Quick Answer
To calculate rainwater storage needs for a family, consider the household's average water consumption, the amount of rainfall in your area, and a safety margin to account for droughts or other disruptions. Multiply the household's daily water usage by 365 to determine the annual water needs, then divide by the average annual rainfall to determine the required storage capacity.
Understanding Water Consumption
To accurately calculate rainwater storage needs, start by determining the household’s average daily water usage. This can be done by reviewing water bills, observing water usage patterns, or using a water meter to track consumption. A general rule of thumb is to assume an average household uses 100-150 gallons per day. For a family of four, let’s assume a daily water usage of 120 gallons.
Calculating Storage Capacity
Next, determine the average annual rainfall in your area, measured in inches. In the United States, the average annual rainfall varies from around 20 inches in the driest regions to over 60 inches in the wettest. For this example, let’s assume an average annual rainfall of 30 inches. Convert the rainfall to gallons by multiplying the rainfall in inches by the square footage of the roof that collects the water. For a 1,500 square foot roof, the annual rainfall would be approximately 45,000 gallons.
Determining Storage Capacity Requirements
To determine the required storage capacity, divide the household’s annual water needs by the available rainfall. Using our previous example, if the household uses 43,800 gallons per year and the available rainfall provides 45,000 gallons, the storage capacity required would be approximately 1,200 gallons. However, to account for droughts or other disruptions, it’s recommended to add a 20-30% safety margin to the storage capacity. In this case, the required storage capacity would be around 1,440 gallons.
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