Power Supply Question

[macholan]

Posted my build a few days ago and appreciate all of the great feedback, but I have an additional question.

The short version:

Does a higher watt power supply always run at the higher level or only use the power currently needed by the computer?

The long version:

I'm trying to build my first computer and am being very patient with parts purchasing to deal hunt where possible. The power supply I originally chose was from the Lifehacker's Midrange Build, found here.

The power supply is the CORSAIR Professional Series HX650 (CMPSU-650HX) 650W ATX12V v2.2 (http://www.newegg.com/Product/Product.aspx?Item=N82E16817139012)
which right now is $94.99 after a coupon code and rebate.

I just noticed on slickdeals that the
OCZ Fatal1ty 750W Modular Gaming (http://www.newegg.com/Product/Product.aspx?Item=N82E16817341041&cm_sp=Cat_Power_Supplies-_-Spotlight-_-17-341-041)
is $59.99 after a coupon code and rebate.

I know my build doesn't need more than 650 watts, but I was wondering what the downfalls of getting the bigger power supply would be since it costs less and has the same warranty and reviews on NewEgg (and Amazon).

If it sucks extra power constantly, it is obviously not worth it, but if it can run at a lower rate and gives me a better deal and more opportunities for upgrades later I'd bite at it.

Thanks again for your time!

 

[ChiefWizard]

I'll try to make this short; all power supplies will leech a small amount of power even if there is zero load, but no it does not suck the full wattage it's rated for at all times. It will only pull as much power as it needs to supply the demand of your system. So if both are, for example, 80+ bronze certified at (just an arbitrary example) 85%, then they will both consume the same amount of power in your system (ideally).

The wattage rating is the MAXIMUM the power supply can safely provide to your system if needed, NOT how much power it draws.

Hope that clears everything up.

-cheers