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Rainwater Harvesting Payback Calculator
Rainwater systems reduce potable water consumption for non-potable uses.
Payback years
22.4
Annual water savings
$3,348
Gallons captured / yr
279,000
How the math works
Gallons = sqft × rain inch × 0.62 × efficiency. Savings = (gal/1000) × rate.
20k × 30 × 0.62 × 75% = 279k gal × $0.012 = $3,348/yr. $75k / $3,348 = 22.4 yr.
How to Use
- Enter roof sqft collection area.
- Enter annual rainfall inches.
- Enter cistern system cost.
- Enter water + sewer rate.
- Enter capture efficiency %.
- Read payback.
Frequently Asked Questions
System components?
(1) Collection surface: roof typically. 1 inch rain × 1,000 sqft = 600 gallons. (2) Gutters + downspouts: direct to tank. (3) First-flush diverter: discards dirty initial water. (4) Filter/settling tank: removes debris. (5) Storage cistern: 500-50,000 gallons depending on scale. (6) Pump + distribution. (7) UV treatment (for certain uses). Cost: $2-15/gallon capacity, plus $5-25/sqft infrastructure.
Use applications?
Landscape irrigation: most common, no treatment needed. Toilet flushing: minimal treatment, plumbing code varies by city. Laundry: gray water OK with filter. Cooling tower makeup: good application. Industrial process water: case-by-case. Potable uses (drinking, food, showering): advanced treatment required, expensive. Most cost-effective: irrigation + toilet flushing.
Typical savings?
Commercial building 40,000 sqft roof in Austin (30 inch rain): captures ~700k gallons/year. Water + sewer avoided: $10-20k/year. System cost: $50-200k. Payback 5-20 years typical. Better economics: heavy-rain regions (Pacific Northwest, Southeast), hot climates with high water costs (TX, AZ, CA). Worse: desert rain-scarce regions, low water rates.
Incentives + codes?
Federal: none specific. State + city: Texas, California, Arizona, Oregon, Washington have active programs. Austin Water: 50% rebate up to $5,000 for residential. Denver Water: rebates up to $5,000. Portland BES: 100% cost of rainwater barrels. New Mexico Water Harvesting: $500-2,000 tax credit. Some states prohibit rainwater harvesting (historically Colorado, now modified). International Green Construction Code (IgCC) encourages adoption.
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