I was just reading this thread, and concur with Leslieann. You need to calculate total power consumption/requirements. Keep in mind the efficiency rating of the power supply. Basic PSUs will provide 80% efficiency. This means a standard 80+ 450w PSU will provide 360w MAX power.
As Tp4tissue said, you have that backwards but also, you never should base it on total power because you cant. I realize it's how it's done, but it's actually a stupid way of rating power supplies.
This isn't directed solely at you, just more of a general guideline for power supplies.
While the PSU is rated for 450 watts, it can't be split however you want, you have x amount for each voltage. It gets even more tricky when you have multiple 12v rails, in which case you have to make sure you have enough for each rail. And you need to pay attention to the rails on cheaper models especially, since many cheap Chinese models will short you on the 12volt rail and put a ton of power on the 5volt. I saw one psu that had almost as many watts for 5volt as it did for 12volt. It was silly.
Another consideration, but not so much in this case, is the rating itself, MANY cheaper power supplies are over-rated by as much as 30% or more. I call it Chinese math, since they seem to have their own way of rating things.
On top of that, as they age, they lose capacity, it's rumored to be as much as 15-20% the first year, 10% the second and 5% the third, some are better, some are worse.
And, ideally, you try to never exceed 80%, this keeps the fans/heatsinks from being pushed too hard and wearing out or making noise and you always want a bit extra. You never know what direction things will go, for example, today we have USB 3.1 on the horizon and it has the potential to consume quite a bit of power or maybe you decide to add a few extra drives. $20 now can save you from having to spend another $100 next year because you needed more power.
There's a reason some call it a black art, and why many people just recommend going overkill.
After doing the math over and over again, I just follow a simple formula. If it's a:
Simple system (no video card) I'll use a good 400-450 watt
Video card with no connector 450-500
single power connector on the video card, 500-550
dual connectors, 650-750
Dual video cards, 750-850
Is this overkill? That depends, but on average, each are over powered by 50-100 watts or so and that is by design. The power supply in my desktop was 10 years old, I only replaced it because of another problem I was having, otherwise it would still be in it (it's in my file server now). How many parts in your box are 10 years old?
So what happens if you under power a system?
You might think it won't work, but that's actually not the case. Components can run on less power than specified (usually you only get a max rating) and when under powered, it can manifest itself in odd ways. I had a psu that had the 5volt side begin to fail but the system still ran stable. I found it because I was using a 6 foot extension for my usb 3.0 ports on my file server and the PSU wasn't able to provide enough amperage over the whole length, it ate about 4 usb sticks before I figured out what was happening. This was a system I had thrown together from scraps, so it had a cheap psu in it.
