PSU Buyers Guide
What PSU do I need - Quick.
Overclocked, overvolted quad core, running high end overclocked SLI. 2 raptors and 2 normal drives, a sound card, fans, a 12V watercooling pump then you need a 700W PSU.
Overclocked, overvolted dual core, running a single high end overclocked graphics card you need a 500W PSU.
Quad core, high end graphics card you need a 500W PSU.
Dual core, mid range graphics card you need a 400W PSU.
(You could go lower, but PSUs are thin on the ground in this range)
And just because someone is bound to ask...
A heavily overclocked, heavily overvolted Intel i7 (the current 130W TDP), with 2 4870x2 cards, both of those heavily overclocked, running 6 sticks of overclocked, overvolted RAM, with 2 old 3.5 inch Raptors, and all the other bits... You need a 900W PSU. In other words a 1KW PSU.
Unless otherwise stated the PCs have standard peripherals attached - 4 fans, keyboard, mouse, 2 appropriate memory sticks, 2 SATA hard drives. Add more fans, USB devices, hard drives etc without worry, they have negligable effect.
Here's a good calculator.
Why should I buy the right PSU?
Reliability. You just don't want your PSU going pop. Nor do you want to have to bug hunt your PC hardware trying to find out what the problem is. Irregular voltage supply won't be good for components.
OcUK don't sell bad brands of PSUs. They sell top end, high end, midrange and value brands, however the value brands are from reputable manufacturers.
The more efficient your PSU is, the less heat it makes, and the less need for a noisy fan running.
Generally speaking the better a PSU is the more efficient it is, and the more efficient it is the less watts are wasted. Watts cannot be truely wasted, they have to come out as something. Noise, heat or light. If your PSU is noisy and it's not the fan, or it glows then something is badly wrong. It all comes out as heat. Your PSU doesn't like heat, and it causes components to become inefficient, leading to more heat. Almost all PSUs deal with this by blowing air on the internal components with a fan. There was a period a few years ago when many PSUs were being made without fans. They seem to have vanished because they just couldn't give the juice that emerging graphics cards and multi core CPUs demanded. Around that time everything had been single core, and graphics cards had only started needing big amounts of power. Finally a big 12cm fan can operate inaudibly and dissipate heat more effectively than a fanless PSU.
What about Generic PSUs?
Generic means a PSU not sold under a reputable brand. A few years ago nearly all PSUs were generic. These days many still are. They are generally bad. They're inefficient, unreliable, their rated output is higher than they could possibly ever deliver. The cables are too short. The electronic components are of poor quality and not rated for the heat that the PSU will run at, nor rated long enough to be fit for the purpose.
The all important 12V rail.
Your PC likes the 12V rail more than any of the other power rails in your PSU. Many generic PSUs cannot supply enough power on the 12V rail.
A quick calculation for the combined output of the 12V rails is that the Amps should be 0.08 of the total rated Watts. In other words if the PSU is rated at 500W then the combined 12V rails should supply 40A. A good PSU will supply this. An 'budget', but acceptable PSU might supply perhaps 0.06 of this.
Generic PSUs sometimes are able to perform to their rated specifications, by ensuring that they do this they can't be returned as faulty. However if they are rated at 480W and the 12V rail can supply 15A then no PC could draw 480W from them and remain within specifications.
Generic PSUs also state specifications that are fiction. The reason they get away with this is because someone building a beefy gaming PC believes they should buy a 800W PSU when in fact they never draw more than 350W.
Generic PSUs also have a lack of components internally. To save money they cut the corners with voltage regulation etc. This means the voltage wobbles, which causes instability.
Finally not all generic PSUs look "generic". They aren't all grey boxes. Some of them are dressed up with LED fans, shiny black cases, braided sleeving etc. But they still suck.
But a review said it was good! Lots of PSU reviews suck badly. There's plenty of inept reviews out there from people who bought and product and found it worked for them. Just because some idiot drew 250W from an 800W PSU and thought it was reasonably quiet and it didn't cost much doesn't make it a good PSU. Good places to read reviews are.
Hardware Secrets
Jonny Guru
Toms Hardware
Silent PC Review
The best (and it's funny) PSU review I've read of generics is over at Jonny Guru.
Can I realistically save money with efficiency?
