Ecology & Environment
Rainwater Harvesting Impact Calculator
Calculate annual rainwater collection, demand offset, avoided water charges, cumulative savings, and net operational-versus-embodied carbon impact.
Annual harvest potential-
Annual demand offset-
Annual offset in cubic metres-
Entered demand offset share-
First-year avoided water charge-
Avoided charges through horizon-
Annual operational carbon difference-
Operational carbon saving less embodied carbon-
Decision view
Annual water allocation and carbon payback
Annual water allocation and carbon paybackUsable demand offset is separated from surplus collection while cumulative operational carbon is compared with embodied carbon.
Exact scenario comparisonAnnual rainfall (mm) changes while all other entered assumptions remain constant.
| Annual rainfall (mm) | Annual harvest potential | Annual demand offset | Annual offset in cubic metres | Entered demand offset share | First-year avoided water charge | Avoided charges through horizon | Annual operational carbon difference | Operational carbon saving less embodied carbon |
|---|
How to use Rainwater Harvesting Impact Calculator
- Enter effective roof area, annual rainfall, and runoff efficiency.
- Enter annual non-potable demand and water charge assumptions.
- Enter municipal, pumping, and embodied-carbon values.
- Compare the live annual water balance with cumulative money and carbon paths.
Calculator guide
Understanding Rainwater Harvesting Impact Calculator
Rainwater harvesting impact begins with a physical water balance: roof area and rainfall create a collection potential, runoff efficiency reduces it, and annual demand caps the usable offset. Financial savings and carbon effects must then be calculated from that usable volume rather than from rainfall that cannot be used.
Calculation method
How the calculation works
Model rainwater harvesting impact by capping collectible rainfall at entered demand, converting it to avoided charges, and separately reconciling operational-carbon assumptions with system embodied carbon. Demand caps the usable offset before financial and carbon assumptions are applied to collected water. Convert one millimetre of rain on one square metre to one litre, apply runoff efficiency, cap the result at annual non-potable demand, then value and carbon-account only the usable cubic metres.
Detailed calculation process
Turn roof rainfall into usable water, savings, and net carbon
The default case uses 180 m2 of effective roof, 850 mm annual rain, 82% collection efficiency, 120,000 L demand, and a 15-year horizon.
General formula: H = A R etaU = min(H,D)V = U/1000S_1 = V c_wS_n = S_1[(1+g)^n-1]/gC_a = V(c_m-c_p)C_net = nC_a-C_e Area times rainfall directly produces litres because 1 mm over 1 m2 equals 1 L. Efficiency reduces that physical potential, demand limits usable water, the growing-series formula totals avoided charges, and net carbon subtracts the one-time embodied amount from cumulative operational savings.
What each symbol means
Worked substitution with the default inputs
The defaults offset all 120,000 L of annual demand and avoid $6,640.67 over 15 years, while remaining 182 kg CO2e short of embodied-carbon payback.
Water and carbon decision view
Compare annual utilization with cumulative carbon payback
A water-allocation bar shows collection used versus surplus, while a horizon curve tracks cumulative operational carbon against embodied carbon.
Worked situations
Practical examples
- The roof can collect 125,460 L, but annual demand caps use at 120,000 L.
- The first-year avoided charge is $384.
- At the entered carbon intensities, 15-year net carbon remains -182 kg CO2e.
Better inputs
Useful tips
- Use effective connected roof area rather than total parcel area.
- Keep annual demand in litres and tariff in currency per cubic metre.
- Interpret financial and carbon payback separately.
Before relying on the result
Limitations and common mistakes
- This annual balance does not model rainfall timing, tank overflow, drought sequence, or treatment losses.
- Tariffs, pumping energy, and carbon factors are entered assumptions.
- Water quality, plumbing separation, maintenance, and local rules require project-specific review.
Reference
Key terms
- Runoff efficiency
- Entered share of roof rainfall reaching usable collection.
- Demand cap
- Annual use cannot exceed entered non-potable demand.
- Operational carbon saving
- Municipal intensity minus pumping intensity for usable water.
- Embodied carbon
- One-time entered system life-cycle carbon amount.
Important note
Calculated from the entered environmental values using the displayed model. Measurement quality, local conditions, system boundaries, and source data affect interpretation.
Frequently asked questions
Why is usable water lower than harvest potential?
Annual demand caps the amount treated as offset.
Why is net carbon negative?
Fifteen years of entered operational savings are still less than embodied carbon.
Does the page size a tank?
No. It is an annual impact model, not a continuous storage simulation.
What happens when rate growth is zero?
The growing total reduces to first-year saving multiplied by years.