Example or Effeciency ratings by Chart
here is one of the top model PSU on hte market by Ultra..
550W, notice the peak effeciency is about 310W
Now notice the 800W, peak effenciency is about 430W
Here is my 1600W PSU
Peak effenciency is about 880W
so my optimal running for my system is with gaming and graphics maxed out with my current rig.. I can however still upgrade and push into the 930 to 950W range and still be comfortable.. I can push even further and lose some effeciency without getting into degredation ranges..
On the charts, the lower the bar gets, the more punishment you are doing to the PSU, and as such, the less life you will have with the unit.
Where people see effeciency in these charts, I see wear and tear..
on the 550 PSU, at peak operation, it is 79% effecient, thus 21% of that is work or wear.. but that equates into minimal degredation of the unit..
Now look at the upper limit .. It is only 73% effecient .. a 6 % increase to wear and tear.. or 27% constant work .. as such the PSU will fail much faster than running it at effecient peak.. and remember, degredation is cumlative..
so if after 3 months of running max you lose 3% effeciency rating running the system at near peak (within the 90% range of the PSU capabilities).. so that 79% will be 76% and the peak end will be 70% or 30% wear and tear.. that 30% is increased wear and as such, next 3 months you will lose approx 5% total effenciency..and that is in 6 months
You subtract that from your max rating.. 550- 5% = a loss of 27.5W peak.. as such the PSU is now rated for 522.5W.
The effect is cumulative and compounding... However degredation doesn't accumulate near as fast so long as you stay outside the 10% max rating. the 5% loss wouldn't be present if you were running Optimal effeciency until about 2 years down the line.. on my Ultra.. I only have about a 4.2% effenciency loss and the PSU is not quite 2 years old.. I figure in Feb, my effencienfy rating will have degraded by about 5 to 5.2 %.. so no where near in danger of my PSU dying..
Remember the PII systems and P III, they came with 200W PSU's, but the systems only pulled 140W max with the best graphics and sound on the market.. they overpowered the systems.. as such, my old P II and P III are working just fine.. though the PSU's are nearing the end of their life cycle.. Today in computer companies, they put PSU's at the limit of the system power requirements.. as such, the PSU may last 3 years and then needs to be replaced.. which is where they make money. they degrade quickly.
These are things that they don't teach you in Computer repair classes.. they teach you to test a PSU to see how much power it is putting out, however that is rarely tested under load.. but if the unit doesn't hit peak, pop another supply in that is rated the same..
Unfortunately, most pre-built systems come with underpowered PSU's in them and most shops do the same.. reason.. repair , warranty, part sales, return business..
Makes sence, good business model.. However in practice it is poor.. when you build your own system, throw out what School tought you.. they teach you market practice, not practicality or comparrison.
This is why I stated I speak from experience.