Before you size a tank, a roof or a well, work out how much water your household really uses, because every other water decision depends on that number.
Why the number matters
Without a pipeline, every litre has to be collected, stored and moved by you. The book lists the jobs rainwater is expected to do: garden and crop irrigation, livestock watering, bathroom use, laundry, drinking, a fire suppression reserve and topping up greywater. An estimate that leaves out half of those will leave you short in the first dry spell.
Build a simple water audit
- List every use. Drinking and cooking, washing, toilet use, laundry, garden, animals, cleaning and any reserve you want to hold back.
- Measure instead of guessing. Time a tap or hose into a bucket of known size, or count buckets per task for a week. Real figures for your own habits beat generic averages.
- Total it per day, then multiply by the number of people and by the length of the longest dry spell you expect.
- Separate seasonal from constant use. Garden and livestock needs usually peak when rainfall is lowest, which is exactly when supply is weakest.
Compare demand with supply
Once you know your daily use, check it against what your site can supply. For rainwater, a rough ceiling is roof area in square metres multiplied by annual rainfall in millimetres, which gives litres (see rainwater collection planning). The book's example of a large roof in a wet climate shows how quickly an average household's needs can be met on paper, but your roof, rainfall and seasons will differ.
Reduce before you build
- Fix leaks and dripping taps, which waste water silently.
- Reuse suitable greywater for the garden where local rules allow (greywater planning).
- Match watering to the season rather than a fixed routine.
Add a margin
Leave headroom for guests, a hot summer or a failed collection season, and plan storage around the longest stretch without rain, not the average week.
Once you have your number, the Off-Grid Water guide shows how collection, storage and treatment fit together. For a wider view of living within your resources, see Off Grid Survival for Beginners.