Storage is the cheapest insurance in a water system. A tank smooths out the difference between when water is available and when it is needed, and sized correctly it turns an intermittent supply into a dependable one. Undersized, it is just expensive plumbing.
Start with daily demand
Add up the real daily use: people, animals, irrigation, cleaning. A household might run 1 to 2 m³ a day; a small livestock farm several times that. The number does not need to be perfect, but it needs to be honest.
Then decide your autonomy
How many days must the tank carry you if the supply stops? For a home on a municipal line, half a day of buffer is usually enough. For a solar-pumped farm, the right answer is often two to three days: enough to ride through cloudy weather and pump downtime without anyone going dry.
Elevation is free pressure
Every 10 metres of tank height gives about a bar of pressure. Put the tank on a tower or a hill and gravity becomes a silent pump that never breaks, never burns out and works during power cuts. For solar systems this matters twice over: stored water replaces stored electricity at a fraction of the cost.
The mistake we see most is buying the tank that was on offer rather than the tank the demand called for. Multiply daily use by the days of autonomy you actually need, then round up. Tanks are cheap. Empty ones are not.