Are PSU calculator sites accurate?

[lehtv]

This is completely wrong. A PSU doesnt use the total amount of wattage it can supply, it only uses as much as is requested by the parts attached to it.

That's not what he meant. The clue is in the previous sentence: "the efficiency of a unit is not uniform across it's output". Meaning, with an oversized unit, you're not going to be drawing enough power to end up in the halfway mark (50% load) where efficiency is optimal.

 

[BonzaiDuck]

I think that's the key to all of this. You can add up all the possible power-draws of devices within the computer case, and assume the worst case has them all working full-bore simultaneously, but that will never happen.

You could spend a lot of time and trouble "measuring" an average "usage-profile" for yourself.

But the peak efficiency for good PSU's is somewhere around the median or -- as has been said -- 50% of the rated PSU output.

The other thing, and I said it elsewhere in these forums recently, the PSU is rated for its sustained, mean output. This would make arguments about graphics card power spikes less relevant for good PSUs, and more relevant for PSUs of questionable rated output. If an XFX rebranded Seasonic 750W unit is capable of sustaining output up to 950W, it tells you that quality PSUs are rated conservatively.

That's why I believe I can "stand pat" with a 650W Seasonic with two OC'd GTX 970's in SLI. Although I won't exceed the rated output, anyone who does would have lower efficiency than the lower level measured just below that output, but the decline is probably a continuum. Conversely, if you use a PSU rated at 850W or 1,000W, and your range of power-consumption within the PC is between 85W and 300W, the PSU will be operating at below rated/peak efficiency.

I like to say it's a "Goldi-Locks, Three-bears and porridge problem." Which PSU is "just right?" :biggrin:

 

[TemjinGold]

For quality PSUs, I wouldn't sweat hitting 50% so much because quite honestly, the efficiency difference from 20 to 100% isn't really much at all. What I would be more concerned with is NOT hitting 20% at idle (i.e. unit is too large) because that is where you will be losing.

 

[BonzaiDuck]

For quality PSUs, I wouldn't sweat hitting 50% so much because quite honestly, the efficiency difference from 20 to 100% isn't really much at all. What I would be more concerned with is NOT hitting 20% at idle (i.e. unit is too large) because that is where you will be losing.

Maybe I got Oldheimers and can't remember if I already said it. The low-end of power consumption is indeed something to look at. The other end of it is a major determinant of the actual PSU investment. If you accept the expansion limits of the computer you built, you can cut it pretty close if you choose a quality power supply.

And I can't believe the junk PSUs I bought 14 years ago. It was like an afterthought: "I just need a PSU with enough wattage and I'll spend as little as possible."

Wrong. You need the PSU with enough wattage, you need to get the best PSU you can, and THEN you need to find a way to spend as little as possible to get it.

So . . . choosing a quality power supply -- that's the key . . . that's the key . .

 

[click4dylan]

I registered just to let you know that I ran an RS-700 PSU 24/7 (literally, only shut it down a few times to clean the system) from 2011 to 2014 with an i7 3770k overclocked to 4.5 ghz, 4 sticks of ddr3 ram, ssd and 4 hard drives, and a GTX 580 maxed out on voltage and clock speed to 950 mhz, and then upgraded to a a GTX 770 4GB, sound card, network card, and watercooling, with tons of fans and never had a problem with it. I replaced it in december with a Rosewill Fortress 650w and was absolutely amazed at how much better quality and heavier it was compared to the RS-700. I never even maintained the RS-700, i took it apart after i got the rosewill to clean it out and use it in another machine. And here's what I noted. It was fairly well built from what I could see. But there is one transformer in it that is unusually small, definitely not for a 700W unit. It was so small in fact that it appeared that it was overheated a few times as it was discolored a bit. But there was never any signs of problems with the machine and the PSU is still being used in another system as of now. So no you don't need to worry about if it will work with your system. Just keep in mind that there are far better quality ones than this.