By collecting rain from a roof during rain events and storing it in a barrel or cistern, homeowners can create an alternative water supply to irrigate their home gardens and landscaping that will not overpump the groundwater or increase the water bill. In this example, a typical home garden would be 400 ft2 on which the homeowner plans to irrigate using a 55-gal heavy-duty plastic barrel with a spigot located near the bottom of the tank. The objective of this study was to develop a spreadsheet-based model using daily data to determine how the watering habits of home gardeners affect the amount of available supplemental irrigation water and cost savings using a typical 55-gal rain barrel, thus resulting in a more realistic cost-benefit analysis. The model allows for multiple, user-selected criteria such as the size of the barrel, number of barrels, harvest efficiency of the guttering system, size of the garden, the catchment area, and the watering habits of the homeowner (such as how many days without precipitation have occurred before they feel the need to water), which were used to develop seven different scenarios. To optimize rainwater use and cost benefits, the following parameters are recommended: catchment area of 600 ft2, 90% harvest efficiency by reducing leaks and other problems with guttering and rain barrel, threshold of 0.10 inch for a wet day, minimum of only 2 dry days before using the water in the barrel, and one overflow barrel. In this case, a homeowner in Knoxville, TN, can harvest an average of 1,570 gal per season (range of 1,076–2,076 gal), at an average cost savings of $22, and thus recover the cost of the two barrels in 3–6 years.