Hydroelectric energy generation is one of the most firm (continuous) sources of electricity and, if you have the necessary geography, highest output. Typical high output generating systems are composed of an inlet pipe from a stream or lake, a significant drop in elevation, and a turbine. If you have available flows of 200 GPM (gallons per minute) and a head (drop in elevation from inlet) of 100 feet, you can generate around 1200W (watts) continuously. The same system will only produce 20W with 20 GPM and 25 feet of head. Current cost for the high output turbine unit is approx. $1500. For those without an elevated water source on their property, other systems exist to harvest hydro energy from streams with a simple submersible propeller unit. These systems can produce from 24W in a 2 MPH stream to 96W in a 10 MPH stream. These units cost around $1200.