The energy in falling water can be a reliable and economical source of electricity for homes and businesses. This natural and recurring energy is a form of solar energy, since it’s the sun that drives the hydrological cycle, evaporating water, which condenses in the clouds and rains back down on us. While much of the potential energy is lost as the rain falls miles from the clouds to the land, there is still plenty to use as it makes its way down the mountains and hills to the oceans.
Hydropower is a combination of vertical drop (“head”) and flow. There must be energy in the water that powers a hydro machine. The water has to be flowing downhill or at least moving—there is no energy in still water. These two parts of hydropower equally influence how much energy we can get.
To make significant energy:
Unlike wind and solar energy, water power is limited by the source. You can always add more solar-electric modules or wind generators if you have a good site for these technologies. But at a hydro site, once you are using most of the available flow and head, you cannot add more collectors to reap more energy. Solar and wind systems are limited primarily by space available for the collectors, and the homeowner’s budget. While budget is always an issue, home hydro projects are most often limited by the actual resource (head and flow) on site.
For a typical small system, a useful formula for estimating available power is:
Head (in feet) × flow (in gallons per minute) ÷ 10 = power (watts).
This formula assumes an output efficiency of 53%, which is typical for small systems. For example, a system with 120 feet of head and 55 gpm of flow could yield about 660 watts of output. Multiplying by 24 hours means 15.8 kilowatt-hours (kWh) per day of potential production. This amount of electricity can provide an energy-efficient home with plenty of modern conveniences.
Sizing a system requires knowing how much energy you are using now and how much you’ll use in the future. Load analysis is a key part of any renewable energy system design process, and a hydro system is no exception. Calculating your current energy use, making a detailed load list of planned future energy use, and accurately determining the number of kWh needed per day will help you make a good plan for your hydro system.
Once you have a good load estimate, work a little harder to see if you can reduce it through conservation and efficiency measures. While some hydro systems produce a surplus of energy, it still makes sense to whittle your usage before you start. That may allow you to pare down all the system components—pipeline, turbine, transmission, storage, and electronics—which will shrink the purchase and maintenance costs. There’s usually no better money and time spent on an energy system than figuring out how to use less energy.
It’s also worth considering whether your hydro system will power your home completely, or if it will be supplemented by the utility grid, another renewable source, or even an engine-generator. Many hydro sites are limited—there is only so much water, and that might disappear in the summer or other dry times. Then, solar energy can be useful, since the dry periods usually coincide with the sunny times. If you’re off-grid, this makes for a rain-or-shine system. If you’re on-grid, you don’t need to make all your own energy, and this could affect your design choices as well.