Rainwater is one of the most accessible off-grid water sources: it scales with your roof area and local rainfall, and it arrives clean at the point it lands. The catch is everything that happens after — what it picks up from the roof, how you store it, and how the system behaves when it freezes.
How much you can actually collect
The arithmetic is simple: catchment area in square metres multiplied by rainfall in millimetres gives roughly that many litres, minus losses. Knowing your roof area and local rainfall gives a realistic sense of what a wet season can bank for you.
Keep it clean from roof to tank
Rain is clean falling but picks up dust, debris and droppings from the roof. A first-flush diversion that discards the initial dirty run-off, plus screens on gutters and tank inlets, keeps the stored water far better. Assume harvested water needs treatment before drinking.
Store enough to bridge the dry spells
Storage should cover the longest realistic gap between meaningful rainfall, with a margin. Running dry is not a minor inconvenience off-grid, so err toward more capacity than you think you need.
Winter-proof the whole path
Gutters ice, downpipes block, and a full tank that freezes solid can rupture. Insulating or sheltering tanks, keeping a reserve indoors that cannot freeze, and planning for the freeze rather than being surprised by it are what keep the system alive in the cold months.