How Do I Size a Rainwater Cistern?

Many skeptical homeowners living in areas with limited amounts of rainfall might doubt a cistern's ability to store sufficient water for an entire household. A few simple calculations will allow you to determine the amount of rainwater that your home can capture. These calculations will help define the optimum cistern size to supply your home with an autonomous supply of potable water.

For example, let's consider an average-size home in the United States. The average annual rainfall is just about 30 inches per year. The USGS estimates that the average person uses between 80 to 100 gallons of water per day. Of course, homeowners could radically reduce the amount of water they use by switching to dual flush toilets, low-flow showerheads, and other water-saving devices. So, for a family of four, the high estimate would amount to an annual household water usage of 146,000 gallons.

What Are The Essential Elements of a Cistern?

The home cistern is the backbone of any rainwater catchment system. It can be made from different materials, including cinderblock, reinforced concrete, or precast concrete, fiberglass, or steel. Cisterns are often located underground (even underneath the home). Then, the water is pumped back into the house through a pressurized plumbing system. For potable water inside the home, your cistern and rainwater catchment system should also include the following elements.

Does a Cistern Require Gutter Guards?

The first line of defense against debris making its way into your cistern is a system of gutter guards to keep your catchment system free of leaf and other larger debris.

Does a Cistern Need A First Flush Diverter?

Since roofs can often collect dust, leaves, and other debris, a first flush diverter is another essential element of your rainwater catchment system. It diverts the first water flow from the system. So, the first "gush" of water from a rainstorm will wash all the sediment from your roof and gutter system away before diverting the rest of the cleaner water back to your cistern.

Does a Cistern Require A Filtration System

Your cistern will also need to have to undergo filtration before pumping the water back into your home. This filtration will help to make sure your water source is clean and potable. It will also keep your plumbing clear of clogs caused by the debris. Several types of filter systems for home cisterns include activated carbon filters, floating filters, and UV filters.

How Much Does a Cistern Cost?

Rain Harvesting Supplies is a Texas-based company that also offers complete home cistern packages. Their Raintech Above Ground Cistern Package provides 2,500 gallons of storage for under $2,000 and includes filters, pumps, and connections.

A home cistern will require a significant investment. However, you will also notice immediate savings in a reduced (or even eliminated) water bill. Assuming that you have an average water bill of $50 per month, you could receive up to $600 of savings each year, which would lead to relatively quick payback time.