Water Storage Tank Calculator?

hey there,

Anyone know of a way to calculate the size of storage tank a home will need to supply potable water to a home?  I've already calculated what we can catch from the roof, but I don't think we need a 26k gallon tank.  :D

We will use no more than 100 gal/day for two of us.  We are allowed to use a waterless composting toilet (huzzah!) and that saves a lot.  We won't be doing laundry here, but next door in the main farmhouse.  So, just cooking and showering.  Still, I doubled what that should be just to be safe and arrived at 100 gal/day.

What's the rule of thumb or straight up calculation for this?

Thinking to put in in-ground tank below frostline because it freezes here in Massachusetts.  Extra water can go to other tanks for the orchard and small Z1 garden where the house will sit.

Thanks heaps,

My signature has details about capturing and using rainwater.
But what size roof and  what rainfall do you have?
You dont need to hold one years supply in the tank.
I have 450mm rainfall and have  a 20,000L tank.
Underground tanks are a bigger cost but great for freezing areas.
If you were building I would suggest a tank in the basement or even a barn with a small heater.

I've calculated that we can expect somewhere in the neighbourhood of 25K gallons of water per year on the roof.  We'll have somewhere in the neighbourhood of 1000 sqft of roof, and somewhere around 42-43" of precip a year.  We're super fortunate.  

John C Daley wrote:My signature has details about capturing and using rainwater.
But what size roof and  what rainfall do you have?
You dont need to hold one years supply in the tank.
I have 450mm rainfall and have  a 20,000L tank.
Underground tanks are a bigger cost but great for freezing areas.
If you were building I would suggest a tank in the basement or even a barn with a small heater.

This is the calculator I've been using while designing our rainwater catchment in Arizona:

Rain Harvesting Calculator with monthly estimates and including drawdown (your usage)

It it so useful.  You enter your location, and it uses figures with the monthly rainfall.  You enter your tank size and catchment area size and catchment efficiency.  Then you put in your monthly usage!  This way you can see when your tank would be full, empty and in between with those input and output figures.  

You can change the tank amounts and see at what point you would have an optimal size.  Even though it seems like bigger tanks mean more storage, especially in dry areas like where I am, with limited catchment you only catch so much.  So this will also tell you if you need to up the catchment area somehow, or supplement.

This is very useful if you are gardening.  In our area, we do get monsoons, but there is a very dry period in spring through early to mid summer.  We have 1560 sq ft catchment area. We were looking at getting 2500 gallons of water storage.  

By using that calculator I was able to tell that without us supplementing a 2500 cistern with well water during each of those months, it will run dry pretty quickly under full irrigation because we don't get enough monthly rain.  So now we are going to increase the storage tanks, and be able to supplement them fewer times.  We don't have a way to increase catchment in that location very easily at the moment, but may someday so we want to plan ahead for that. Who knows, maybe a carport?  

This calculator is helping us plan ahead much better by visualizing monthly usage, rather than just yearly.  In a place with consistent monthly rain, this may not be as much of an issue, but I live where if it rains, it pours - or it's dry as bone.

Well this is cool!  Thank you so much for this link.  I can't wait to see what it says.

Kim Goodwin wrote:This is the calculator I've been using while designing our rainwater catchment in Arizona:

Rain Harvesting Calculator with monthly estimates and including drawdown (your usage)

Using the calculator, it looks like a 2500 gal tank would get us by with a prefill from the main well.  But if we went up to 3000 and also collected from the studio roof immediately adjacent, we'd have more than enough to get us through even an unusual drought.  Plus it would give us the option to have a little wee washing machine if we later decide to have one (especially convenient in the winter).  Overflow and grey water can go to the Z1 and shared Z2.

Cool!  Now to look around for tanks that don't have a really long lead time.

Nissa Gadbois wrote:Well this is cool!  Thank you so much for this link.  I can't wait to see what it says.

Kim Goodwin wrote:This is the calculator I've been using while designing our rainwater catchment in Arizona:

Rain Harvesting Calculator with monthly estimates and including drawdown (your usage)

We've bought a few brands of tanks now.  Here is a great list from Brad Lancaster where I learned a lot more about tanks and the quality of different tanks on the market in the US:
Brad Lancasters experiences and recommendations for water tanks

What I've learned that also applies: the cheap tanks use less plastic, have thinner walls, and only have a 1 or maybe 3 year warranty.   Friends of mine bought a tank from Home Depot, one of these with the 1 year warranty, and it feels like a big plastic bag. Only a slight exaggeration...it has very flexible walls. So though they may be cheap to buy, they will likely be expensive to own in the long run.

On Brad's list above, I've bought a tank from the private company New Mexico Water Tanks on Brad's list above.  
New Mexico Water Tanks - a small tank manufacturer, local to SW
That tank has a 10 year warranty.  The walls are so thick and dense, I believe it will last many, many years beyond that time.  It's black, but it is paintable with elastomeric paint (that's like paint for cement or stucco). The company was very nice to work with and they also explained how to set the tanks and how to paint them. Some people have put murals on their tanks and even make them disappear in the landscape!  I want to try that!

The only other tank I've found that has that same warranty (10 year) is Enduraplas.  Those aren't on Brad's list above, I don't know why, they seem very good.  You have to buy through a semi-local distributor, but some of the distributors don't even charge shipping! That makes it well worth it because buying a tank online often means paying the same amount in shipping - i.e. if the tank is $2000, it may be $2000 to ship it to you.  Our nearest Enduraplas distributor is 100 miles away and delivers here for free. We are ordering from them next rather than the NM company I just mentioned because Enduraplas has tanks in other dimensions we wanted and they deliver directly to our location. Enduraplas sells lots of types of tanks and there is one with thicker walls that is under their agricultural products.  See Flat Bottom tanks if you want the real premium product.  Their retail line and commercial lines each have the 10 year warranty.  This Enduraplas tank below also comes in tan, so we won't have to paint it.

Enduraplas tanks with 10 year warranty

Underground tanks are harder to find. There are only a few manufacturers, Snyder/Norwesco is one.  There are others.  The materials are way tougher than aboveground tanks and will last a long time.  They are expensive.  We have these, too, for our home's water so it stays cool.  Very few people want or are willing to spend the extra money for these (plus installation), so I won't go into it unless asked. It's easy to search online after all...!

Thanks so much for the resources!  I think I'm going to go for an outdoor tank  for the sake of cost savings.  The digging of a cellar hole, lining with concrete or stone, etc. might just be too much for the budget.  Certainly undesirable in terms of disruption to the site.  Piers are allowable here, so that's my preference. And that would mean an upright, outdoor tank.

Now I'll be looking for ideas to insulate it against our cold winters.  Right now, I'm wondering if ecofoil insulation like they use on proper yurts could be a good option.  Later, perhaps, we could create a stone enclosure for beauty and durability, and marginal heat gain (NNE side).  Stone, we've got.  And because this structure would be added later and isn't critical to the overall build, we can take our sweet time.

You get to insulation when you know what building you will have.
Dont forget to look at 'first flush' rainwater diverters.