Estimate how much potable water rainwater harvesting can displace over a month and a year. Enter the reuse demand, available supply, operating days, system availability, and current water-and-sewer rate. The calculator limits savings to the smaller of usable supply and demand, then reports avoided volume, cost savings, and unmet demand.
Daily offset = min(demand, supply) × availability; monthly offset = daily offset × operating days
Annual cost savings equals monthly offset × 12 ÷ 1,000 × the entered utility rate.
The main result is the annual potable-water volume displaced under the entered availability. It is capped by both source supply and non-potable demand.
This is a volume and utility-bill estimate. It excludes fixed utility charges, energy, consumables, maintenance, and rate-tier effects unless those are reflected in your entered rate.
Demand of 180 gal/day, supply of 150 gal/day, 30 days/month, and 95% availability gives 150 × 0.95 × 30 = 4,275 gal/month, or 51,300 gal/year.
Reuse cannot displace more potable water than the eligible non-potable demand.
Use the marginal combined water and sewer charge that actually changes when consumption falls.
The calculator multiplies the smaller of supply and demand by the availability percentage before annualizing it.
Use the actual days for an operating month and adjust the annual figure separately if the system does not operate all year.
Fixed charges, tiered rates, minimum bills, and changing demand can make billed savings differ from a simple volumetric estimate.
| Input concept | Planning use |
|---|---|
| Volume or flow | Use the same gallon basis throughout the calculation. |
| Efficiency or allowance | Enter percentages as displayed, from 0 to 100. |
| Operating condition | Base it on the intended design period, not an unrelated annual peak. |