Quick Answer
Calculate the right size for your rainwater storage tank by estimating your annual rainfall, determining the roof catchment area, and accounting for water usage and losses. A general rule of thumb is to aim for 1,000 to 2,000 gallons per 1,000 square feet of roof space. This will provide enough water for basic needs.
Calculating the Rainwater Harvest System Capacity
To calculate the required storage tank size, start by determining your annual rainfall in inches. In the United States, the average annual rainfall is around 30 inches. For a specific location, consult a local precipitation data source. For example, if your area receives 40 inches of rainfall annually, and your roof catchment area is 1,500 square feet, you can estimate the total rainfall collection potential.
Assessing Water Needs and Losses
Next, consider your water usage patterns and account for potential losses due to evaporation, overflow, and leakage. A general rule of thumb is to allocate 50-70% of the total rainfall collection potential for household use, with the remaining 30-50% available for irrigation, toilet flushing, and other non-potable applications. For example, if your annual rainfall collection potential is 6,000 gallons, allocate 4,000 gallons for household use and 2,000 gallons for irrigation and other non-potable uses.
Choosing the Right Storage Tank Size
Based on your estimated water needs and losses, select a storage tank size that meets or exceeds the calculated requirements. Consider factors such as tank material, size, and shape, as well as the site-specific constraints, such as space availability and local building codes. For instance, for a household with a 4,000-gallon water usage allocation, a 6,000-gallon storage tank would be a suitable choice, leaving room for overflow and potential future growth.
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