WaterCalcLabRainwater ROI Calculator
Estimate simple annual payback using eligible annual demand and actually available annual supply.
Your result
Bars compare only results in the same unit group. They are normalized, not a time-series forecast.
Planning result based on your inputs.
How this tool works
Estimate simple annual payback using eligible annual demand and actually available annual supply.
net savings = displaced m³ × tariff − maintenance − kWh × energy price; payback = capital/net savingsExample calculation
The prefilled form is a worked example. Its base-unit inputs and expected outputs are shown below; use the reset button to restore it.
| Input | Example |
|---|---|
| Available supply in the same period | 15000 |
| Demand in the same period | 20000 |
| Avoidable volumetric price | 3 |
| Initial investment | 2000 |
| Annual maintenance | 50 |
| Annual energy use | 40 |
| Electricity price per kWh | 0.3 |
Planning notes for this calculator
Compare an initial rainwater investment with a simple annual net-saving scenario. The tool reports payback time rather than a discounted financial return or guaranteed investment outcome.
Choose meaningful inputs
- Available supply in the same period
- Use annual usable supply after realistic storage and timing allowances, not just gross roof rainfall.
- Demand in the same period
- Use annual demand that rainwater can replace, matched to the supply period.
- Avoidable volumetric price
- Use the avoidable marginal water charge. Fixed charges that remain are not a saving.
- Initial investment
- Include the initial installed project cost, not only the tank price.
- Annual maintenance
- Enter annual recurring non-energy cost. Do not also include pumping electricity here if entered below.
- Annual energy use
- Use annual electricity consumption in kWh.
- Electricity price per kWh
- Enter the electricity price per kWh in the same currency as the investment.
Read the result correctly
Annual replacement is bounded by supply and eligible demand. Avoided water charges minus maintenance and electricity give net savings. Positive savings allow initial cost divided by net savings; zero or negative savings have no finite simple payback. Even a finite result may exceed equipment life and excludes financing, replacements and the time value of money.
Common mistakes to avoid
- Do not present gross avoided water cost as net profit.
- Keep annual energy, annual demand and annual maintenance on the same basis.
A useful next step
Test a lower supply, a lower avoidable tariff and higher operating costs before evaluating affordability or lifecycle cost.
Follow a worked scenario
A separate illustrative scenario; these values do not change when you edit the form above.
- 40 m³ × 4 = 160 annual avoided units; maintenance 30 + electricity 40 × 0.30 = 42 annual operating units.
- 160 − 42 = 118 net units/year; 2,000 ÷ 118 = about 16.95 years simple payback.
Frequently asked questions
Why can the payback result be not applicable?
If modeled annual net savings are zero or negative, the investment is not recovered through this constant-saving scenario. It is not a division error or an infinite return.
Can I use this to compare all investment options?
It is a preliminary simple-payback comparison only. Options with different lifetimes, financing, replacements or residual values require lifecycle or discounted cash-flow analysis.
Continue your plan
Rainwater Savings Calculator
Bound potential mains replacement by both available rainwater and eligible demand in the same period.
Open calculator ↗Costs & savingsWater Cost Calculator
Calculate a single volumetric tariff plus a fixed charge in the same billing period.
Open calculator ↗Costs & savingsAnnual Water Savings Calculator
Calculate annual replacement potential for a specified number of active days. Enter annual usable supply.
Open calculator ↗Related reading
Costs & savingsRainwater System Payback Period
Simple payback divides initial investment by annual net cash savings. Net means avoided consumption charges minus relevant recurring costs. Gross roof collection is not an annual cash saving. Estimate water that actually replaces billed use, check the applicable tariff and keep assumptions dated. The calculation is a planning scenario, not a guaranteed investment return.
Read guide ↗
Costs & savingsRainwater Harvesting Costs
Describe connected roof area, usable storage, permitted end uses and the delivery endpoint before requesting quotes. Ask each installer to price the same scope. A barrel-and-hose arrangement and a buried, pumped building system are different projects. A useful budget makes those differences visible rather than combining them into one unsupported international average installation price.
Read guide ↗Sources & method references
- US EPA WaterSense: Understanding your water bill
Volume units and billing structures. Enter your own applicable supplier prices.
- Texas Water Development Board: Rainwater Harvesting Manual
Collection, storage and system-planning context. Geometry and unit conversions on this page are stated explicitly.
Method version 1.1 · Explanations and examples reviewed: 2026-10-11