How much rainwater storage can reduce your water bill?
Rainwater collected from a roof can replace some mains water used for gardens and outdoor cleaning.
A larger tank does not automatically produce proportionally larger savings: rainfall and demand occur in different months, and unused water must either carry forward or overflow.
This calculator compares two effective tank capacities against the same roof area, monthly rainfall and non-potable demand.
It helps you identify how much extra water a second proposal may supply and whether its additional installation and maintenance costs are justified by the entered assumptions.
Decisions supported by the comparison
- Estimate annual mains-water replacement and the share of entered demand covered.
- Locate overflow and shortage months before choosing storage capacity.
- Compare quotes with the same scope, then send the monthly water-balance CSV to an installer for daily design verification.
The physical model is location-independent, but this interface uses m², mm, m³ and Korean won (KRW) in both languages.
It does not convert currencies or import foreign tariffs, rainfall records or subsidy rules.
Prepare the inputs and their sources
Connected roof area and efficiency
Use the horizontal projected area draining to the tank, in m².
Total building floor area and sloping roof material area are different quantities.
Combine runoff, first-flush diversion and filtration losses in one collection factor; do not deduct the same losses twice.
The initial 100 m² and 80% are editable assumptions, not a measured roof or guaranteed collection performance.
Monthly rain and actual demand
Enter January through December rainfall in mm for a consistent weather station and observation period.
Dividing annual rainfall equally across 12 months removes the seasonal pattern that storage is intended to address.
Enter garden and cleaning or other non-potable demand separately in m³ per month.
Count only water that would otherwise come from mains supply; extra consumption created by the project and water already supplied by another source are not mains-water savings.
Capacity and complete installed quotes
One m³ equals 1,000 L.
Use effective usable storage, which can differ from the external or advertised tank volume.
Align tank, foundations, delivery, installation, pump, pipework, electrical work and tax scope between quotes.
Enter confirmed support only; support greater than its quote blocks the calculation.
Pending applications are not confirmed reductions in your investment.
Avoided water rate and maintenance
Enter the mains-water volumetric charge actually avoided, in KRW/m³.
Fixed charges may remain payable, and sewer charges are not automatically deducted by this tool.
If the reduction crosses tariff tiers, separately compare the actual before-and-after bills.
Include recurring power, cleaning and filter costs in annual maintenance.
Initial zero prices mean unconfirmed data, so financial results remain withheld until the confirmation box is selected.
The monthly water-balance formulas
One mm of rainfall over one m² represents one litre.
Let A be connected roof area, P monthly rainfall, and c the efficiency percentage divided by 100.
Both tanks start January empty, with opening storage S = 0.
Inflow Q = A × P × c ÷ 1,000 (m³)
Demand D = garden demand + cleaning / other demand
Mains-water replacement U = min(D, S + Q)
Closing storage E = min(tank capacity C, S + Q − U)
Overflow O = max(0, S + Q − U − C)
Shortage M = D − U · Next month starts with E
Zero capacity means no installation: use and closing storage are zero and inflow is treated as overflow.
Each month must conserve water: opening storage plus inflow equals use plus closing storage plus overflow.
Annual inflow must likewise equal annual use plus overflow plus December closing storage.
Demand coverage is annual use divided by annual demand; a zero-demand case has no coverage percentage.
An optimistic monthly bound, with important timing limits
The model assumes rainfall and demand can be aligned within a month and caps the water remaining at month-end.
It allows repeated replenishment, so monthly use can exceed tank capacity and even a very small positive tank may appear unrealistically effective.
Rain arriving late in a month cannot actually supply demand that occurred earlier, even if monthly totals balance.
Results therefore estimate an optimistic supply bound, not daily supply reliability or a certified optimal tank size.
Step-by-step use
- Measure connected roof area, select a loss-inclusive efficiency and record the rainfall station and observation period.
- Enter all 12 rainfall and non-potable demand totals.
Set unused-month demand to zero, but do not turn a missing rainfall observation into a dry month.
- Enter effective capacity, complete quote, confirmed support and annual maintenance for tanks A and B.
- Enter the avoidable water rate and analysis period, then confirm the prices when checked.
- Read annual replacement, shortage months and the B−A differences in investment, annual net savings and horizon net benefit together.
- Record the source verification date and download both water balances.
Ask the installer to check daily supply, pump and pipe capacity, structural conditions and maintenance requirements.
Worked example: more water saved can still mean slower payback
The fictional example uses 100 m² of roof, 80% efficiency, 950 mm of annual rainfall, 48 m³ of garden demand and 12 m³ of cleaning demand.
Monthly rainfall is 0, 0, 20, 50, 100, 150, 300, 200, 100, 30, 0, 0 mm.
Garden demand is 0, 0, 2, 4, 6, 8, 10, 10, 6, 2, 0, 0 m³, and cleaning demand is 1 m³ every month.
The avoided water rate is 1,000 KRW/m³, confirmed support is zero, and the comparison period is 10 years.
These are synthetic teaching inputs, not a representative climate or market quote.
