How large a tank is right for me?

Once you have decided you want to capture rainwater, your next decision will be about the size of the tank you need. Consider the mathematics: the area of your catchment (roof) surface multiplied by the annual rainfall multiplied by 0.623, which is the amount of rain in inches per square foot.

This number is the theoretical number of gallons of rain it is possible to capture annually. But this is not the answer to the question of how big a tank you should buy. It only answers howlarge a tank you would need if you harvest all the rain you get in a yearwithout using any of it. Your tank size can be significantly smaller. The answer to the question is driven by several other questions: How much water do you need? Howmuch you can afford to spend? How much water buffer (i.e. water security) do you want?

The first question is the easiest to answer. It is directly related to how you are going to use the rainwater. Is it outdoor landscape watering? Is it toilet flushing? Is it irrigation and a backup water supply? Regarding size, bigger is better, and also more expensive— but you may also want to plan for future expansion.

The next issue is water security. Is your area prone to droughts? How much is your landscaping worth? Do you believe climate change will increase droughts in your area? Will this be your only source of water? Is one month of water backup enough? Thinking about how a water shortage could affect you should be part of your decisionmaking process.

Harvesting rainwater is the right thing to do for many, many reasons, but how much can be a little more complicated. Consider all of the above aspects in making your decision.

Doug Pushard, founder of the website www.HarvestH2o.com, has designed and installed residential rainwater systems for over a decade. He is a member of the Santa FeWater Conservation Committee, a lifetime member of the American Rainwater Catchment Systems Association, and an EPAWaterSense Partner. He can be reached at doug@HarvestH2o.com.