I think it's 15 watts.
The GC-TITAN RIDGE and GC-MAPLE RIDGE Thunderbolt 3 add-in cards allow 100 watts for if you want to charge a laptop (but they need a separate power input from a PSU for this capability).
Anyone know a good USB power tester that allows 10 Gbps USB or 40 Gbps Thunderbolt pass thru?