85% is a common default (evaporation, overflow, first-flush loss).
The math:
Gallons = Roof area (sq ft) × Rainfall (in) × 0.623 × Efficiency
You can collect
635.5
gallons (2,405.5 L)
That’s about
13
standard 50-gal barrels
At 85% collection efficiency, from 1,200 sq ft.
PSI = 0.433 × height (ft)
Water pressure
1.3 PSI
Under 3 PSI: enough for gravity-fed drip irrigation, not much else.
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