The fastest way to waste money is to buy a tank before defining what the water will be used for and where overflow will go.
1. Define the end use
Outdoor irrigation, toilet flushing, laundry, commercial process water and drinking water have different quality, treatment, plumbing and approval requirements. Begin with the least complex approved use that still has value.
2. Estimate supply
Measure the roof area draining to the collection point and use local rainfall data. Apply realistic collection losses and account for first-flush diversion.
3. Estimate demand
Build a weekly and seasonal demand profile. Storage should serve real demand; unused water simply creates overflow and long water age.
4. Test storage and dry periods
Use dry-period demand as an initial screening value, then use monthly or daily water-balance modelling for larger projects.
5. Design the full-tank condition
Assume the tank is full before a heavy storm. Overflow must protect the building, neighbouring property, septic systems, slopes and public paths.
6. Check rules and maintenance
Contact the appropriate plumbing, building, public-health, drainage and environmental authorities. Assign maintenance before installation.