A few points:
Batteries are rated in Amp hrs (Ah), meaning the number of Amps the battery will deliver over a given time period before discharging completely. However, rechargeable batteries are generally destroyed by doing this, so the number cannot be used to predict mileage without knowing a lot more about the system. Battery-driven systems generally shut down (and batteries are damaged) below a critical battery voltage, not after a set number of Amps are drawn, and the relationship between Amps pumped out and battery voltage is complex (at least to a lay person like me) and nonlinear. In general, however, a 100 Ah battery will run your gizmo about twice as long as a 50 Ah battery, between charges.
Watts of electrical output for the battery is just the Amp draw of the motor (at a given output) times the voltage of the system, which is essentially constant. However, that number is meaningless in terms of bike power or range without knowing the efficiency of the motor over its speed range.