I know that PSUs work best at 50% of their rated load, and that under 20% they are ineffecient. However at what point are they harmful? I recall reading that at some point you start getting ripples in the rails and/or voltage regulation issues, but I can neither find, nor recall at what point this happens if it happens at all. I'm looking to buy a new PSU and was thinking that if it's just a matter of 10 or 20 extra USD then I'd be apply to have a quieter, cooler PSU, but not at the expensense of my system components. I consider the PSU the most important part of a PC largely due to having once purchased only what was cheapest. This lead to me going through hardware like crazy until I had DFI support (may they rest in peace) tell me ``lol, what type of n00b are you?!" which was just about the most constructive thing I ever heard from a tech support rep.

I think that is a misconception. Modern PSUs can scale quite well regardless power output: their efficiency is within 2-3% of rated efficiency (e.g. bronze 82-85% or plat 89-92%). Of course 50% seems most efficient place to be but I wouldn't claim it being inefficient.

nstgc

Nada1
I think that is a misconception. Modern PSUs can scale quite well regardless power output: their efficiency is within 2-3% of rated efficiency (e.g. bronze 82-85% or plat 89-92%). Of course 50% seems most efficient place to be but I wouldn't claim it being inefficient.

I'm not worried about efficiency. It's power quality that I care about.

Oh, I don't completely understand your question. Are you asking: at which power output/level PSUs tend to fail/start behaving oddly, and if so, does quality of the PSU matter? I don't have any experience about that — never had any issues although though I haven't ran them any odd loads. To be honest, I doubt any reliable brand would start behaving 0-100% load (if item is not faulty). 

http://www.jonnyguru.com/...uestion&ndfaq_id=3

Larger PSU's generally have higher quality components which can be attributed to lower ripple, noise, etc. The components are beefier and will remain cooler under loads than if you buy a smaller wattage PSU that is 90% utilized under full load. The PSU I have listed in the specs in my signature is totally overkill in terms of wattage, but I never get to hear its fan :D

If your PSU under load is using 20% or less of its rated wattage, then that is typically a unit that is just too big. Why? Because most PSU's don't do well with ripple and efficiency at such small loads. Where they usually really shine is between 50 and 75% of there rated wattage.