watts is amps x volts.

a 12V 7.2ah battery can (technically) create an elecrtrical current of 1 amp at 12V for 7.2 hours though it isnt very good for the batteries to be discharged this fast.

that is why the c20 rate is recomended. c20 is the amp hours divided by 20 so for example if you are using a 7.2 ah battery the optimal discharge rate would be 7.2/20 = 360ma (which is 4.32 watts).