Storage can bridge a short flow deficit; it cannot create water, prove sustainable well yield or make unsafe water potable.
1. Separate source, storage and delivery
The well or supply line replenishes water over time, the storage vessel shifts that water to a different time, and a booster or pressure system delivers it to fixtures. Give each function its own design value.
A pressure tank controls cycling and provides a small drawdown between switch settings. It is not normally the large buffer used to cover a sustained difference between household demand and well yield.
2. Establish a defensible source rate
Use a pumping test, operating history or professional assessment appropriate to the well and aquifer. A two-point recovery observation inside the casing is useful evidence, but it is not automatically a sustainable source rate.
Private-well owners are responsible for construction, maintenance and water safety. Keep well condition and water testing in the decision rather than treating storage as a substitute for source protection.
US EPA · Learn about private water wells ↗3. Define the peak event instead of one vague daily total
List the fixtures and processes that genuinely overlap and assign a realistic duration. A morning shower peak, irrigation cycle and treatment backwash are different events and may not belong in one sum.
The buffer calculator subtracts source contribution from demand during that period. Repeat it for each important event and consider whether the tank has enough time to recover before the next one.
4. Protect a deliberate reserve and water quality
Do not count every nominal litre as usable. Allow for control levels, pump protection, sediment, inaccessible volume, uncertainty and any required emergency or fire reserve.
Use a vessel and fittings suitable for the water use, exclude contamination, provide a secure cover and plan inspection, cleaning, overflow and drainage. CDC guidance emphasizes protecting cisterns from storm contamination and cleaning them after flood exposure.
CDC · Cistern safety before, during and after disasters ↗5. Coordinate pumps, controls and treatment
Set source-pump protection, high and low level controls, overflow routing, backflow protection and booster operation as one system. Check whether filters, softeners, UV equipment and backwash flows can pass the required peak rate.
Commission with measured fill, draw and recovery times. Record tank levels and pump starts through representative days so the design can be adjusted from evidence rather than an assumed average.
Keep the three capacities separate
| Capacity | Primary question | Evidence |
|---|---|---|
| Source flow | How fast can water arrive? | Pumping test or operating history |
| Usable storage | How much timing deficit can be bridged? | Measured control levels and reserve |
| Delivery flow | How fast can fixtures receive it? | Pump curve, pressure and pipe losses |
| Treatment flow | How fast can water be treated? | Equipment-rated service and backwash flow |
Worked example
Bridge a 45-minute morning peak
Usable storage is 800 L above the protected reserve. The source can contribute 12 L/min while the household peak is 25 L/min for 45 minutes.
Peak demand = 25 × 45 = 1,125 L Source contribution = 12 × 45 = 540 L Storage required = (25 − 12) × 45 = 585 L Margin = 800 − 585 = 215 L
The event fits the stated usable storage with 215 L left, but the next step is to check recovery time, delivery pressure, treatment capacity and repeated events.
What to have ready
- Defensible sustained source rate and water-level observations
- Fixture groups, process flows and realistic event durations
- Tank working levels, protected reserve and overflow route
- Booster curve, treatment flow limits and control sequence
Common mistakes
- Using a brief casing recovery observation as sustainable yield.
- Treating a pressure tank as bulk water storage.
- Counting nominal tank volume without control levels or reserve.
- Sizing from one daily total while ignoring overlapping peak flows.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.