How this estimate is calculated
Annual harvest is roof footprint × annual rainfall × a region volume-factor × collection efficiency — typically 75–85% for a well-maintained gutter and downpipe system. Household demand is household size × per-person daily use, scaled to a year.
The per-person default here is the off-grid baseline, not a grid-tied average: — per person per day covers drinking, cooking, hygiene and laundry on efficient fixtures with a composting or low-flush toilet. Raise it toward — per person per day only once a garden or livestock enters the picture — that bigger number sizes catchment and refill capacity, not the emergency reserve this tool defaults to.
Tank size starts from daily demand × your autonomy window (30 days is a common starting point), then scales up by the headroom factor — airspace for ice expansion in a freeze climate, plus the gallons below a tank’s outlet a pump can never reach. Default is 20%, roughly a 5–10% dead-storage loss stacked with 10% freeze headroom; drop it toward 0% for a buried, frost-line cistern with a bottom-center outlet.
What this estimate doesn’t capture:
- Seasonal rainfall variation — a dry season can leave a full-size tank empty even when the yearly average looks generous.
- First-flush diversion and filtration, both needed before catchment water is safe to drink.
- Real-world efficiency losses from roofing material, gutter sizing and roof pitch.
- Local plumbing and health-code minimums for potable water storage.
For the full baseline-to-nameplate formula and a worked example, read how much water storage you really need off-grid.