By Jonathan Saucedo, Co-Founder of Tank Supply Direct
An off-grid water tank needs to be big enough to cover the longest period when your water source supplies less water than your household uses. That period might be the two weeks between water deliveries, the hour every evening when everyone showers and the well falls behind, or a dry summer with no rain to collect. So the right tank size depends on where your water comes from.
You also need a backup water source, such as a water hauler you can call when the well pump breaks, so the tank doesn't run dry when your main source stops.
At Tank Supply Direct, we sell holding tanks and cisterns to rural landowners, homeowners on wells and people building off-grid, and we talk these setups through with them on the phone. One of the most common mix-ups we help with is expecting a storage tank to push water to the faucets with pressure. A storage tank can't do that. Instead, a pump moves the water to the house, with a pressure tank (a small tank that holds water under pressure) between them.
How much water does an off-grid home use per day?
An average American uses about 82 gallons of water a day at home, according to EPA WaterSense. The USGS puts indoor use at 80 to 100 gallons per person per day, with toilets and showers using the most.
Off-grid households often use a lot less, because every gallon has to be hauled, pumped or caught. The Texas Water Development Board's rainwater manual says a water-conserving household uses between 25 and 50 gallons per person per day. Low-flow fixtures, shorter showers and a composting or low-flush toilet are what get you into that range. If you aren't sure how careful your household will be, plan with 50 gallons per person rather than 25. It's easier to have water left over than to run out. (If you only want an emergency reserve for outages, our water storage tank buying guide covers that much smaller amount.)
How big should an off-grid water tank be?
An off-grid water tank should hold your daily use multiplied by the number of days your source can't refill it, plus some extra in case the refill comes late. Here's how to work out that number.
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Figure your daily use. Multiply the number of people by gallons per person per day. Use 25 to 50 gallons for a careful household, or around 82 if you expect to live the way most homes on city water do. Add any livestock or garden watering on top.
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Decide how many days of reserve you need. This comes from your refill source. With hauled water, it's the number of days between deliveries. With rain, it's the longest dry stretch you expect. With a well, plan on at least one full day's use, so the tank covers the hours when you use water faster than the well can supply it.
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Add a buffer. Trucks run late, pumps break and dry seasons run long. A few extra days of water gives you time to fix a problem before the taps go dry.
- Round up for water you can't use. The outlet sits a little above the bottom of most tanks, so the last few inches of water stay in the tank. Check where the outlet sits on the tank you're looking at, and size up to cover the water below it.
Here's a hypothetical example. A couple living in a cabin uses about 50 gallons each, so 100 gallons a day. Their water hauler comes every two weeks, so they need 14 days times 100 gallons, or 1,400 gallons. They add a week of buffer in case a delivery slips, which brings them to 2,100 gallons. Rounding up for the water below the outlet, a 2,500-gallon tank would cover them.
Before any buffer, the totals look like this:
|
Daily use |
1 day (well) |
14 days (hauled water) |
90 days (dry season) |
|---|---|---|---|
|
100 gal (2 people, careful use) |
100 gal |
1,400 gal |
9,000 gal |
|
200 gal (4 people, careful use) |
200 gal |
2,800 gal |
18,000 gal |
|
328 gal (4 people, average use) |
328 gal |
4,592 gal |
29,520 gal |
The dry-season totals are large, but you don't have to store them in one tank. You can split the total across several smaller tanks connected together, so 18,000 gallons could be a few tanks side by side. If you'd rather use one tank, our water storage tanks go up to 20,000 gallons.
Tank material is a separate choice from size. Polyethylene (plastic), steel and fiberglass each suit different sites. Our comparison of poly, steel and fiberglass tanks explains which fits where.
Hauled water, a well or rainwater: where will your water come from?
Your refill source decides how often the tank fills, what can stop it from filling, and how much storage you need. Most off-grid homes rely on one of three sources, and here's how they compare.
|
Source |
How it refills the tank |
What can stop it |
Storage it calls for |
Upkeep |
|---|---|---|---|---|
|
Hauled water |
Large loads on a schedule |
Late or missed deliveries, bad roads, a booked-up hauler |
Days between deliveries, often a few thousand gallons |
Booking deliveries, cleaning the tank with bleach after loads |
|
Well |
Slowly, all day |
Using water faster than the well produces it, pump failure, power outages |
At least a full day's use |
Looking after the well pump and its power supply |
|
Rainwater |
Only when it rains |
Dry seasons and droughts |
The longest dry stretch, often three months or more |
Cleaning gutters and screens, keeping debris and algae out |
Hauled water
Hauled water fills the tank in large loads, either from a water hauler's truck or from a tank you fill in town and drive home. A delivery truck pumps the water in through a fill inlet on the tank. The risk is that you depend on someone else's schedule. A truck that can't reach you after a storm, or a hauler who is booked up, means you're living on what's left in the tank.
