availability this side of the pond is a little limited.

*correction - I only have two ears. They're 30-something years old. Each.