Rainwater Harvesting ROI Calculator

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. Adjusting one input at a time toward extreme values shows which ones move the result most.

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.