Here's some information about the power requirements of the board:
http://elinux.org/Jetson/Jetson_TK1_Power
The board itself typically uses 5W, though you could peak at 15W if you were fully maxing out the CPU and GPU capabilities. However, you can draw more power with peripherals - SATA, HDMI, USB3, flash memory can all draw a significant amount of power. I would measure the current the board actually draws for your usage case, and see if you can get away with a lesser power supply. I suspect you would be able to get away with using the 12V 1.5A terminals on the VRM.