Rainwater Harvesting ROI Calculator
Rainwater system payback.
Calculate rainwater harvesting ROI from tank size, install cost, rainfall, roof area, and water price. Enter tank capacity litres and see the result instantly.
What this tool does
This tool calculates rainwater harvesting ROI from tank capacity, install cost, rainfall, roof area, and water cost.
Enter Values
Formula Used
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Disclaimer
Results are estimates for educational purposes only. They do not constitute financial advice. Consult a qualified professional before making financial decisions.
Rainwater harvesting captures roof runoff for non-potable use (toilets, irrigation, washing). Installations cost 2-8k for residential, 10-50k for commercial. Annual savings depend on roof area, rainfall, and water price. Most systems pay back in 8-15 years for residential, 5-10 years for commercial with higher water bills.
5,000-litre tank, 4,000 install, 800mm annual rainfall, 100sqm roof. Captures roughly 64,000 litres/year (80% efficiency). At 3 per 1,000 litres = 192 annual savings. 20-year savings: 3,840. Break-even but environmental case strong. Commercial installations with higher water rates and bigger roofs see much better economics.
Best returns: large roof area, high water cost region, commercial users with toilet/cleaning demand, off-grid properties (no mains alternative). Less viable: small roofs, low rainfall regions, low water prices, infrequent use that doesn't justify tank capacity.
A worked example
Try the defaults: tank capacity of 5,000, install cost of 4,000, annual rainfall of 800, roof area of 100. The tool returns -10.00%. You can adjust any input and the result updates as you type — no submit button, no reload. That's the real power here: seeing how sensitive the output is to one or two assumptions.
What moves the number most
The result responds to Tank Capacity (litres), Install Cost, Annual Rainfall (mm), Roof Area (sqm), and Water Cost per 1000L. Not every input has equal weight. Flip one at a time toward extreme values to feel which ones move the needle most for your situation.
The formula behind this
Annual capture = roof × rainfall × 80% efficiency. Capped at tank × 12. Savings = litres × price. Everything the calculator does is shown in the formula box below, so you can check the math against your own spreadsheet if you want.
Beyond the number
Carbon, health, and local air quality don't show up on the calculator but often drive the decision. The financial figure is a lower bound on the value; the rest is whatever you'd pay for the non-financial benefits.
What this doesn't capture
Carbon reduction, health benefits, and local air quality have real value the financial figure doesn't price. The calculation gives the money side honestly; for the full picture, note the non-financial benefits alongside.
5,000L tank, £4,000 £ cost, 100sqm × 800mm = -10.00%.
Inputs
This example uses typical values for illustration. Adjust the inputs above to match a specific situation and see how the result changes.
Sources & Methodology
Methodology
Annual capture = roof × rainfall × 80% efficiency. Capped at tank × 12. Savings = litres × price.
References
Frequently Asked Questions
What can rainwater be used for?
Maintenance requirements?
Better in commercial use?
Building regulations?
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