How much storage do I need? The amount of storage you need depends on your use and the rainfall in your area. We use a “water budget” approach in planning your storage needs. A water budget takes several factors into account, starting with an estimate of the monthly expected water demand calculated from your planned use for the water. This is matched against the area’s rainfall history and averages (historical amount and days between rainfalls).
You can think of storage as being like a bank account, with rain making the deposits and water demand making the withdrawals each month. If you are going to depend entirely on rainwater, the storage size you need would be that which has water in the “bank” all through a worst-case rainfall year.