The wattage drawn from a battery depends on the load you give it.
A fully charged lithium battery is 4.2 volts.
If you put a 1 ohm load, your wattage will be 4.2 x 4.2 / 1 = 18 watts.
If it is a regulated mod, the mod electronics attempts to draw the power that you set.
If it is a dual battery...