Originally Posted by fengshui
what happens if the sole purpose of the pc is rendering or gaming? in other words, 99% of the time the components are drawing the max power. wouldn't this make a bigger psu more efficient? i think you should've mentioned this in the op. there is simply no way that an equal amount of time is divided between the range of voltages to use the average voltage for calculations. plus, if ur about to drop a high end, efficient psu, it is more than likely a high end gaming pc. and it will be at max load most of the time playing games.
He did mention it. You want your average power draw to be at roughly 50% of the PSU's capacity. This means that if your computer is pulling let's say 500W from the PSU 24/7 (as discovered by multiplying a wall meter reading by your PSU's advertised efficiency, like .80 or .85 or .90 or .95), then your minimum PSU recommendation would be at around 850W (continuous, not peak) in order to maximize your average efficiency. So, all you really need to know is approximately how much power your computer is pulling from the PSU on average in order to size the new PSU for maximum average efficiency.
So, to take this in the other direction, let's say that your computer only pulls 500W for a couple of hours per day and the rest of the day it's pulling anywhere between 50 to 125W from the PSU. Well, this would mean that you would want to get a 550 to 650W PSU (continuous, not peak). Anything bigger is just a waste of money unless you have definite plans to make upgrades that would increase your power draw quite a bit.
So, what do you do if your computer is pulling 500W for about 12 hours per day? Get a good 650W PSU.
As you can see, it's all about your average power draw. Too many people say, "Oh, my computer pulls 500W when I'm gaming, therefore I should get a 1000W PSU in order to maximize my efficiency because everyone says that you want your power draw to be at about 50% of the PSU's capacity". This would only be true of that 500W power draw is a 24/7 power draw and also planned to be like that for several months or even a few years. If that person is only gaming for a few hours per day, then they will maximize their average efficiency by using a good 550-650W PSU.
Now, the next question is: what about leaving room for future upgrades? That requires a lot of questions and answers, and so I can't address that in this post. Some future upgrades wouldn't require more power at all (some would even reduce the power consumption), others would require a LOT more. Some future upgrades are just pipe dreams, wishful thinking. Some people even mention future upgrades just so they can trick you into recommending a much bigger PSU than they really need so that they can get that 1000W+ PSU when they know that they'll never need anything more than 550-650W (they know that you'll never recommend a 1000W+ PSU unless they make you think they'll need it - some people are really that stubborn). You have to be part expert, part detective and part psychologist or mind reader.
Anyway, so yeah: go by your average power draw and put that average power draw at roughly 50-75% of the PSU's capacity in order to maximize your average efficiency. This will also result in getting the highest quality power output from your PSU as possible. If the power draw is too small on the PSU, like 50-125W from a 1000W PSU, then you're not only killing your efficiency, but you're also getting "dirtier" power! This can result in all kinds of weird problems and it can also shorten the life of both your computer and your PSU. Therefore, when it comes to PSUs, bigger is not always better. I see way too many people these days with like a 1200W+ PSU powering a system that has a mainstream CPU that's overclocked on air with just one video card! I saw someone yesterday with exactly that kind of a rig being powered by the 1500W SuperNOVA. This kind of a setup is becoming way too common.
To get back on topic once again just to close this post: always go by what your average power draw is, not what your peak is. After all, what if that peak is only happening for 2-3 hours per day and the rest of the time your computer is only pulling 50-125W from the PSU? This gets worse if you leave your computer on 24/7 because guess how much power it's pulling when it's just sitting there doing nothing. Yep: 50-125W.
Edited by TwoCables - 2/17/14 at 9:12am