Quick Answer
To calculate drawdown, identify your daily water usage, determine the total system volume and pipe size, and use a formula or online calculator to find the correct tank size, considering factors like water pressure and flow rate.
Understanding Drawdown
Drawdown refers to the decrease in water level in a tank due to flow from the tank. Calculating drawdown requires knowing your daily water usage, as this directly affects the required tank size and piping system.
Calculating Daily Water Usage
For example, assume a family of four with a daily average water usage of 100 gallons per day. You can use an online water usage calculator to determine this value based on the number of people, activities, and appliances. Record the total daily usage to proceed with the calculation.
Determining System Volume and Pipe Size
Next, consider the total system volume, including the tank, pump, pipes, and other components. For a small off-grid setup, a 1,000-gallon tank might be sufficient. Calculate the pipe size based on the required flow rate. A 1-inch pipe can handle approximately 10 gallons per minute (GPM) of water. For a 100 GPD household, you’ll need a 1-inch pipe with a flow rate of at least 10 GPM.
Applying the Drawdown Formula
To calculate the correct tank size, use the following formula: Drawdown (DD) = (Daily Usage in gallons) / (System Volume in gallons * Efficiency Factor). The efficiency factor accounts for the tank’s ability to supply water under varying pressure conditions. A common efficiency factor is 0.5 to 0.8, depending on the tank type and application. For a 1,000-gallon tank with an efficiency factor of 0.6, the drawdown would be approximately 33.3 gallons (100 GPD / 1,000 gallons * 0.6).
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