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Annual yield is roof area times rainfall times a runoff coefficient of 0.8-0.9 for pitched hard roofs. Tank size is a compromise between yield and demand: sizing much beyond about 18 days of average yield or 30 days of demand gives diminishing returns because the tank rarely fills or empties fully.
Annual yield
Yield (L) = roof area (m2) x rainfall (mm) x runoff coefficient
Tank size
Size is the lesser of about 18 days of average yield and 30 days of demand
Rainwater harvesting for potable use requires treatment, backflow prevention and compliance with drinking water regulations. Non-potable systems must be clearly labelled to prevent cross-connection.
Because cost rises linearly with volume while the additional demand met falls away sharply. A daily rainfall time-series simulation gives the true yield-reliability-storage relationship.
First-flush diversion and screening as a minimum. Potable use requires filtration and disinfection with verification, and cross-connection to mains supply must be prevented by design.