Rainwater harvesting — frequently asked questions
Straight answers on mandates, cost, and savings. For the exact formulas behind the estimate, see how we calculate.
- Is rainwater harvesting mandatory in Karnataka and Tamil Nadu?
- Yes. Both states have residential rainwater harvesting mandates. In Bengaluru, BWSSB requires RWH for existing sites of 60×40 ft and above and new buildings of 30×40 ft and above. Tamil Nadu has had a statewide RWH requirement for residential buildings for years, administered through CMWSSB in Chennai.
- What is the penalty for not installing rainwater harvesting?
- In Bengaluru, non-compliance attracts a penalty of 25% of your water bill for the first three months, rising to 50% of the bill after that — charged for as long as you remain non-compliant. Avoiding this recurring penalty is part of what an RWH system saves you, and our calculator factors it in.
- How much does a rainwater harvesting system cost?
- For a typical home, budget roughly ₹25,000 as a base plus about ₹200 per square metre of roof area, plus ₹10,000–15,000 for each recharge pit on a hybrid system. A small home setup often lands around ₹40,000–60,000; larger plots and societies cost more. Your vendor's site survey confirms the figure.
- How much water can my roof actually harvest?
- More than most people expect. One millimetre of rain on one square metre of roof yields one litre. After runoff losses and filtering, a 100 m² flat RCC roof in a city with 900 mm annual rainfall harvests on the order of 50,000–55,000 litres a year. Run the calculator for a figure based on your roof and city.
- What is the difference between storage and recharge?
- Storage captures rainwater in a tank for direct use — flushing, washing, gardening. Recharge directs rainwater into the ground through pits or wells to lift the local water table and revive borewells. A hybrid system does both, which is what we usually recommend.
- Does RainbowHome install rainwater harvesting systems?
- No. RainbowHome never installs. We help you estimate your harvest and savings, compare vetted RWH vendors, and line up financing if you need it. The installation is done by the independent vendor you choose.
- Can my apartment society install rainwater harvesting?
- Yes — and societies often see the strongest economics because the cost spreads across many flats while tanker savings are large. Our society calculator computes the harvest, recommended system size, total cost, and the per-flat cost share that helps win committee approval.
- How accurate is the water estimate?
- It follows the BIS IS 15797:2008 standard and uses city-level rainfall data. It is a planning model, not a quote — catchment routing, plumbing runs, and soil percolation rate move the real number. See the methodology page for every formula and assumption.
- Will rainwater harvesting help my borewell?
- A recharge-focused or hybrid system directs rainwater into the ground rather than letting it run off, which helps stabilise and slowly raise the local water table that your borewell draws from. It is one of the few things an individual household can do to push back on falling groundwater.
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