In order to calculate the amount of power available at your site, you will need the following information:
The vertical distance (in feet) from the water intake to the turbine. Pressure is measured in terms of "head".
The simplest way to measure the head is with an altimeter, available from sporting goods stores. You can also use topo maps, available online through Google Earth or similar sites.
If none of the above are available, you can also sight along a level, and go up 5' at a time, with the help of an assistant.
The volume available, measured in gallons per minute, or gpm. This may vary greatly in areas with seasonal rain falls. In these situations, it is necessary to measure both the minimum and maximum flows.
With the use of a short piece or pipe in a temporary dam, you can time how long it takes to fill a bucket (5g is good) to get a flow measurement.
The amount of power you will be able to generate is directly proportional to the amount of head and flow you have available at your site.
The other factors to consider are the size and the length of the pipe from the water intake to the turbine, and the distance from the turbine to the power shed.
To see how the size and length of the pipe will affect your head, check the Head Loss Chart.
To calculate the amount of wire loss, go to the Wire Loss Chart.
Once accurate head and flow figures are available, the potential power of your site can be calculated by using the following formula:
Head (ft) x Flow (gpm) / 10 = Watts per hour