Annual water balance and economics for fictional 1 m³ and 3 m³ tanks| Measure | Tank A · 1 m³ | Tank B · 3 m³ |
|---|
| Installed quote | 500,000 KRW | 800,000 KRW |
| Annual maintenance | 10,000 KRW | 15,000 KRW |
| Annual inflow | 76 m³ | 76 m³ |
| Annual replacement | 54 m³ | 55.6 m³ |
| Annual overflow | 22 m³ | 20 m³ |
| Demand coverage | 90% | 92.67% |
| Shortage months | 6 | 4 |
| December closing storage | 0 m³ | 0.4 m³ |
| Annual net savings | 44,000 KRW | 40,600 KRW |
| Simple payback | 11.36 years | 19.70 years |
| 10-year net benefit | −60,000 KRW | −394,000 KRW |
Tank B replaces another 1.6 m³ annually, but its extra 5,000 KRW in maintenance makes annual net savings 3,400 KRW lower.
Its initial investment is also 300,000 KRW higher, giving a B−A difference of −334,000 KRW in ten-year net benefit.
Neither option recovers its investment within the ten-year horizon.
The 0.4 m³ remaining in tank B at December-end is unused inventory and is not counted as water already saved.
Interpret savings, payback and horizon benefit
Annual savings and simple payback
Gross annual savings equal annual replacement multiplied by the avoidable volumetric water rate.
Net savings subtract annual maintenance.
Only positive net savings support a simple payback calculation: divide the installed quote minus confirmed support by annual net savings.
Zero net investment with positive savings gives zero-year payback.
Non-positive savings have no payback, while a positive payback longer than the selected horizon is explicitly marked beyond the analysis period.
Net benefit over the selected period
Multiply annual net savings by analysis years and subtract the initial net investment.
This repeats the empty-start first-year result, holding rainfall, demand, prices and maintenance constant.
It does not value carryover into the following year, interest, discounting, inflation, equipment replacement or residual value.
A pump replacement or rising maintenance costs can therefore delay actual payback beyond this simple estimate.
Practical scenarios and tank-selection tips
Seasonal garden irrigation
Heavy summer rain may not coincide with the days plants need irrigation.
Check how much water carries into the following month and whether it reduces later shortages.
Garden demand should represent supplemental irrigation after direct rainfall, so rain falling on the garden does not generate an overstated water-saving claim.
Weekend homes and outdoor cleaning
Occasional visits should not use the same monthly demand as an occupied home.
Consider stored-water condition, access for cleaning, winter operation and drainage.
A small increase in annual water replacement may be outweighed by installation space and maintenance burden, which can justify revising the quote scope.
Increasing tank capacity cannot create inflow in a dry month.
High overflow alone also does not establish a benefit from more storage if there is no later demand.
Compare an observed dry year and a typical-year dataset separately to see how sensitive the proposal is to rainfall assumptions.
Korean installation rules require a separate applicability check
Entering a roof area does not determine a statutory installation duty or legal minimum tank size.
Article 4 of the Enforcement Rule of the Act on Promotion and Support of Water Reuse includes a roof-catchment-area × 0.05 m minimum storage provision for applicable facilities, with a different basis for golf courses.
That provision is not a universal economic sizing formula for every domestic rain barrel.
Facility type, size and construction conditions under Article 10 of the Enforcement Decree, together with local requirements, need separate review.
Non-potable identification and maintenance
For applicable facilities, Article 4 also addresses identification to prevent drinking-water use, inspection and cleaning at least twice a year, and retention of usage, inspection and cleaning records for three years.
Tank volume does not establish potable-water safety or treatment quality.
Confirm first-flush treatment, pipe identification, cleaning access and the actual operating plan with the installer.
Frequently asked questions
Does enough annual rainfall replace all mains water?
No.
Seasonal shortages, overflow and limited storage still matter, and monthly totals do not reproduce daily rainfall timing.
Is monthly use above tank capacity a mistake?
The optimistic model permits repeated replenishment within a month.
Check small-tank performance with daily rainfall, demand and pump flow before deciding capacity.
Does the calculator estimate a rainwater subsidy?
Enter only a confirmed support amount.
Historical Seoul subsidy percentages and installation rates are not applied as current policy.
Are both water and sewer charges saved?
Only the entered mains-water volumetric rate is used.
Verify fixed charges, sewer charges and any tariff-tier changes separately.
Will a larger tank pay back sooner?
Not necessarily.
If additional investment and maintenance exceed the value of extra water, payback becomes slower.
Read the B−A differences and horizon net benefit together.
What do zero prices and initial rainfall zeros mean?
Financial results are withheld before price confirmation.
Initial rainfall and demand zeros are starting placeholders, not a missing-observation model; replace them with complete source data.
Is the CSV sufficient to approve a design?
The CSV records inputs, assumptions and monthly balances.
It does not confirm daily supply, structural or pipe design, water quality, installation duties or local compliance.
Sources and update basis
Technical and legal references were checked on September 10, 2026.
The Korean National Law Information OPEN API confirmed Enforcement Rule MST 279837, Article 4, effective November 11, 2025, and Enforcement Decree MST 286137, Article 10, effective June 3, 2026.
This reference check does not certify the rainfall, water rates, quotes or support amounts entered by a user; record their own sources and dates.