# WATER POWER BASICS

“Innovative Renewable Energy Systems Since 1980”

The energy in moving water is determined by the head and the flow. The head is the vertical distance that the water moves through. This is the same as the pressure and is measured in meters, feet, psi etc. The flow is the quantity of water and this is measured in gallons per minute, litres per second, etc. The power that might actually be generated is found from the product of the head and flow. So if we multiply the head in feet by the flow in gallons per minute and divide by ten, we get the output in watts in a typical system. The metric version is head in meters x flow in litres per second x five = watts. These formulas give an efficiency of about 50% which is typical of a small system. This means that half of the energy in the water is converted to electricity. Unlike wind and solar power, water power is very site specific. So each site must be carefully evaluated and a system installed that matches the conditions. Sites can vary greatly—from very high heads to no head sites like tidal ones where there is essentially no head and only velocity.
Higher heads are usually easier to work with than lower ones. The equipment can be smaller and the pipeline delivering the water can be smaller in diameter since less water is needed. At the lower head extreme, you will find tidal machines that look like underwater wind machines.
In most cases, we are trying to make electric power. There are many other uses for water power, such as water pumping and grain grinding, but we will only concern ourselves here with electric power. Larger systems (> 5 kW or so) can have generators connected directly to the loads which is what the utilities do. But the systems that we will be dealing with are smaller and often generate less than 100 watts. This can still be very useful to
those living off-grid.
In these systems, the power is stored in batteries and then an inverter makes AC (alternating current) which is typically used to operate standard appliances. A charge control device is needed to prevent the batteries from overcharging. In this way, this excess power is usually sent to an air or water heater. Many people supply their hot water needs with these systems.
The site I will show you at my residence has 1600 feet (500 meters) of two inch (50 mm)polyethylene pipe and 140 feet (43 meters) of head. The maximum flow that the pipeline can use is about 30 gpm (2 l/s). This is sent to the house about 400 feet away at 240 volts.
This is converted to 48 volts using transformers. About 300 watts is generated continuously. This corresponds to the magic number for operating a typical off-grid household. Experience has shown that this is enough to power lights, refrigerator, freezer, washing machine, well pump and entertainment for a family of four.

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