The power consumption depends on the chosen output board. It could vary from 7W to 12W - depending on the quantity and type of mounted relays.
In the idle mode (all relays off) boneIO consumes about 2W.
With Relay Board 32x5A (hf46f relays) on full load (all relays on) boneIO consumes about 7W.
With Relay Board 24x16A (zetler relays) on full load (all relays on) boneIO consumes about 10W.
To power the boneIO you need a 24V DC power supply, which will give at least 15W of power.
Prefered models of PSU are:
boneIO uses LM2596 Buck Converter to power BeagleBone Black (5V) from 24V Main Power Supply. To power BBB with LM2596 you can use voltage from about 6V to 35V but be careful - relays have direct connection to main power supply so they will not work correct if powered with inproper voltage. If you want to work with, for example 12V power supply, you have to change relays to the 12V version - everything else will work just fine.
External PSU is used for powering the inputs from outside the main power supply - for example with 48V. This function is not working properly yet in v0.3.