How to Use Harvested Rainwater
Harvested rainwater serves a wide range of residential purposes, from watering your garden to flushing toilets and even drinking water with proper treatment. Understanding the water requirements, setup complexity, and treatment needs for each use helps you design a system that matches your goals and budget. Below we detail 10 common uses for harvested rainwater, ranked by daily water consumption.
Water Use Comparison
The table below compares all 10 uses by daily water requirement, whether the use is seasonal or year-round, and whether potable-grade treatment is needed. Click any use for a detailed guide covering setup requirements, best practices, and state-specific recommendations.
| Use | Gallons/Day | Season | Potable Required |
|---|---|---|---|
| Pressure Washing | 100 gal/day | Year-round | No |
| Lawn Irrigation | 75 gal/day | Growing season | No |
| Garden Irrigation | 50 gal/day | Growing season | No |
| Car Washing | 50 gal/day | Year-round | No |
| Livestock Watering | 30 gal/day | Year-round | No |
| General Outdoor Use | 30 gal/day | Year-round | No |
| Toilet Flushing | 20 gal/day | Year-round | No |
| Laundry | 15 gal/day | Year-round | No |
| Pool Top-Off | 10 gal/day | Growing season | No |
| Drinking and Cooking | 1 gal/day | Year-round | Yes |
Outdoor Uses: The Easiest Starting Point
Outdoor water uses are the most accessible applications for harvested rainwater because they require minimal treatment and simple delivery systems. A basic rain barrel with a spigot is sufficient for most outdoor tasks. Garden irrigation is by far the most popular use, as plants actually prefer rainwater over treated municipal water — it is naturally soft, slightly acidic (which helps nutrient absorption), and free of chlorine and fluoride that can stress sensitive plants.
Lawn irrigation is the most water-intensive outdoor use, consuming 75 gallons per day per 1,000 square feet of turf. If you are watering a large lawn, you will need a pump system and significant storage capacity (1,000-2,500 gallons minimum). Many homeowners find it more practical to reduce their lawn area and focus rainwater on high-value uses like vegetable gardens and ornamental plantings. General outdoor use, car washing, and pressure washing are intermittent activities that work well with modest rain barrel setups since they do not require daily supply.
Indoor Non-Potable Uses: Bigger Savings, More Setup
Using harvested rainwater indoors for toilet flushing and laundry represents a significant step up in both water savings and system complexity. These uses operate year-round, providing consistent savings regardless of season. Toilet flushing alone accounts for roughly 30% of indoor residential water use, making it the single most impactful indoor application.
Indoor use requires a dedicated non-potable plumbing system (typically purple-colored pipe per code), a pump-and-pressure-tank assembly, sediment filtration, and a backup connection to municipal water with proper backflow prevention. Most jurisdictions require a plumbing permit for indoor rainwater connections. Despite the higher upfront cost, the year-round nature of indoor use means faster financial payback compared to seasonal outdoor uses. A family of four using rainwater for toilets and laundry can offset 35-40% of their total indoor water consumption.
Potable Use: The Full Treatment Path
Using rainwater for drinking and cooking is entirely feasible but requires the most rigorous treatment and the strictest attention to system design. Only certain roof materials are acceptable (metal, clay tile, concrete tile, and slate — never asphalt shingle or wood shake). The treatment chain includes a first-flush diverter, 20-micron pre-filter, 5-micron sediment filter, activated carbon filter for organic compounds and taste, and UV sterilization for pathogen elimination. Some systems add a final 0.5-micron absolute filter for additional safety.
Over 60,000 residents in Hawaii rely on rainwater catchment as their primary drinking water source, proving the viability of the approach when properly implemented. Regular water quality testing (at least annually for coliform bacteria, pH, and basic chemistry) is essential. The per-person water demand for drinking and cooking is modest — about 1 gallon per day — so the treatment system handles relatively low volumes. The investment in filtration equipment ($500-$2,000) is justified by the quality and reliability of the resulting water supply, especially in areas with unreliable or poor-quality municipal water.
Combining Multiple Uses
Most rainwater harvesting systems serve multiple purposes simultaneously. A homeowner might use the same tank for garden irrigation, car washing, and general outdoor cleaning. The key is to calculate the total daily demand across all uses, account for seasonal variations, and size your tank accordingly. Our calculator allows you to check multiple uses at once and see the combined demand, supply balance, and recommended tank size for your specific combination.
When combining indoor and outdoor uses, consider installing two separate tanks or a single large tank with two outlets — one gravity-fed for outdoor use and one pump-fed for indoor applications. This ensures that high-demand outdoor watering does not deplete the supply needed for indoor non-potable fixtures. A tank-level switch can prioritize indoor use by restricting outdoor access when water levels drop below a set threshold, ensuring you always have water for essential indoor functions.