When a psu says it has 18 Amps per rail, thats AVAILABLE power, not constant, or full time power, its available. 6 pins are 75 watts, also available, not constant, just like the PCI slot, which also supplies 75 watts, its there if you need it. Thats why, one thing to check is total output vs available. Most psu ratings may say 18 amps per rail at 12volts each, and youd think thatd add up to be 36, but most the time it doesnt, as youll often see around 75%-80% of that number for combined total output, or actually 26-29 amps