How do you calculate amperage for the gpu?

I have 2 12v rails each 18amps and below them there's written 36 so does that mean If that one pci-e 6pin cable has 18 (each since I have 2 of them) or if the other cable isn't used one has 36? man my head hurts... never really understood the differences between amps, watts, volts, ohms, coulombs, farads and all that stuff, all I know is that I need 24amps for my gpu that gets power from a 6pin cable.
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  1. power (watts)= voltage * amps

    so 500 watts is 100v*5amps

    500 watts divide 5 amps = 100v

    500 watts divide 100v = 5amps


    18 amps * 12 = 216watts


    bassically thats the minimum you need, as long as your powersupply can output more than that you don't even need to bother checking

    thats the reason i bought a 750watt psu (ocz). bassically i was being lazy and didnt bother working out power
  2. 1) The graphics power reccomendation is for the entire system and is not based on how many rails the psu has, it's based on the psu's total 12v output
    2) A 6 pin connector is "officially" rated for 6.25 amps ( it can provide much more )
    3) The graphics card also draws some of it's power from the pcie slot
    If the psu provides 36 amps combined and the card manufacturer reccomends 24 amps total you have nothing to worry about
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