More head is usually better because the system uses less water and the equipment can be smaller. The turbine also runs at a higher speed. At very high heads, pipe pressure ratings and pipe joint integrity become problematic. Since power is the product of head and flow, more flow is required at lower head to generate the same power level. More flow is better, even if not all of it is used, since more water can remain in the stream for environmental benefits.
Net head is the pressure available after subtracting losses from pipe friction.
Most hydro systems are limited in output capacity by stream conditions. That is, they cannot be expanded indefinitely like a wind or PV system. This means that the sizing procedure may be based on site conditions rather than power needs. Only an impoundment of sufficient size to direct the water into the pipeline is required. Power is generated at a constant rate and if not used, it is stored in batteries or sent to a shunt load.