Windows 7 - Recommended Power Supply Wattage Calculator

From ASUS, a guidline minimum PSU calculator. With upgrades and general playing around with hardware you can find maybe your present PSU could use an upgrade too. With my setup it shows I need 600W and I have a 650W, so maybe I should get a bigger one.
Another from Antec with more options to select.

I like to do a bit of simple math with the actual effective output of a power supply. When the AC from the wall is converted to DC, some of the energy is lost to heat. This is where the 80Plus certification comes in. The higher the cert, the better the efficiency. To be safe, I like to take the advertised wattage and multiply it by 80% (or .8). In your case, 650 watts is 520. That's what I do when I shop for supplies so I can always make sure I have a little bit of padding so I'm not always pushing the supply to capacity.

If I use one of those calcs, I always add a minimum of 150W to whatever it says.
One thing people always overlook is, how many USB devices will be connected at any given time.
This includes, Keyboard and Mouse. Then add Cameras, mp3 players, external tumb drives and hard drives, web cam, e-reader, Cell Phone. Will there be adding an Internal HDD, or Optical Drive like a BD-Writer.

What happens when it is decided to upgrade the Video card that may require more power.

Those calcs are Ok, but they are only a guideline, and should never be taken at face value.

Yup I agree. Usually these calcs are handy since most device mfr's don't give a power rating.
As for headroom, the ASUS site gave me a figure 166W higher than did Antec.

That's why I always recommend to add 25% to the result of those calculators. It gives you some kind of safety-buffer

That Calculator says I need a 750W PSU .....
And Im not sure thats considering my OC.

Antecs said 530W w/the OC.

Make sure you put in some reasonable value for capacitor aging - that significantly affects the wattage you need. Some of these default to no aging - resulting in an underestimate.

This is the one that I use;
eXtreme Power Supply Calculator

Personally, I feel that most people way over power their computers. Most people who have 750w+ PSU's, simply won't ever come close to using all of that power. But hey, if it makes them feel better or more futureproof, go for it. I've got a 620 in my box, (Quad Core 9550, 8GB of RAM, 1 SSD, 1 hard drive, 1 9800GTX+, 3 x 120mm case fans, 1 DVD burner) and I was toiling between a 520 or a 620. I went with 620, but could have easily done the 520....heck a 450 probably would have done it...but I couldn't find a modular 450 at the time.

I run an Antec True Power New 650 and before my current setup, I had the following parts in my box,
Q6600 @ 3.02Ghz, 4x2GB DDR2-1066, GTX285, 9600GT, 9 7200 RPM Hard Drives, BD/HD-DVD/DVD-Burner, 2x 90mm Fans, 2x 120MM Fans, 5 External USB Drives, G15, G13, Performance MX

My current Setup consists of the following,
i7 870 @ 2.93Ghz, 4x4GB DDR3 1333, HD5850, 5 7200 RPM and 2 Green Drives, BD/HD-DVD/DVD-Burner, 3x 230mm Fans, 1x120mm Fan, 5 External USB Drives, G15, G13, G700,

A few of those Calculators said I needed almost a 1000watt PSU.

The Asus Calc says 800watt for my Current Setup and 1000 Watt for my old setup.

The Antec Calc says 495watt for the current setup and 756watt for the old setup

Actually, there are a lot more factors that go into it.
The system in my specs had a 650W. Which is also what most of these calcs recommend for my system.

My Main issue, and with just a Keyboard, Mouse and one other USB Device.......
But, change any of the USB's on the calcs to max and the results don't change much.

I had an older External HDD that did come with a Y Power USB Cable, Power+Data and Power ends
If I plugged in just with the Power+Data cable, it would work fine
But if I added an External HDD or thumb drive, it would then start clicking and stop working,
It require the other power cable.
After adding the 750W, I do not have that issue any longer.