Pretty sure it depends on the slot but most fully wired pcie x16 slots will give 75w. Don't take it as a given though so read what it says next to the slot on the motherboard.
Ripped from wiki
Power
x1 cards are limited to 0.5 A at +12 V (6 W) and 10 W combined.
x4 and wider cards are limited to 2.1 A at +12 V (25 W) and 25 W combined.
A full-sized x1 card may draw up to the 25 W limits after initialization and software configuration as a high-power device.
A full-sized x16 graphics card may draw up to 5.5 A at +12 V (66 W) and 75 W combined after initialization and software configuration as a high-power device.[22]: 38–39
The main 12 V power supply for the PCIe slot is pins B2, B3 (side B) and pins A2, A3 (side A). Power standby 3.3 V is pin B10 and A10. PCIe x1 cards can receive up to 25 W and x16 graphics cards can receive up to 75 W.
Pretty sure it depends on the slot but most fully wired pcie x16 slots will give 75w. Don't take it as a given though so read what it says next to the slot on the motherboard.
Ripped from wiki
Power
x1 cards are limited to 0.5 A at +12 V (6 W) and 10 W combined.
x4 and wider cards are limited to 2.1 A at +12 V (25 W) and 25 W combined.
A full-sized x1 card may draw up to the 25 W limits after initialization and software configuration as a high-power device.
A full-sized x16 graphics card may draw up to 5.5 A at +12 V (66 W) and 75 W combined after initialization and software configuration as a high-power device.[22]: 38–39
The main 12 V power supply for the PCIe slot is pins B2, B3 (side B) and pins A2, A3 (side A). Power standby 3.3 V is pin B10 and A10. PCIe x1 cards can receive up to 25 W and x16 graphics cards can receive up to 75 W.