sorry that sounded like i was being a smart ass about calculating it yourself mentioning a calculator. i actually meant probably the minute power needed to run a calculator, watch, small electronic device with minimal processing. lemme do a quick calc for you with some assumptions. guessing flow rates and being a little generous:
assuming using the most efficient turbine available, with a guestimate of max flow rate through a 20mm pipe of 1 cubic meter of water per hour, with an inlet to outlet drop of 1m through the turbine, you will generate 2.7 watts of power. ok that could run a lot of calculators since mine says on the back 0.0002 watts but yeah, that is enough power to run a single bright LED bulb.