By Sarah Chen · environmental engineer · water, waste and everything that flows downhill
“After losing three days of water to a 200-litre tank that was simply too small, I now size pressure vessels for every drop that falls.”
By Sarah Chen · environmental engineer · water, waste and everything that flows downhill
“After losing three days of water to a 200-litre tank that was simply too small, I now size pressure vessels for every drop that falls.”
Look, if you're off-grid in Australia and you've got a pump, you need a pressure tank. No tank? Your pump will crap itself in 3 months. Done. Let's get to sizing.
Sizing a pressure tank correctly is the single most useful thing you can do for an off-grid well pump. Get it wrong and the pump short-cycles, runs hot, and dies years early. Get it right and the system quietly does its job for the next two decades.
This is a working guide: how pressure tanks actually work, the three numbers you need to size one, drawdown versus total volume, the common tank types, and a sensible starting point for a typical 2–4 bedroom Australian off-grid home.
A pressure tank sits between the well pump and the house plumbing. It holds a captive air charge (pre-charge) on one side and water on the other. When a tap opens, water leaves the tank first, the pressure drops, and only when it drops below the cut-in setting does the pump switch on to refill it.
Two consequences follow from this:
That's the whole job: protect the pump, smooth the pressure, deliver a usable reserve.
This is where most sizing errors happen.
Drawdown depends on three things: tank total volume, pre-charge pressure, and cut-in/cut-out pressure. Change the pressure switch from 30/50 to 40/60 and the drawdown changes too.
Practical examples from the common residential tanks:
| Tank | Total Volume | Approx. Drawdown @ 30/50 |
|---|---|---|
| Amtrol WX-202 | 20 gal | ~5.7 gal |
| Water Worker HT-32B | 32 gal | ~9.3 gal |
| Wellmate WM-9 (composite) | 30 gal | ~9.6 gal |
| Amtrol WX-250 | 44 gal | ~14.0 gal |
When you're choosing a tank, drawdown is the number that matters, not the marketing label.
To size a pressure tank you need three key data points: pump flow rate (GPM), cut-in/cut-out pressure switch settings, and target run time.
Read it off the pump nameplate or the installer's paperwork. A typical off-grid submersible in this part of Queensland sits somewhere around 5–10 GPM at working pressure, but smaller surface pumps and solar-driven pumps can be as low as 1–3 GPM.
A pressure switch tied between the pump and tank is typically set at a PSI range of 30/50 or 40/60. 30/50 is the standard residential default. 40/60 delivers stronger showers but cuts drawdown roughly in half on the same tank.
In metric terms (handy for 230V/50Hz installations): 40 PSI ≈ 2.76 bar, 60 PSI ≈ 4.14 bar. The bar conversions are correct, but the pressure switch itself is still labelled in PSI in Australia.
Most manufacturers suggest a pump run time of less than one minute when the motor is under 1 HP. Aim for at least 1–2 minutes of runtime per cycle. If the pump fires for 10 seconds and stops, the tank is too small for that pump — or the pump is oversized for that tank.
The exact formula uses Boyle's Law on the air charge, but a working rule for residential off-grid is:
1. Decide your peak household demand in GPM (count showers, taps, dishwasher, washing machine running simultaneously).
2. Decide your pressure switch setting.
3. Pick a tank whose drawdown comfortably exceeds 30–60 seconds of peak demand.
4. Confirm the pump can refill that drawdown in 1–2 minutes at its rated GPM.
If the pump's flow rate (e.g., 3 GPM) is lower than combined faucet demand (e.g., 5–7 GPM), the pump becomes the limiting factor and upsizing may be needed — either a bigger pump, a bigger tank, or both.
For most 2 to 4 bedroom homes, a pressure tank in the 40 to 85 gallon range is a sensible starting point.
Bladder tanks (e.g., Well-X-Trol Pro Series) are the industry standard for residential wells and separate air and water with a replaceable rubber bladder to prevent waterlogging.
Captive-air / diaphragm tanks work on the same principle but use a fixed diaphragm instead of a replaceable bladder. Common in smaller domestic and caravan applications.
Fiberglass/composite tanks (e.g., WellMate) will not corrode and weigh about half as much as steel tanks; they are recommended for acidic water or easy installation.
Galvanised steel tanks are the budget option. They work but the internal coating eventually fails, and stainless steel tank connections resist internal corrosion better than the galvanized steel nipples used on budget tanks.
For off-grid work where the tank sits outside or in a pump shed, composite tanks are worth the premium — half the weight, no rust.
Short cycling means the pump turns on and off too frequently. It reduces minimum run time, puts more strain on the motor, and shortens pump life.
Signs of a failing/waterlogged tank include rapid pump cycling, surging and dropping water pressure, and a tank that feels heavy or "dead" when tapped. If you tap the tank and it sounds like a full water drum instead of a hollow thump, the air charge is gone or the bladder has failed.
Undersized tank is the number one killer of well pumps. Verify pump flow rate and pressure switch settings before selecting tank size.
Pressure tank pre-charge must be set below cut-in (commonly ~2 PSI below) for the system to function.
Check it with a tyre gauge at the schrader valve on top of the tank, with the pump isolated and the house pressure bled to zero. Wrong pre-charge is the most common reason a "new" tank behaves like a waterlogged one.
Residential pressure tanks commonly range from 20 to 120 gallons, and multiple 20-gallon tanks can be tied together in series to increase pressurized storage. Two smaller tanks in series can be easier to handle than one large one — useful when the only access is a narrow shed door or a ute tray.
Quality pressure tanks last 15 to 25+ years, while cheap tanks from box stores often fail in 5 to 7 years. A correctly sized tank reduces pump cycles, keeps the pump cooler, and delivers steady pressure to downstream fixtures, irrigation, wash bays, livestock barns, and industrial water lines — all common draws on a Central Queensland property.
In Australia, the following work around a pressure tank should be done by, or at minimum inspected by, a licensed tradesperson:
A homeowner can legally check pre-charge, swap a pressure gauge, replace a pressure switch on a like-for-like basis in some states, and install a tank on a level base — but the moment you open the 230V side, call a sparky.
What size pressure tank for a 3-bedroom off-grid home?
For most 2 to 4 bedroom homes a pressure tank in the 40 to 85 gallon range is a sensible starting point. Match drawdown to your peak simultaneous demand.
Is a bigger tank always better?
Up to a point. A larger tank gives more drawdown and fewer pump cycles. But it also costs more, takes more space, and if the pump is undersized, the pump ends up idling for ages between cycles. Match tank to pump.
30/50 or 40/60 pressure switch?
30/50 is standard residential. 40/60 gives stronger shower pressure but halves drawdown on the same tank. Higher pressure switch settings also stress fittings and small-bore pipe more. Stick with 30/50 unless you have a specific reason to change it.
How do I know if my tank is waterlogged?
Tap it. A healthy tank sounds hollow. A waterlogged tank feels heavy and "dead" when tapped, and the pump cycles rapidly. Other signs include surging and dropping water pressure.
Can I use two small tanks instead of one big one?
Yes. Multiple 20-gallon tanks can be tied together in series to increase pressurized storage. Useful for transport and access.
How long should a pressure tank last?
Quality pressure tanks last 15 to 25+ years. Cheap tanks often fail in 5 to 7 years.
Worth a watch: Off Grid Water Pressure Tanks · The Ready Life