These days whenever you try to buy/build a PC, you constantly see demands and suggestions for large power supplies. 400W seems to be for low-end machines, Real Men need at least 500W, but if you intend to do gaming, better get at least 650W, etc. In one MB manual that I'm considering to buy I even saw a suggestion to buy a 1000W PSU if you intend to install a serious VGA.
But how much of this is true, and how much of this is sweet-talk by marketing weasels? From personal experience I can tell that for years I've been happy with 300W PSU's, and now, when I recently put together a fairly mid-to-high-end gaming machine it runs just fine on a 400W PSU. And that's with a Phenom II X2 550 Black Edition, MSI Radeon 4850 OC, 2x1GB Corsair DDR3 RAM, 2xHDD, 1xDVD-RW, plus a bunch of lesser power-consumers like fans and USB peripherals.
Am I risking something? Can the system just go up in smoke one day because I've selected a PSU too weak? Or is 400W really enough for most needs and the whole wattage-hype is way too overrated?
Note - I do understand that there are cheap low-quality PSUs out there that boast a power rating way higher that they can actually support. I'm not talking about those. I'm talking about decent PSUs from manufacturers with a good reputation that can be trusted to deliver close to the power they promise.
65 Answers
In your shoes, I would probably start by adding up the consumption ratings for every component in the system. I try not to put too much stock in the nebulous claims of "you need at least X watts for a good machine." I prefer to determine my power needs based on my components since no one else is going to use the exact same combination of components. Keep in mind that you'll want to add a little "extra" capacity for those times of "peak" usage.
Also keep in mind that even high-quality parts can be subject to failure of an individual component, so there's nothing to guarantee that a (true) 1000W PS is going to be "safer" than another.
In my experience I don't think I've ever had a PS die because it was underpowered, but rather because one of many seemingly innocuous capacitors blew its top prematurely.
Core i7 920 clocked at 3.4, 6GB RAM, 3 drives (one 10k raptor), DVD drive, 9800GT OCed, runs from a 500watt PSU. I got some power readings before my power meter died. Idle is just under 200 watts, full CPU load puts that up to about 300 watts. I didn't get figures for full graphics and CPU load but it must be less than 500 watts because that is what my PSU is.
For years I have been saying a similar thing.
However, it may be worth putting your machine under a stress test - either video rendering or playing a high end game / just make the CPU and GPU go to 100% for as long as possible as it is random failures during gaming / tasks like this when I always suggest "Is your PSU high enough".
Also, I agree, there are some cheap ones which are rated higher than they can provide, but typically there is just unknown brands or unbranded and known brands - the known brands are not really better than any others.
The only thing to be on the look out for is 80%+ certified PSU's which if you use your PC 24x7, may have some cost savings.... and generally are of good quality.
1That's a pretty low-end system you have there. Dual core, 2 GB (seriously? in 2009?) of RAM, etc. I would indeed expect 400 Watts would be sufficient. I used to run a 4 GB system with 7 hard drives, two optical drives, an 8800 GTX video card, hardware RAID, etc. on a 700 Watt power supply. 400 Watts would not have been sufficient. A modern mid-range system can probably do fine with 400 Watts but when you start looking at dual optical drives, high end video cards (though the new Radeons are fairly efficient), the possibility of upgrading to SLI, 6 - 12 GB of RAM, etc., 400 Watts is really too low.
You want to go with a high quality and 80%+ certified PSU. You'll likely find the price difference between a quality 400 Watt power supply and a quality 600-700 Watt power supply is not that significant, often on the order of $10 - $25. Certainly more expensive than the garbage you get 'free' with a case, but who's willing to risk that? You still pay a lot more for 1000+ Watt power supplies. Those aren't worth the money unless you really need them, and you should know if you do.
It is certainly the case that many people don't need more than 400 Watts. Anyone building a low end system like you listed should probably not bother spending extra on a high end power supply. But it is more important when you look at mid-range or high-end systems.
6Manufacturers lie about PSU output. Go for a 80+ certified PSU of low wattage; it will give the nominal output, won't dissipate too much heat, and you'll save on electricity bills. Here is a guide.
