MARK:
17 FEET *0.3048 = 5.2 METERS.
(120 GPM * 3.78 LITER) / 60 SEC = 7.6 LITER/SEC
- 27 INCH *2.54 CM / INCH = 31.2 CMS DIAMETER
- 25 DIA / 2 DIA = 2.6 RATIO
GROSS WATTS = 5.2 METER * 7.6 LITER/SEC * 9.81 GRAVITY = 387.7 Watts
Water Velocity = squarerroot (2 * 5.2 meter * 9.81) = 10.1 meter / sec
Actual RPM at peak power
(Water vel) / 2 = 10.1 / 2 = 5.05 meter / sec = 505 cms / sec
Turbine RPM = ((Water vel) / 2) / ( PI * Turbine diameter) = 505 / ( 3.14 * 31.2 cms)= 5.2 RPS = 5.2 Revo per SEC ) * 60 sec / minute = 309.1 RPM
Turbine RPM * 2.6 ratio = 803 RPM
With that alternator efficiency and the speed multiplier you may get 40 % power out or 387.7 watts * 0.4 = 155 watts, and I am guessing that you may get around 120 watts.
Missing from the equation the pipe lentgh to calculate the water friction losses for lower head.
Regards
Nando