The potential power of your system would be mass of water/ time multiplied gravitational acceleration multiplied by the height of the fall. If you use Kg for mass, m/s^2 for acceleration of gravity and meters for height, then you get an answer in Joules per second, or Watts.
For your situation:
1 gallon of water has a mass of a bit greater than 3.78 Kilograms
The acceleration of gravity is approximately 9.8 meters per second per second
43 inches is equal to just over 1.09 meters
The potential power output is therefore:
3.78 Kg/second * 9.8 m/s^2 * 1.09m = 40.3 Kg*m^2/s^3 = 40.3 Watts
I am not an expert on the efficiency of water wheels, but if you could manage 72% efficiency in the conversion of this energy into electrical energy that would be about 2 amps into your battery at 14.4 volts. I don't have much knowledge about the efficiencies of water wheels, but the numbers do not seem out of the realm of possibilities for some modest charging of your battery. Rich