On most rural properties the pump is the last decision, not the first. How much water you can capture, where you store it, and how the tanks are plumbed all shape what the pump has to do. Getting the storage right often removes a pump problem before it exists.
How much water can you capture?
The calculation is simple and worth doing before buying anything.
Litres per year = roof area (m²) × annual rainfall (mm) × 0.85
The 0.85 accounts for losses — first flush diversion, evaporation, splash and overflow during heavy falls. Some sources use 0.8; on a well-guttered steel roof 0.85 is realistic.
Roof area is the plan area, the footprint the roof covers as seen from above, not the sloped surface area. Rain falls vertically, so the horizontal projection is what catches it.
Example
A 200 m² roof in a district receiving 600 mm a year:
200 × 600 × 0.85 = 102,000 litres per year
That is a genuine 102 kL of supply — but it arrives unevenly, which is the whole reason for storage.
How much storage do you need?
Storage exists to bridge the dry gap, so size it against the longest dry period you expect, not the annual total.
Storage needed = daily use × longest expected dry spell in days
Typical daily use
| Use | Litres per day |
|---|---|
| Drinking and cooking, per person | 5–10 |
| Full household use, per person | 120–200 |
| Household on water-efficient fixtures, per person | 90–130 |
| Garden, per 100 m² in summer | 200–500 |
| Cattle, per head | 40–80 |
| Sheep, per head | 4–8 |
| Horses, per head | 40–50 |
Example
Four people at 150 L/day each is 600 L/day. Planning for a 90-day dry spell:
600 × 90 = 54,000 litres
So around 55 kL of storage, ideally more for margin. Against a 102 kL annual yield that is comfortable — the roof can refill it, and the storage carries the gap.
Where yield and storage do not match, you have three options: increase catchment, reduce demand, or add another source such as a bore.
Tank layout and its effect on the pump
One large tank or several smaller ones?
Several smaller tanks connected at the base behave as one large tank and are usually easier to site, transport and install. They also let you isolate one for cleaning without losing the whole supply.
Connect them at the base with balance lines, sized generously — undersized balance pipe means the tanks equalise slowly and the pump effectively draws from one.
Tank height matters
Every metre the tank outlet sits above the pump is a metre of suction lift the pump does not have to produce. Every metre below eats into the 7–8 m practical suction limit.
Wherever the ground allows, put the pump below the tank outlet and let gravity feed it. A flooded suction primes instantly, primes reliably, and eliminates the most common cause of pump failure. It costs nothing but thought at layout time.
Header tanks
Where a tank can sit high — on a hill or a stand — gravity may supply useful pressure with no pump at all. Each 10 m of height gives 1 bar. A tank 10 m above the house gives about 1 bar at the tap: enough for garden use and basic supply, not enough for a modern shower.
The common arrangement on hilly properties is to pump from a low storage into a header tank, and gravity-feed the house from there. It reduces the pump’s duty cycle enormously, since it fills on a level switch rather than starting for every tap.
Choosing the pump from the layout
| Layout | Suits |
|---|---|
| Tank above pump, house nearby | RM-JP100 or RM-JP100-E |
| Tank at ground level, pump beside it, exposed position | RM-SJP800 stainless |
| Several outlets used simultaneously, constant pressure wanted | RM-DJSM1025 multistage |
| Long run to the house, or a rise | RM-JP200 — 50 m head, 130 L/min |
| Filling a header tank from low storage | RM-TF1 for volume, on a level switch |
Work through the head and flow numbers in Understanding Flow Rate, Head and Pump Curves before settling on a model.
Water quality
Storage design affects what comes out of the tap.
First flush diversion. The first rain off a roof carries dust, leaves and droppings. A first flush diverter sends that initial volume to waste. Roughly 0.5 L per m² of roof is a common allowance.
Leaf guards and inlet screens. Keep organic matter out. What does not go in does not need removing later.
Draw from above the base. Sediment settles. A tank outlet at the very bottom draws it straight into the pump. Set the outlet slightly above the floor, or use a floating suction filter that draws from just below the surface where the water is cleanest.
Light exclusion. Any light entering a tank grows algae. Check that inlet and overflow screens are intact.
Overflow. Must be screened against mosquitoes and vermin, and directed away from footings.
Where the water is for drinking, appropriate filtration and disinfection should be specified for your situation — this is worth local advice, since requirements vary by state and by water source.
Protecting the pump from an empty tank
Tanks run out. A pump running dry destroys its seal within minutes.
Options:
- A controller with run-dry protection such as the RMPC-10 or RMPC-30 — the simplest approach, and it handles start/stop as well.
- A low-level float switch in the tank that cuts power before the outlet is exposed.
- A tank level gauge so you can see it coming.
The last is worth having regardless. Knowing you are at a quarter full changes decisions; finding out when the shower stops does not.
Planning checklist
- Roof catchment area measured (plan area)
- Local annual rainfall figure obtained
- Annual yield calculated
- Daily demand estimated honestly
- Longest expected dry spell considered
- Storage sized, with margin
- Tank position chosen — pump below the outlet if at all possible
- Balance lines sized where multiple tanks are used
- First flush diversion and inlet screening planned
- Overflow routed and screened
- Outlet positioned above sediment level
- Run-dry protection specified
- Pump selected against calculated head and flow
- Pressure tank or controller decided — see Pressure Tank or Pump Controller?
