How do you size a PSU?

[holabr]

How do you determine how many watts in a PSU you need for any given system? I'm planning to build a new system using a AMD processor (either x4 or x6) with 4GB of DD3 memory, 1 SATA HDD, 1 SATA DVD writer, a Legacy PATA HDD and possibly a SSD. I will use a board that has video and audio built in but may upgrade with a CrossFireX video card. Are there other considerations? Does the OS make a difference? I'm thinking of one of the following:

Antec BP550 Plus 550W
SeaSonic M12II 520 Bronze 520W
CORSAIR CMPSU-400CX 400W

Are the overkill or insufficient? Which is the best value?
Thanks.

 

[Rifter]

CORSAIR CMPSU-400CX 400W will be perfect for that system.

 

[holabr]

Rifterut,

I noticed that you used an 850W PSU in your system. I'm trying to understand what is in your configuration that requires twice the power than the one I am proposing.

 

[Rifter]

I am considering getting another 4890 for crossfire and want the PSU to back it up when i decide to do it.

Since the 4890 is a power hog to begin with running 2 of them will require a beefy PSU, i could have got away with a 750W but decided to get a little bit of breathing room, and i got it on sale for $99 Canadian so at the time the 750W would have actually cost me more.

 

[nipplefish]

http://extreme.outervision.com/psucalculatorlite.jsp

This site is pretty good and should cover all your bases... computers really aren't as power-hungry as many people think. The computer in my sig wants a 500W PSU according to the calculator. My UPS reports a draw of about 480W with LinX and Furmark both running (that includes a monitor, though).

 

[Campy]

I've read that you should have 600W when running cf/sli(recommended by ati/nvidia, not 100% sure about this number though). Of course this isnt to say the cards would use anywhere near 600W it's just to make sure you should be getting enough amps on each psu rail and stable voltages.

 

[ModestGamer]

in analog circut design a general rule of thmub is 130% of max demand minimum. Not a bad policy for a pc.

 

[jonnyGURU]

A computer power supply only puts out as much power as demanded of it, so it doesn't matter if you get a bigger PSU than you need.

PSU's do tend to be more efficient when the load on them is between 20 and 75% of it's capability. Also, a lightly loaded PSU will run cooler and quieter and may last longer because it's not being tasked as hard as it's designed to. So that's a couple good reasons to not get the smallest PSU you can get by with. Of course, the flip side is higher cost.

 

[Meghan54]

I've got an HX850 in my system. Kinda overkill, but I tend to lean towards what JG posted above.

1. Really cannot buy too big, within constraints.

2. I prefer my rig to top out at full draw at about 60% of my ps's rated output. That's around the sweet spot for efficiency, noise, heat production in ps's, typically.

3. My system, an i7 920 @ 4GHz @ 1.32V, 4870 Toxic, 2 1GB hd's, watercooling via dual DDC2 pumps, and my system idles around 240+W pulled from the wall. With a few web pages open and listening to ESPN, it "idles" right at 300W from the wall. Full draw, the highest I've seen pulled from the wall, was right under 600W, and at an assumed 90% eff., that's like 450W from the ps.....just over half output and most efficient spot.

That's why I overbought. It runs quiet, cool, and I never worry bout excess noise, heat, or much of any other stress really being put on the ps, even if I decide to Crossfire a pair of 5850's or whatever.

 

[theAnimal]

was right under 600W, and at an assumed 90% eff., that's like 450W from the ps.....just over half output and most efficient spot.

540W by my math, which seems quite high for that system.

 

[Modelworks]

I really don't like to use overall watts as a decider for what size supply to buy. Instead I prefer to break it down to how many amps each voltage needs and purchase based on that. Using watts alone can put you in a situation where you add a high power video card and find out it would have worked out fine if they had just put 2A more on the +12V instead of the extra capacity on the 3.3 and 5v. One supply may say 500W and have low 12V but high 3.3 or 5v while another has just the opposite.

 

[holabr]

OK. I'm going to buy today. Newegg has two PSU that look promising to me at the $50 to 60 range after offers and rebates. They are:

OCZ ModXStream Pro OCZ700MXSP 700W
Antec EarthWatts EA650 650W Continuous Power

Other than one is modular and the other not, how do they compare. Which would you buy? The other posts above convinced me to go bigger.

 

[ElFenix]

you're going to be way down on the efficiency curve with that much peak available.

if i were buying one of those two it'd be the antec, based on reputation. they usually source good units from good OEMs.

 

[HOOfan 1]

You are only using onboard video. Meaning you are not interested in gaming, and even if you upgrade to a videocard, it likely won't be a high power gaming card. I would just go with the Seasonic M12II 520 Bronze in your first post. It is certainly a better power supply than the OCZ ModXstream.

 

[TemjinGold]

With onboard video, that system will likely PEAK under 150 watts (normal operation you're probably looking at under 40 watts.) Any of the PSUs in your original post will be more than enough. The two you are now considering will be worse because you won't be anywhere NEAR 20% usage under normal operation, meaning it will be HORRIBLY inefficient. It would be like hooking a calculator up to a power generator.

 

[holabr]

When I used the PSU calculator at

http://extreme.outervision.com/psucalculatorlite.jsp

nipplefish mentioned in his post, I came up with 470 watts. That is a long way from the 150 watts TemjinGold figures I will need.

Also, from a cost standpoint the Seasonic M12II 520 Bronze is actually about $25 more than the Antec or OCZ higher power units in my more recent post. The CORSAIR CMPSU-400CX 400W is really the most inexpensive at about $30.

 

[jonnyGURU]

TemjinGold is mistaken. A quad core Phenom is going to use 65 to 140W of power alone. A 400W is a good choice, but it leaves you no overhead for your future graphics card plans.

 

[holabr]

I know you guys said a 400 watt would be sufficient for my system but considering that I may want to add a higher performance video card in the future would it be a good idea to buy an Antec Earthwatts EA650. I can get one new for $53 shipped.

 

[lord_emperor]

1a. Precise way: Use that PSU calculator linked above.

2b. Ballpark way: Add up the manufacturer's stated power usage for for the video card and CPU, add a few more watts for slush room then multiply by 1.33 so your PSU will run at around 75% capacity.

2. Buy a power supply that's on sale and is equal to or greater than the result from above.

 

[Fayd]

1a. Precise way: Use that PSU calculator linked above.

2b. Ballpark way: Add up the manufacturer's stated power usage for for the video card and CPU, add a few more watts for slush room then multiply by 1.33 so your PSU will run at around 75% capacity.

2. Buy a power supply that's on sale and is equal to or greater than the result from above.

your "ballpark way" is gonna be a hell of a lot more precise than your "precise way".

 

[Fayd]

You are only using onboard video. Meaning you are not interested in gaming, and even if you upgrade to a videocard, it likely won't be a high power gaming card. I would just go with the Seasonic M12II 520 Bronze in your first post. It is certainly a better power supply than the OCZ ModXstream.

i'd just go with the corsair. he's overthinking this.

i've had a corsair in my rig for 3 years, ever since they introduced them.

i've built 3 more rigs for others based on corsair PSU's. never a problem.