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Greywater Reuse Water Savings Estimator

This estimator calculates how much potable or purchased water could be displaced by reusing greywater over a selected period. It compares recoverable greywater supply with eligible non-potable demand and uses the smaller value as the achievable reuse volume before storage and timing losses. The tool is useful for conservation studies, project screening, and utility-cost planning when you already have estimates of source water and end-use demand. It also reports how much eligible demand remains after reuse and the percentage of demand covered. Because it treats the period as a single water balance, it does not simulate hour-by-hour production, storage limits, treatment downtime, or seasonal changes in irrigation demand.

Inputs

L/year
L/year
%
Result
—
Estimated water savings
Available reusable supply—
Eligible demand coverage—
Remaining eligible demand—

1. Enter recoverable supply
Use the greywater volume that could reach the reuse system over the selected period.

2. Enter eligible demand
Include only end uses permitted and connected to receive treated greywater.

3. Set system availability
Account for planned or expected downtime with a percentage between 0 and 100.

4. Keep the period aligned
Supply and demand should both be monthly, annual, or another matching period.

5. Review savings and coverage
Use the remaining demand to estimate how much conventional supply is still needed.

Available reusable supply = Recoverable supply × System availability Water savings = min(Available reusable supply, Eligible demand) Demand coverage (%) = Water savings ÷ Eligible demand × 100

Availability is entered as a percentage and converted to a decimal. The model assumes recovered greywater is used whenever eligible demand exists.

What the result means

Estimated water savings is the volume of eligible demand that can be served by available greywater during the selected period.

Actual savings can be lower when supply and demand occur at different times or when storage, water-quality controls, or operating constraints limit reuse.

Given
Recoverable supply = 240,000 L/year; eligible demand = 210,000 L/year; availability = 95%.

Calculation
Available supply = 240,000 × 0.95 = 228,000 L.
Water savings = min(228,000, 210,000) = 210,000 L.
Coverage = 210,000 ÷ 210,000 × 100 = 100%.

Result
210,000 L/year

The annual supply is sufficient to cover all entered eligible demand in this period-level balance, though short-term mismatches can still require backup water.

Can the calculator show more than 100% demand coverage?

No. Savings are capped at eligible demand, so coverage cannot exceed 100%. Surplus recoverable greywater is not counted as additional water savings unless another eligible use is added.

What should system availability include?

Use it to reflect periods when the reuse system is not operating because of maintenance, treatment outages, bypass, or other downtime relevant to your project.

Should potable uses be included in eligible demand?

Only include uses that the system is specifically designed and permitted to supply. The calculator does not determine whether a particular use is allowed.

Why might daily backup water still be needed at 100% annual coverage?

Annual supply can exceed annual demand while short-term supply falls below demand. A time-step analysis is needed to evaluate those mismatches.

How does this differ from recovery rate?

Water savings measures displaced conventional water relative to end-use demand. Recovery rate measures how much collected greywater remains usable after losses.