450W from a 550W PSU might be pushing it a bit, but it really depends on the PSU, some are perfectly capable of supplying >80% of their maximum load continuously and efficiently, others are not.

Take a TX650M for example,


From a 230V input it's maximum efficiency is ~88% (at 50% load it's at it's optimal), but at ~75% load it's efficiency has only fallen to 87% and even at 100% load (not recommended) the efficiency is still 85%. A better model would have 90% efficiency at 90% load,



and a top model could be 92% efficiency at full load,



If you are worried about the 550W PSU, look up it's efficiency. If you think you need to reduce the load stick with stock CPU clocks.

BTW. Haswell needs a PSU that can deliver 0.05W to support it's C6 and C7 power states (but you can configure a system to stay at a high clock).

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Nice PSU graphs. I have an HX850, so I feel it should be pretty efficient when I get two GTX 660s in there. It looks like a target of 50% is great for budget PSUs, but a high end PSU like the AX860i would more easily handle additional load.

Hm ok does anything changed in technology? I heared years ago, the psus with 80.83 85 certificate are running only at this rate in the upper usage. So i have as example buyed an 530w psu for my dual 570gtx setup here. The other psus in other machines dont have much headroom either. But as i read here this is not the case anymore, good to know O.o

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I'm running 2 x GTX 570 and a OC'd i5 4670k with a 550W be quiet! psu at the moment.
No problems so far, the machine draws about 450-500W under full load.
I don't understand why some people on Google recommend a 850W psu for this setup.

If your system is pulling 500W on a 550W PSU, it will run less efficient, it will run hotter, with a shorter lifespan and it may result in some errors of GPU WU's.
The advice is to use a PSU with plenty of head room. If it works at 50-70%, so a 1000-850W PSU in your case, is the best for performance, efficiency and heat production.

It's not a 24/7 crunching machine, so it doesn't matter.
500W is the peak with 100% and both GPU's at 99%.
I'm only doing GPU WU's every few days and while crunching with the CPU it draws ~ 150W.

Looking at those PSU efficiency charts, we're looking at around 3% difference in efficiency comparing between the peak ~45% load and at 90% load.

The bigger difference is probably to make sure you get a good power supply in the first place. There's a 5% difference between 80Plus Bronze and 80Plus Gold if you live in a country with 115V power. It widens to 7% difference if you're in a 230V country. There are also Platinum and Titanium grades of power supplies... though price at that level is another issue.

Of course once you get the quality of the PSU pinned down, second most important thing to try is to find a PSU where you're operating at 50% load...

More details here:
http://en.wikipedia.org/wiki/80_Plus

Example of my four years old PSU, 20-50% mean the range between 130-310 watts. Buying a new one with 90% (4-6% better than now) starts at 75 Euro (PSU with same net power) or 300 kWh (0,25 Euro per kWh). With 5% increased efficiency that is according to my current wattage level (~170 watts) 170*5% = 161 or ~10 watts. Price of 300kWh to reduce 10 watts requires a minimum runtime of 30.000 hours or (current daily runtime in my case max. 14 hours) 2143 days or 5.8 years. It´s too costly than it´s worth. Maybe I should switch to a platinum PSU. Or simply reduce the DC runtime at all.

For a good crunching system (say with 2 high end GPU's or 3 mid-high end GPU's) with a power draw of say 500W, you would notice the difference more. The more energy efficient PSU would result in a cooler system, and might save you money in the long run.
To work on my examples and compare a descent TX650M with the high end AX860i, you can see at 500W (230V) the efficiencies are 86% and 94% - a difference of 8%.
8% of 500W is 40W. Running 24/7 you would save 0.96KW.
At a cost of £0.16/KW that's about 15pence a day or £56/year.
The cost of these PSU's in the UK is about £90 for the TX650M and £144 for the AX860i - so it would roughly pay for itself in a year.
Obviously with less power draw and cheaper electric it would take longer, but in some countries the better PSU would pay for itself within 6months or less.

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