Ecology & Environment
Rainwater Harvesting Offset Calculator
Cap annual harvest at entered demand, value the offset at the water tariff, subtract maintenance, and compare horizon benefit with installed cost.
Annual harvest potential (L)-
Annual demand offset (L)-
Demand offset share-
First-year avoided water charge-
First-year saving after maintenance-
Simple payback at first-year net saving-
Gross avoided water charges through horizon-
Maintenance through horizon-
Net benefit after maintenance and system cost-
Decision view
Rainwater demand offset and payback
Rainwater demand offset and paybackAnnual harvest is capped at demand before avoided charges, maintenance, payback, and horizon value are calculated.
Exact scenario comparisonAnnual rainfall (mm) changes while all other entered assumptions remain constant.
| Annual rainfall (mm) | Annual harvest potential (L) | Annual demand offset (L) | Demand offset share | First-year avoided water charge | First-year saving after maintenance | Simple payback at first-year net saving | Gross avoided water charges through horizon | Maintenance through horizon | Net benefit after maintenance and system cost |
|---|
How to use Rainwater Harvesting Offset Calculator
- Use effective roof area and local rainfall.
- Enter only demand the system can legally and practically serve.
- Check tank timing before relying on annual totals.
Calculator guide
Understanding Rainwater Harvesting Offset Calculator
Rainwater value is limited by usable demand: collecting more annual water than can be used does not create additional avoided charges.
Calculation method
How the calculation works
Cap annual rainwater offset at entered demand, translate the offset into avoided water charges, and compare system cost with maintenance and a transparent rate-change scenario. Multiply roof area, rainfall, and collection efficiency for annual litres, cap at demand, convert to cubic metres, and project avoided charges with the entered rate change.
System review
Check annual economics against physical storage
A favorable annual balance still needs a month-by-month tank assessment.
Worked situations
Practical examples
- One millimetre on one square metre equals one litre.
- Offset cannot exceed entered demand.
- Simple payback uses first-year net savings only.
Better inputs
Useful tips
- Model dry years.
- Check tariff blocks and sewer charges.
- Budget pump, filter, and tank replacement.
Before relying on the result
Limitations and common mistakes
- Tank size, rainfall timing, overflow, drought, treatment, energy, rebates, financing, equipment replacement, and tariff complexity are excluded.
- Annual balance can overstate usable offset.
- Water quality rules remain external.
Reference
Key terms
- Harvest potential
- Annual roof rainfall multiplied by collection efficiency.
- Usable offset
- Lesser of harvest and entered demand.
- Avoided charge
- Offset volume multiplied by entered water rate.
- Simple payback
- Installed cost divided by first-year net saving.
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 cap harvest at demand?
Only water that replaces entered consumption produces the modeled saving.
Does payback include rate growth?
The simple payback does not; the horizon result does.
Is sewer savings included?
No.
Does the page size a tank?
No.