Going Off Grid in the 21st Century: Rain Water Storage Tanks

In our last few articles, we have covered all the aspects from harvesting the rain water to pre filtering it and delivering it to the tank. This leads us to another important aspect which is where are we going to store it? Is one tank better than another, what are the options, should I just have one tank or more than one? How much storage do I need, and what happens if???

Water Quality:

When one starts thinking about storing anything, the first question many ask is how much will it hold? While this still applies to water, a better question in this situation is – how much usable water does it hold? As we alluded to in our last article, one needs to use a calming inlet to prevent disturbing the sediment (1” – 3” depending on age, size, filtering, etc…), the inverse also applies. Namely you do not want to be pulling water from the very top (you also lose 1 – 3”) where oils or other items might be. The freshest and cleanest water is in between those two layers.

That is why most companies utilize submersible pumps, don’t put the outlet pipe at the very bottom, &/or utilize a floating extractor. A floating extractor is a basically a tube connected to a floating buoy. The tube opening / inlet basically is about 6” under water.

Along the lines of water quality, one must also remember that it is possible that the chemicals &/or materials the tank is made up of can leach into the water. Just think of all the uproar over BPA being used for drinking cups, or how barrels help give certain types of alcohol their unique taste.


Sizing is an important factor at almost every turn, from determining the sizing of your water pump to how much rain you can actually harvest. In many ways it is like harvesting any other crop; it doesn’t do the farmer any good to plant a huge field, harvest it only to find out his storage area can only hold half of it.

In our case, not only do you need a tank that can hold what comes in, but one that is capable of holding enough usable water to meet your needs till the next rainstorm can replenish it. Based on how your numbers come in, I would consider adding in a 10% or greater buffer if possible which should also help take care of the water quality issue above. While the examples below are overly simplified, this should hopefully give you a starting point to work from.

The Prepper Model – For this one I am going to use the typical Prepper model (as I don’t know your needs) which is to stock 2 gallons of water per day per person. So for a family of 5 who plan on being in a bug-out shelter for 6 months would need to have a minimum of 1830 gallons of usable water. (Daily Need x Period = Minimum Tank Size or in this case 10 x 183 = 1830)

Another method is based on capacity – Assuming a 10” annual rainfall on a 2000 SF roof where you typically have at least 1 soaker a year (in this case let’s say 3” in a week) and the remainder spread throughout the year leaves us with two quick options. You can either size the tank based on the yearly rainfall which equals the SF of roof x .623 x total annual rainfall (2000 x .623 x 10 = 12460 gallons a year). The other option is to size it based off soaker where we use the same calculation as above and end up with 3738 gallons of water. Assuming one needs to capture that full amount, you would need a tank that could handle that and whatever water still maybe in there – in this case I would estimate maybe a 20% buffer or simply round up & go with 4500 gallon tank or maybe two connected 2500 gallon tanks.

Tank Choices:

The three most popular choices are plastic, fiberglass, & metal.


  • Can be used above ground & below (No vehicular traffic – just grass)
  • Most affordable choice for homeowners & businesses which makes it great for homeowners just starting out to expand their system as funds become available
  • Generally the easiest to expand / daisy chain together
  • Come in a wide variety of sizes up to 10,000 gallons


  • More durable than plastic & generally buried under ground
  • Unlike plastic, these can be designed / spec’d for vehicular traffic
  • Generally used on commercial projects requiring 10,000 gallons or more
  • Tank sizes do vary but not as much as plastic


  • Generally corrugated & used above ground
  • The most durable choice for homeowners & businesses
  • Depending on structure / design you might not even realize it is a tank
  • Sizes range from 1,000 to 100,000 gallons

Stand-Alone Bladder Style:

  • Wide variety of sizes up to 200,000 gallons
  • Residential & commercial
  • Easily hidden in a crawl space, under a deck, porch, etc…
  • Bladders have been used for years inside tanks so springing a leak isn’t really an issue, especially with today’s newer fabrics & testing.
  • One of the interesting benefits I do see involves enclosed crawl spaces and the waters ability to help moderate the temperature in that space.
  • The only potential issue I see with this type of product revolves around the water quality issue requiring one to pull water mentioned at the start of the article.

Other Popular Choices:

One Tank or Two:

In this case we are not talking about sizing or a backup supply, but rather how it applies to how you use the water. In an upcoming article we are going to cover water filtration which helps guarantee how clean the water is & making it safe for drinking or potable. The amount of water needed in most houses to meet that standard is pretty small so the question becomes do you want to treat all the water the same or just the amount required? Personally unless you are also using the water for gardening or farming it might not make sense to run two tanks & pumps but it is definitely something to consider.

  • http://g35coupeforsale.net/2004-infiniti-g35-coupe-for-sale Alicia Weldingstainlesssteel

    I just read the article about filtering the water and now I found this Rain water tank storage. This blog is really informative.  It gaves me idea on how to save water even during rainy days.  great post!