Use iPhone to power an accessory

What is the maximum power-per-hour can an iPhone provide for an accessory connected to the iPhone to its Lightning port?

I got this:

Apple's power rating for its Lightning cable is 5 Volts DC carrying 1.8 Amps of current equaling 9 Watts of power.

Which sets a max for output (may be lower however).

Use iPhone to power an accessory
 
 
Q