To live off the grid, the average family needs at least 400 gallons of stored water - or, ideally, a constant supply of water. The average American family uses around 300 gallons of water per day, so if you need water for all purposes (not just for drinking), you'll need access to an off-grid well or surface water system that can provide this amount of water without being stretched to its maximum capabilities. You may want to combine several off-grid water systems, such as a rain catchment system and a private well, so you don't put too much strain on one resource.