Buy the Knee, Not the Tank: A Daily Water Balance Locates Where Rainwater Storage Stops Paying

Tank retailers sell capacity, but householders buy reliability: the share of toilet, laundry, and garden demand that rain actually meets. This paper runs a twenty-year daily water balance for a 110 square metre roof in a stated temperate rainfall regime (mean 749 mm per year) serving a stated demand averaging 130 litres per day. Reliability rises steeply at first and then flattens: a 1,000 litre tank meets 87.4 percent of demand, a 3,000 litre tank 99.5 percent, a 5,000 litre tank 100.0 percent, and a 10,000 litre tank 100.0 percent. Doubling from 5,000 to 10,000 litres buys only 0.0 percentage points, while the step from 1,000 to 3,000 buys 12.1. Overflow remains substantial even at large sizes — 27236 litres per year at 10,000 litres — because wet-season rain arrives faster than a steady demand can use it. The sizing rule that follows is to buy the knee of the curve, around 3,000 to 5,000 litres for this roof and demand, and to treat capacity beyond it as purchasing very expensive percentage points. All results are simulated, not measured.