Choose the reporting boundary

Decide whether savings mean less utility water, less total water used or lower expenditure. Rainwater substitution can reduce utility demand without reducing total garden use. More efficient irrigation may reduce both. Keep these measures separate so one benefit is not counted twice. Describe the baseline and the proposed change before presenting a percentage.

Compare equivalent periods

Use the same duration, household occupancy and relevant operating conditions. For gardens, note weather and planted area. A lower summer bill after a wet season does not demonstrate that a new device caused the entire reduction. Metered comparisons establish what changed in volume; attributing the cause may require separate irrigation records and weather-adjusted analysis.

THE EXAMPLE AT A GLANCECompare like-for-like annual use
Baseline mains use120 m³/year
Comparable changed use102 m³/year

The observed difference is 18 m³, or 15%; do not add already-included substitution twice.

Same duration and relevant household/garden conditions.

A volume and percentage example

Suppose comparable metered annual utility use falls from 120 m³ to 102 m³. The observed difference is 18 m³ or 18,000 litres. Percentage reduction is 18 ÷ 120 × 100 = 15 percent. If a separate rainwater meter records 12 m³ of substitution, do not simply add it to the 18 m³ reduction: it may already account for part of that observed change.

Cap a rainwater scenario sensibly

Estimated roof collection is not the same as useful substitution. A preliminary upper bound is the smaller of eligible demand and available usable supply, but storage timing can reduce the actual amount further. A garden needing 15 m³ cannot save 30 m³ of utility water through rainwater alone when its original utility use was only 15 m³.

Translate volume into cost carefully

Apply the tariff that actually changes with consumption. Fixed charges may remain, and wastewater charges may follow separate local rules. Where price blocks apply, calculate bills for both scenarios rather than multiplying savings by one average bill rate. State currency, tariff period and operating costs when reporting financial savings alongside litres.

Savings checklist

  • State the boundary.
  • Choose a defensible baseline.
  • Compare equal periods.
  • Document weather and occupancy.
  • Avoid counting substitution twice.
  • Cap savings by displaced use.
  • Apply the actual tariff structure.
  • Distinguish measured results from estimated scenarios.
  • Keep volume and money in separate result fields.