Many locations can collect sufficient water to meet needs. Often the limiting factor ends up being storage volume. Calculating monthly collection capacities minus monthly demand shows how much storage is needed. The spreadsheet in Figure 4-4 shows the monthly rainfall, collection, usage, and storage calculated for a house in Columbia, Missouri with a footprint of 541 \(ft^2\), a roof efficiency of 0.8, and a storage tank of 800 gallons. Note that this system, assuming it starts full in January the first year, is sustainable with the assumed monthly usages. The tank always has some water remaining, with February of the second year being the lowest at 98 gallons.
When storage is the limiting factor, it is often due to the available size, budget, or weight durability of the storage area. Dialing back usage and increasing collection areas are two ways to lower the needs of the storage.