Outline your roof and see what a tank would catch through twenty years of real wet and dry seasons, what it could supply, and which size pays.
Often, but slowly, and this calculator says so when it does not. A tank pays back fastest where a garden or toilets are fed through a long wet season on a high municipal tariff. On the Highveld the rain comes in summer, when the garden needs least, and the tank sits empty through the dry winter; the right size there is smaller than people expect. The estimate counts the pump’s electricity and the tank’s upkeep against the saving.
Big enough to carry you through the gaps between storms, and no bigger. The size chart on your results shows how much more water each step up actually delivers on your roof; past the point where the curve flattens, extra litres mostly sit empty. For most suburban roofs feeding a garden that is between 2 500 and 10 000 litres.
Not without treatment, and this calculator does not model that. Roof water carries dust, bird droppings and whatever is in the air. It is fine for the garden, toilets, laundry and topping up a pool, which is what the tool sizes for.
For a tap on the tank and a hose to the garden, no. To feed toilets or a washing machine, yes: SANS 10252 requires a backflow preventer wherever tank water meets municipal plumbing, and the changeover should be done by a plumber. Some municipalities ask you to register a tank connected to internal plumbing. The estimate includes the plumber where you tick an indoor use.
Better than on the Highveld, in one way: the rain falls in winter and the tank is full when the dry summer starts. But a summer garden drains a 5 000 litre tank in a few weeks, so the size chart matters more there, not less. The twenty-year run at your address shows exactly how the timing works out.
No. The address goes to a geocoding service, the map position to the providers of the aerial photos, and your location, rounded to about a kilometre, to NASA for the rainfall record. Your outline and your answers never reach us; the estimate is worked out in your browser.