Proof you need another PSU

[dravisar]

I just plugged another GTX 285 into my rig below to use in SLI (first time using SLI since dual Voodoo II's a long, long time ago), and am running a 650w BFG power supply in the current rig. I knew it wouldn't hang, but I had to try.

The old BFG 650 was designed for 8800GT SLI, not 2xGTX285s. It ran Vantage benchmarks for about 10 minutes, then overheated, efficiency dropped, and the computer shut down to lack of power/safety limits in the PSU.

Now going shopping for another, 850+ watt power supply!

 

[Zero82z]

The reason you had issues with that PSU is because it's one of BFG's older lower-quality PSUs. A truly good 650W PSU would be able to handle your system with both video cards without a problem. Each GTX285 uses about 150W at maximum load, and your CPU uses about 120-130W at most (estimating on the higher side). Your total system load is maybe 500W at most, which although a bit high, is still perfectly within the capabilities of a 650W PSU that's actually capable of handling its full rated load under normal temperature conditions. Obviously your BFG PSU isn't capable of that, but don't take that as a sign that all 650W PSUs would behave the same way.

 

[JonGerow]

The reason you had issues with that PSU is because it's one of BFG's older lower-quality PSUs. A truly good 650W PSU would be able to handle your system with both video cards without a problem. Each GTX285 uses about 150W at maximum load, and your CPU uses about 120-130W at most (estimating on the higher side). Your total system load is maybe 500W at most, which although a bit high, is still perfectly within the capabilities of a 650W PSU that's actually capable of handling its full rated load under normal temperature conditions. Obviously your BFG PSU isn't capable of that, but don't take that as a sign that all 650W PSUs would behave the same way.

I'd still upgrade the PSU to more than a 650W if you plan to run two GTX 285's. At BFG, we've brought a lot of solid PSU's to their knees by running both FurMark and Prime on systems using a pair of GTX 285's. Even when the PSU performs the task, it's running at the upper levels of it's capabilities, runs hot and has the fan spinning full tilt.

 

[Benjus]

I'd still upgrade the PSU to more than a 650W if you plan to run two GTX 285's. At BFG, we've brought a lot of solid PSU's to their knees by running both FurMark and Prime on systems using a pair of GTX 285's. Even when the PSU performs the task, it's running at the upper levels of it's capabilities, runs hot and has the fan spinning full tilt.

+1. Not to mention the fact you are nowhere near the optimal efficency range of a 650W if you're running an i7 + 2 gtx 285s.

 

[dravisar]

Only reason I even bothered with the BFG 650w was to see if it would work, my 1000w was delayed until Friday.

It was definitely out of its element (I've had it since the Opteron 165 craze) and needed replacing. The 1kw PSU was overkill, but I got a deal and couldn't pass it up for the "future proof" peace of mind.

I'd be very hesitant to tell anyone a modern 650 watt PSU can handle 2x GTX285's "without a problem" Zero. Someone is liable to either be very pissed off at having a system that doesn't work at best, or a PSU that melted down and fried their system at worst.

 

[keenan]

+Not to mention the fact you are nowhere near the optimal efficency range of a 650W if you're running an i7 + 2 gtx 285s.

Any 80PLUS certified PSU must be 80% efficient at 20%, 50% and 100% of rated load. That basically forces them to aim for a pretty flat efficiency curve. There are going to be areas where efficiency is better, but generally not much more than 5% deviation once you get to a reasonable load (efficiency is generally low at light loading). Though OP's PSU probably isn't 80PLUS, this is pretty much what the 'standard' SMPS efficiency curve looks like. Low efficiency at low load, quickly increasing and flattening out until it gets overloaded.

See for example SPCR's testing of the Corsair TX650: http://www.silentpcreview.com/article813-page4.html

 

[JonGerow]

Any 80PLUS certified PSU must be 80% efficient at 20%, 50% and 100% of rated load. That basically forces them to aim for a pretty flat efficiency curve.

"pretty flat", but not really flat. There's still a curve that peaks at 50% load. This is why to earn a "Bronze" by 80 Plus, for example, the PSU has to be only 82% at 20% and 100%, but has to be 85% at 50% load. Not that 3% makes a huge difference on the electic bill at the end of the day, but....

There are going to be areas where efficiency is better, but generally not much more than 5% deviation once you get to a reasonable load (efficiency is generally low at light loading).

There are actually a number of PSU's that are "only" 80 Plus (as opposed to Bronze, etc.) because although they may hit 85% or better at 50% load, they're only 80% at 20% and 100% load.

Example: BFG's EX-1200 hit 87.58% at 50% load! But only 81.90% at 100% load. That's a difference of 6.32%!

Though OP's PSU probably isn't 80PLUS, this is pretty much what the 'standard' SMPS efficiency curve looks like. Low efficiency at low load, quickly increasing and flattening out until it gets overloaded.

See for example SPCR's testing of the Corsair TX650: http://www.silentpcreview.com/article813-page4.html

Actually, a better place to witness the "curve" is 80 Plus's own website, although they don't test below 20% or at stand by, which is really a shame since some power supplies (*cough* BFG EX Series *cough*) perform very well at very low loads where most computers are most of the time.

You can also look at reviews right here on HardOCP for fairly thorough efficiency results and Jeremy over at jonnyguru.com has begun adding his own "low load" efficiency reports to his reviews.

 

[Zero82z]

I'd still upgrade the PSU to more than a 650W if you plan to run two GTX 285's. At BFG, we've brought a lot of solid PSU's to their knees by running both FurMark and Prime on systems using a pair of GTX 285's. Even when the PSU performs the task, it's running at the upper levels of it's capabilities, runs hot and has the fan spinning full tilt.

I'd certainly recommend something better. However, a good 650W PSU would still be enough for someone running that kind of a system considering the fact that it would rarely be under absolute maximum load.

I'd be very hesitant to tell anyone a modern 650 watt PSU can handle 2x GTX285's "without a problem" Zero. Someone is liable to either be very pissed off at having a system that doesn't work at best, or a PSU that melted down and fried their system at worst.

You may be hesitant, but that doesn't mean it wouldn't be sufficient. I already broke down the numbers in my post above.

 

[vjcsmoke]

A Corsair 1000Watt should do the trick.