Start with net annual benefit

Simple payback divides initial investment by annual net cash savings. Net means avoided consumption charges minus relevant recurring costs. Gross roof collection is not an annual cash saving. Estimate water that actually replaces billed use, check the applicable tariff and keep assumptions dated. The calculation is a planning scenario, not a guaranteed investment return.

Work through a positive example

An illustrative installation costs 2,400 currency units and replaces 60 m³ each year. At an avoidable charge of 4 per m³, gross annual bill savings are 240. If electricity and maintenance total 60 annually, net benefit is 180. Simple payback is 2,400 ÷ 180 = approximately 13.3 years under unchanged conditions.

THE EXAMPLE AT A GLANCEUseful substitution drives payback
Net saving at 60 m³ substitution180 currency units/year
Net saving at 40 m³ substitution100 currency units/year

A 2,400 investment takes 13.3 or 24 years in these simple, undiscounted scenarios.

Avoidable tariff 4/m³; annual operating costs 60; assumptions stay constant.

Handle nonpositive benefits honestly

If only 15 m³ are substituted at 3 per m³, gross savings are 45. Annual operating costs of 60 produce a net result of minus 15. There is no positive simple payback from those inputs. Do not divide by a negative result and display a negative year count as an attractive return. The system could serve other goals, but they need separate evaluation.

Test the influential assumptions

Compare lower useful yield, higher operating cost and major replacement cases. In the positive example, reducing substitution to 40 m³ lowers gross saving to 160 and net benefit to 100, extending payback to 24 years. The change does not require a new formula; it reveals how sensitive the result is to actual annual use rather than optimistic collection potential.

Understand what simple payback leaves out

The basic calculation ignores discounting, financing, future tariffs, inflation, replacement timing, residual value and changing performance. A pump replacement partway through the period can alter cumulative cash flow. If the estimated payback exceeds the relevant equipment life, investigate the full lifecycle rather than reporting the simple number without context.

Payback checklist

  • Use complete installed cost.
  • Calculate actual substituted volume.
  • Apply avoidable tariffs.
  • Subtract recurring costs.
  • Reject positive-payback claims when net benefit is nonpositive.
  • Test conservative cases.
  • Consider replacement and equipment life.
  • Show assumptions alongside the result.