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How Large A Tank is Right For Me?

by Doug Pushard

Rectangle TankOnce you have decided you want to capture rainwater, the question of how big a tank is your next decision. This simple question leads to many other variables to be taken into consideration prior to installing your rainwater catchment system. From a mathematical point of view, how large is your catchment surface (i.e. typically your roof area), multiplied by how much rain do you receive annually, multiplied by .623 (i.e. 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 how large a tank you would need if you harvest all the rain you get in a year without using any of it.

Your tank size will be significantly smaller than this. The answer to the question is driven by several other questions A) How much water do you need? B) How much water buffer (i.e. water security) do you want? C) How much you can afford?

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? It is your drinking water? Is it irrigation and a backup water supply? Knowing your use, helps directly answer the question on how much you will need to harvest.

Landscape watering can be determined by looking at your water bill and/or a little observation. To do the latter, open your water meter (see links below); turn on your irrigation system and run it through an entire cycle; and note how many gallons the meter said were used. Toilets use 1.6 gallons per flush with an average of 5 flushes a day per person per household. A single person generally uses about 70 gallons per day for all activities. Multiple these by the number of days in a month or how many times your irrigation system runs in a month to get a general guestimate of your monthly water demand. Now you know approximately how much water you will use.

The next issue is your water security or reliability. Is your area prone to droughts? How much is your current landscaping worth? Do you believe that climate change will increase droughts in your area? Will this be your only source of water? Is one-month of water backup enough or is three? If water security/reliability is critical than maybe there is a way to install a bigger tank now, rather than later (e.g. install the tanks now, but the rest of the system later). Thinking about how a water shortage could affect you and your willingness to tolerate a shortage should be part of your decision making process.

The last question is how much money you can afford. Bigger is always better, but also more expensive. The answer to this question typically reduces the optimal size of tank. So planning for future tank expansion may be a good approach.

Rainwater supply is the easy part of the equation. How large a tank is a harder question to answer. The rationale for installing a rainwater harvesting system is different and consequently there is no one answer on how big your tank should be. Rarely is the answer all you can possibly capture. It is normally constrained by other factors.

Harvesting rainwater is the right thing to do for many, many reasons but how much can be a little more complicated. However, if you think through your situation and gather the information suggested in this article, the answer that is right for you can be determined.

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