Rainwater Harvesting Tanks, Storage and Demand in South Africa

Available storage, initial water and protected reserve

Enter operational maximum stored litres after any required freeboard, rather than an external-dimension estimate. Initial water is already in the tank and must not exceed that maximum. The optional minimum storage limit stops the model withdrawing water below that level. It is a numerical operating assumption, not a certified fire reserve, pump protection design or guarantee that the water is physically accessible. Water that cannot fit at the time of the event overflows, even if demand would empty some of the tank later.

How the event sequence and demand work

Each row means one rainfall event followed by the entered number of days of demand at the combined daily rate. The model first collects rainfall, diverts the entered first flush, fills available space and records overflow. It then serves the following demand from storage above the minimum limit and records unmet demand. Add a zero-rainfall row to represent a dry interval; set its diversion to zero. Days are durations for demand arithmetic, not weather forecasts. The ordering is explicit and does not model water use during a storm or hourly flows.

Why annual rainfall is not reliable tank sizing

Multiplying annual rainfall by roof area can estimate an annual gross collection potential, but does not establish tank size, drought cover or reliable supply. Rainfall timing, dry spells, starting storage and use between storms can produce very different outcomes for the same annual total. Do not enter a whole year as one storm and treat the simulated overflow or reliability as a daily model. Use representative event or finer time-series records and a suitable design assessment when supply reliability matters. This calculator reports the entered sequence only.

South African systems, JoJo and Pretoria searches

For a home in Pretoria or elsewhere in South Africa, use rainfall records appropriate to the location and period, measured connected catchment and the actual tank operating limits. JoJo and other manufacturers provide model-specific installation information and accessories; this calculator does not reproduce an official brand sizing result or endorse an installer. Obtain current local quotes for the same tank, screening, diversion, support, plumbing, treatment and commissioning scope. No live prices, nearby availability or guaranteed financial saving are generated.

Water quality, intended use and maintenance

Roof runoff can contain contamination. Debris screening and first-flush diversion do not by themselves establish drinking-water safety. Choose treatment, testing and maintenance for the intended use and actual water quality. Inspect and maintain gutters, screens, diverters, tank access and overflow arrangements according to the installed equipment and local conditions. Keep rainwater and potable supply connections appropriately designed to prevent contamination. This volume calculator does not certify drinking water, select a disinfection dose or approve a plumbing connection.

Frequently asked questions

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