A bigger tank also means fewer deliveries. Hauling prices, minimum loads and delivery areas vary a lot by region, so call the haulers who serve your area before you pick a tank size. Ask how often they can come and what their smallest and largest loads are. If you plan to haul water yourself, our utility and hauling tanks come in sizes up to 1,000 gallons. Water weighs about 8.34 pounds a gallon, according to the U.S. Geological Survey, so check what your truck or trailer can carry before you choose a size.
Well water
A well's yield is the most water it can pump per minute without drawing the water level down below the pump. Before you size anything around a well, find out what yours actually produces. A licensed well professional measures it by pumping the well for an extended period and tracking how far the water level drops, as the University of Nebraska-Lincoln Extension explains.
Even a low-yield well usually makes enough water in a day. The problem is the times when everyone uses water at once. A well that pumps just 1 gallon a minute still produces 1,440 gallons a day, Penn State Extension points out. But a typical house can use about 5 gallons a minute when two fixtures run together, such as a shower and the washing machine, according to the New York State Department of Health. In that case the well falls 4 gallons behind every minute, and without stored water to draw on, the house runs short.
A storage tank, often called a holding tank when it sits between a well and the house, fixes that by storing water ahead of time. The well pump refills the tank slowly, often overnight, and the house draws from the tank instead of straight from the well. Penn State's rule of thumb for a low-yield well is a tank that holds about 100 gallons per person. That covers a full day even at average household use, and more than a day for a careful household. A bigger tank also helps if your well pump runs on solar power. A solar pump runs only while the sun is up, so the tank stores what it pumps during the day, and the house draws from that water at night. Dan Fink, writing for Countryside Magazine, puts it this way: "Think of your cistern as a 'battery' of sorts, which buys you time until you need to pump again."
Rainwater
Rainwater fills the tank whenever it rains and stops completely when it doesn't, so a rain system has to hold enough to get you through the dry season. The Texas Water Development Board manual sizes systems "to meet estimated demand for a three-month period without rain." In some climates the dry stretch runs longer than that. For a quick sense of supply, a roof collects about 0.62 gallons per square foot for every inch of rain, so 1,000 square feet of roof gives about 620 gallons per inch before losses. Most installers assume about 75 to 85 percent of that actually reaches the tank, according to the Texas Water Development Board's rainwater FAQ. Gutters and roof screens are a separate project. For the tank itself, you'll find options in our rainwater harvesting tanks collection.
Do you need a backup water source?
A backup source is what keeps a problem with your main source from leaving you without water. If your well pump fails, a hauler can fill the tank while you wait on a repair. If you live on rainwater, a hauled load can carry you through a drought. Plan the backup before you need it: find out which haulers serve your area, and make sure a truck can reach your tank and connect to its fill inlet.
How does an off-grid water tank connect to the house?
An off-grid water tank stores water but doesn't push it to your faucets. A storage tank can't hold the pressure that pushes water through household pipes, so that pressure comes from a pump and pressure tank, or from the height of the tank above the house. In order, from source to faucet, a pumped system looks like this:
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Fill line. Carries water from the source into the top of the storage tank.
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Storage tank. Holds your reserve. A vent lets air in and out as the water level rises and falls. A float control shuts off the fill when the tank is full. An overflow pipe gives extra water a way out if the float fails. It also matters on a rain system, because a roof keeps sending water to the tank during a storm even after the tank is full. Route the overflow pipe so the water drains away from the tank.
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Transfer or booster pump. Draws water from the storage tank and pushes it toward the house. A low-level float switch shuts the pump off before the tank runs dry, which protects the pump.
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Pressure tank. Holds a small amount of water under pressure so the pump doesn't start every time someone opens a tap. Without one, the pump switches on and off constantly. The Connecticut Department of Public Health warns that this short-cycling severely limits the life of a well pump.
- Filtration and treatment. Cleans the water before it reaches your plumbing.
With a well, this means two pumps: one in the well to fill the storage tank, and one to send water from the tank to the house. Our post on why a well water holding tank is not a pressure tank goes deeper on pressure tanks and float switches.
