[Southcross]

well... most batteries aren't deep-cycle, so they tend to not fully recharge if run down completely.

Watts = amps * volts
watts / volts = amps
Batteries have a rating in AH (amps * hours) but most of them time they aren't "defined" unless they are specialty batteries

200/12.7 = 15.75 amps
but as voltage decreases, so theoretically does the Amps consumed to produce the same amount of work.
if the battery voltage drops to:
12v = 16.667 Amps
11.5v = 17.4
11v = 18.2

as the voltage lowers, the life on the battery charge will get lower faster and faster as components attempt to continually draw the same number of Amps...