Yes. But not massive amounts.
First thing to do is forget about the wattage your PC actually uses. It's used by the PC, you can't save on it.
At worst a decent PSU will be 80% efficient. At best it'll be 86% efficient. That's only 6% difference, right? Wrong. I already said we had to forget about the power that gets used. So we're at 20% waste, compared to 14%. 30% difference, rather than 6%.
Percentages don't mean much until you apply them to real world figures. Assuming you use an average of 250W for your PC. It's not an unreasonable figure since the PC isn't going to be doing everythng at once. 250W and to supply that at 80% efficiency the draw on the AC is 312.5W. Wasted power is 62.5W. At best efficiency 290.5W is needed, meaning 40.5W is wasted. Electricity is charged by the kilo watt hour. In other words for every 1,000 watts used you get charged a fee. It varies, but let's use 10p.
The difference between the two efficiencies is 22.5W. Or 0.0225 kilo watts. An hour of that costs you £0.00225. 24 hours in a day 7 days a week 52 weeks in a year costs you £19.66. Over the 3 year life of the PSU that's £58.97. Call it £60 quid. So getting the efficiency right on, as opposed to completely wrong saves you £60 on the life of the PSU. That's not a bad figure considering £60 is the cost of the PSU. I can't speak for you, so it's up to you if that figure is significant or not. Also remember it's a best case/worst case, you're unlikely to run into a £60 cost by accident.
Don't buy a PSU too big. Almost all PSUs I've seen have their peak efficiency at 50%. They tend to be inefficient at tiny power draw (but that makes the waste tiny too). However they are 2-3% less efficient in general, and sometimes more at 20% efficiency. If you have a PC that needs 300W of power on load and 150W on idle, and you have a 1000W PSU which is 88% efficient, then your efficiency at idle could be 84%, meaning all that extra you spent on an efficient PSU is down the drain. Not to mention of course you've spent 200 quid on a beast of a PSU when you needed to spend 60 quid on something more appropriate.
What is 80plus
80 plus is a certification programme to enable PSU manufacturers to brand their power supplies as efficient. The theory is that any reasonable PSU should be 80 plus certified, reassuring the consumer that they're buying something of at least adequate quality. There is also a bronze, silver and gold award for higher efficiency labels. This means the consumer can spend a bit extra, and in theory save money in the long run.
Sadly it seems to be utterly useless. They cerify Huntkey PSUs, however every single Huntkey that Hardware Secrets has reviewed has failed badly. Hardware secrets come out with an efficiency of 71% at load on a Huntkey.
They work from reference units, so it's possible that PSU manufacturers send them beefed up sample units rather than retail. Or perhaps they just bribe them.
The programme is rendered useless if it can't be trusted. And it can't be trusted. I've crosschecked a few decent reviews of PSUs against the "official" 80 plus figures, and they don't match.
What Features do I want?
PFC - Power Factor Correction. If you can buy a PSU it has it. It's legally required to.
Modular - This means that you can select which cables you attach, and which you don't. It means the case ends up tidier. It's not essential. If you want -fully- modular then be careful, as not all modular PSUs are completely modular. This is because you have to have a 24 pin power cable to run any PC, so some suppliers have that one fixed.
Braided/Sleeved - Windowed case? You might want to consider this. You can sleeve your own cables, but it's time consuming.
LED Fans - I'm not going to pass judgement on anyone who wants these. However, if you don't want them make sure the PSU doesn't come with them.
Form Factor - Your case will take a standard ATX PSU, almost certainly. However many of the larger PSUs don't fit this form factor. Be careful if you're buying a larger one in case it doesn't fit!
A note on Multiple Rails
Intel came out with a spec for PSUs that required multiple 12v rails. There's no real advantage to this, but disadvantages. It means you need to watch what's on what rail, and it increases the cost of the unit. However not all suppliers properly adopted it (for those reasons) and Intel has dropped the requirement.
The problem with having 12V rails limited to 20A is if you want to power 3 things at 15A you simply can't.
This means that PSUs with "virtual rails", in other words really only having 1 rail are preferable.
Thus a PSU with just 2 rails limited to 20A each is no good for a dual card system (2x or x2). As such I won't recommend any, except at the budget level where there's little choice.
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