How to size your rain harvesting system
Updated: Feb 8, 2020
Wanting to supply your whole house with rainwater? We've learned a few things about sizing systems over the years, and here's a quick guide to help in your system design.
Please note: This guide is based on average annual precipitation of 36", spread out relatively evenly between all months, as is the case for Ohio, Michigan, Indiana, Kentucky, West Virginia, Western Pennsylvania, Illinois, and much of the Midwest. If your rainfall amounts vary drastically per month, you may want to consider taking your roof sq footage, multiplying by 0.6, and then multiplying that number by the highest monthly precipitation amount (for instance, if May is your wettest month and you get 2.5 inches of rain in May, multiply by 2.5).
Second note: This guide assumes that you are not also using this water to irrigate (with the exception of occasional vegetable garden watering or watering plants around the house). Supplying an irrigation system like a lawn irrigation system uses a lot of water, and the system will need to be drastically upsized to accommodate.
CABINS WITH OCCASIONAL USE
If using a plastic cistern, install between 1200-1700 gallons.
If using a concrete cistern, install between 1000-2500 gallons.
Note: Ohio regulations require a minimum capacity of 2500-gallons for all rain cistern systems that supply a building/residence.
1-2 PERSON HOUSEHOLD
2500-3000-gallon capacity recommended
3-5 PERSON HOUSEHOLD
5500-gallon capacity recommended
6+ PERSON HOUSEHOLD
8,000-11,000-gallon capacity recommended. For most households, unless you have several outbuildings or a really big house, it does not make sense to install more than an 11,000-gallon system at your residence (again, assuming you are in the Midwest U.S. climate)