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Choosing the right size of tank for your project can be tricky. First ask yourself these questions:
1. How much water can you collect and when does it fall?
2. What is your monthly water demand and how does it change throughout the year?
3. What is your budget? A water tank is around 50% of the total cost of a system.
For example, let's say I have a 2,000 square foot roof. That will give me ~ 1,200 gallons of water every time it rains one inch. My average rainfall (North Texas) is 40 inches per year, which means that I could potentially collect 48,000 gallons of water per year.
Let's say my water demand is 2,000 gallons per month, which means I need at least 2,000 gallons of storage to get me through one month without rain. However, a drought in my area may last up to six weeks with no rain and then only intermittent rain for another month. To be safe, I should have 3 months worth of water stored (from the rainy season) to be able to outlast a significant drought event.
In this scenario, I would choose a tank around 6,000 gallons.
In areas where it rains more often, a smaller tank will work just fine. If you have sufficient rain, but your water demand is very high, a larger collection area is needed.
If you have sufficient rain, but it all seems to fall during a few weeks of the year, a larger tank is needed.
If in doubt, get the largest storage tank you can afford. In my experience, I have never seen a collection tank too large. It is amazing how much water can be collected during a peak rain event!
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