1. 300 W is the output power of the generator. Power output has nothing to do with the size of the water flow and the output voltage of the generator.
2. The rated output voltage of the generator is related to the design. Its concept is: the rated voltage value that can be output at the rated speed. For example: for a 220 V generator, the rated output voltage should be 220 V; for 380V generator, the rated output voltage should be 380V and for 36V generator, the rated output voltage should be 36V;
A homemade 220 V generator requires the following:
220 V generator (housing, stator core, stator coil, rotor, permanent magnet, bearings, blades cooling, etc.).
With a generator, you also need equipment to drive the generator: gasoline engine, diesel engine, fan blades, hydraulic turbine, etc.
IIt is impossible to make your own generator, so it is best to buy a ready-made one.
Disassemble a small DC motor from an old used tape recorder or DVD player, and install a small fan blade on the motor shaft, or buy it directly and make sure that it comes with a sun hat with a small fan, no batteries. Solder a light-emitting diode to the motor terminal, preferably red (catchy and low starting voltage). Use the wind to blow the fan blades to turn the engine.