Rainwater harvesting systems range from the simple and inexpensive to the elaborate and costly. If you are considering a rainwater harvesting system it is important to calculate the total amount of water than could be captured, stored, and used in a year with typical rainfall patterns. Using your local water and wastewater rates you can calculate the value of the saved water and determine if the system is cost effective.
On the simple and inexpensive end of the spectrum are rain barrels. A 57 gallon rain barrel with a cover, overflow hose, and hose bib connection costs about $100. If this barrel can be filled and used 20 times during the irrigation season approximately 1,100 gallons of water can be saved. At a value of $3 per 1000 gallons you can see that these devices would not be particularly cost effective, but they can save some water.
A mid-sized rainwater harvesting system may employ a series of rain barrels or a small storage tank. These systems have about 500 gallons of storage capacity and cost $350 - $500. Polyethylene storage tanks are available for between $0.30 and $0.60 per gallon. If a 500 gallon system can be filled and used 20 times during an irrigation season then 10,000 gallons of water can be saved valued at approximately $30 (depending upon local rates).
A larger residential rainwater harvesting system might have storage capacity of 2500 gallons. At a price of $0.50 per gallon, a 2,500 gallon polyethylene storage tank costs $1,250. Total installed cost for the system could be as much as $2,000. With one inch of rainfall, a 3,000 square foot roof catchment area with 80% efficiency can provide approximately 1,440 gallons of water to the storage tank. In an area that receives 30 inches of rain per year, approximately 43,000 gallons of water could theoretically be captured, stored, and used. The value of the saved water would be approximately $130 (at $3 per 1000 gallons).