The size of the water tank you need for an off-grid living will depend on several factors, including the size of your household, your water usage habits, the availability of water from your water source, and the climate in which you live. As a general rule, it is recommended to have a water tank that can hold at least 1 gallon of water per person per day for drinking, cooking, and personal hygiene purposes. It would mean a minimum water tank size of about 120 gallons for a family of four. However, this is just a rough guideline, and you may need a larger or smaller water tank, depending on your specific circumstances. For example, if you live in a hot, dry climate, you may need to store more water to account for increased usage due to evaporation and higher temperatures. Similarly, if you have a large family or use a lot of water for irrigation or other purposes, you may need a larger water tank to meet your needs. It is essential to carefully consider your water needs and the capacity of your water tank to ensure that you have enough water to meet your needs while also maintaining the overall sustainability of your off-grid system.