We don't carry well pumps, booster pumps or pressure tanks for house water, so size those with the pump maker. They'll ask how many gallons per minute you need, how high the pump has to lift the water, and what power you have to run it.
Can you use gravity feed instead of a pump?
Gravity feed works when you can put the tank high enough above the house, because every foot of height adds about 0.43 psi of water pressure, according to the University of Arizona Cooperative Extension. The height is measured from the water's surface in the tank to the faucet. The size of the tank doesn't change the pressure.
Getting useful pressure by gravity takes a lot of height. A tank whose water sits 23 feet above your faucets gives you about 10 psi. Home well systems commonly run between 30 and 50 psi or between 40 and 60 psi, according to the Washington State Department of Health. Getting close to 50 psi by gravity alone would mean a water surface about 115 feet above the house. You can get that kind of height if your tank sits on a ridge well above the house. On flat ground, you can't, so you'll need a pump. Fink recommends gravity feed only for agricultural uses.
Where should an off-grid water tank go so it doesn't freeze?
In cold climates, every part of the system that holds water has to stay above freezing, because frozen water stops your supply and can crack fittings and pipes. Frost depth varies a lot by location, so check yours with your local building department or extension office before you dig.
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The tank. Bury it deep enough that the ground keeps it from freezing, or set it inside a heated, insulated space. The University of Alaska Fairbanks Cooperative Extension describes both setups for cold-climate water storage. In milder climates, an above-ground tank can work as long as the lines are protected.
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The lines. Bury the fill and supply lines below the frost level, or insulate them where they run above ground.
- The pump and pressure tank. Keep them somewhere heated.
If you plan to bury the tank, choose one made for burial and check the product listing to confirm it. Our cistern collection is a good place to start comparing, and we can confirm which models suit your site when you call.
How do you keep stored off-grid water safe to drink?
Keeping stored water safe comes down to two things you control: where the water comes from and how you look after the tank. If you have water hauled, it should come from an approved source. The Minnesota Department of Health, for example, requires hauled drinking water to come from an approved source and names a community public water supply as the best one. Rules for haulers differ from state to state, so ask any hauler where their water comes from and check what your state requires.
Disinfect the tank by adding bleach when it's new, after repairs and after each load of hauled water. A county health department guide lists those three times. Bleach strength varies from bottle to bottle, so ask your local health department how much to add and how long to leave it in the tank before you use the water.
Choose an opaque tank, one that doesn't let light through, because algae needs light to grow. For drinking water, choose a tank whose product listing says it is rated for potable (drinking) water. Rules for drinking water tanks vary by state, so check with your state or local health department before you install one.
What should you know about getting a large tank delivered?
Most large tanks ship by LTL freight, which means the tank shares a truck with other freight and arrives curbside at a scheduled appointment. Curbside means getting the tank off the truck and to where it will sit is up to you. At a rural home without a forklift or tractor, that often means you need a liftgate, a powered platform on the back of the truck that lowers freight to the ground. More than four in ten of our shipments need a liftgate, so it's a common request. Nearly a third of our shipments go to homes rather than businesses, and homes usually have no loading dock or forklift. Tell us about your site when you order, so the delivery is set up for what you have on hand.
Tell us too if a freight truck may have trouble reaching you, such as on a rural road, a steep or soft driveway or a tight turn. We coordinate the delivery details with the freight company, and knowing about access problems ahead of time lets us pass them along. In some cases, depending on where a product ships from, you may be able to pick the tank up yourself at the manufacturer's location instead. Call us to check.
When the tank arrives, follow these steps:
- Inspect the tank with the driver still there.
- Photograph any damage.
- Write the damage on the Bill of Lading, the delivery paperwork the driver hands you, before you sign it.
- Report the damage to us within 48 hours.
We file the freight claim with the carrier for you, but only if the damage was noted on the Bill of Lading. Without that note, there's no claim and no replacement. We don't install tanks, so line up your own help for setting it, burying it or plumbing it in.
On freight cost, our promise is simple: we won't ship your order or charge you any added freight without your approval. Freight cost depends a lot on how far the tank travels, so every product page shows where the tank ships from. That lets you see why the freight costs what it does.
Get help sizing your off-grid water tank
Once you know your daily use, your main source and your backup, you're ready to pick a size and type. You can browse our water storage tanks online, or call me and the team at 833-465-8265, Monday to Saturday, 8:00 to 5:00 Central. You can also reach us through our contact page. We'll go over your numbers with you, help you choose the right tank for your site, and tell you plainly if a tank you're looking at won't do what you need.