The establishment period is the missing term
Every conversion brochure runs the same sum: area times a water saving per square foot times a price, divided into the net cost. It gives a clean number and it is optimistic, because it assumes the new planting arrives at its settled water use on the day it is planted.
It does not. A new planting has a small root system in a limited volume of soil, and until the roots reach out it has to be watered often and generously — frequently more often than the lawn was, sometimes for two full seasons. Whether that comes out above or below the old lawn use depends on how thirsty the lawn was, and the calculator prints the break-even point directly: if the settled planting takes 14 inches a year against a lawn that took 40, the establishment use has to exceed 286 percent of settled before the early years actually cost more than the grass did.
Below that threshold the early years still save, just less. Either way the payback shifts right, and a budget built without the term will be short by the difference for the first two years.
Which half of the saving is cash
The calculation adds two things that behave differently. Water is cash: the volume drops and the bill drops, subject to the tier structure. Mowing time is a valuation, and whether it is cash depends entirely on who was doing it.
| Who mowed | Hours saved a year | Cash effect |
|---|---|---|
| A paid service | 12 | Real, and it stops appearing on a statement |
| You, on a Saturday | 12 | None — the Saturday changes, the bank balance does not |
| You, with fuel and blades | 12 | Fuel, sharpening and machine wear only |
The field is there because both are legitimate ways to look at it, but it is worth being clear which one you are entering. A payback of 5.8 years that becomes 20.2 when the labour valuation is removed is really two different answers to two different questions.
The rebate is somebody else's decision
Rebate rates, caps, eligible area, what has to be planted, what has to be removed, whether a pre-inspection is required and whether the programme is still funded are all set by the paying agency, and they change. Nothing on this page states any of them, and a rate typed into the form is a placeholder until it has been confirmed in writing by the body that would pay it.
The one procedural point worth flagging is sequencing. Most programmes require the application, and often a site inspection, before the existing lawn is removed. A conversion done first and applied for afterwards is a common and expensive mistake, and no amount of arithmetic recovers it.
What moves the answer most
Run the sensitivity rather than trusting the point estimate. In order of leverage on a typical residential conversion:
The water price comes first, because it multiplies the whole saving and because it is the input most likely to change over a payback period measured in years. Prices have generally moved upward and sometimes sharply, so a payback computed at today's rate is a conservative one. The gap between lawn and settled use comes second, and it is a real measurement you can take from meter readings either side of a season rather than an assumption. The rebate comes third: it is large, but it is a one-off that shifts the starting point rather than the slope. The labour valuation comes fourth and is the least defensible, which is why it is worth looking at the answer with and without it.
Questions people ask
What should I enter for what the lawn was using?
The applied depth over a year, and the best source is your own meter rather than a table. Read the meter before and after a single irrigation run to get gallons per run, multiply by the runs in a season, and divide back to inches over the lawn area — one inch over 1,000 square feet is 623.4 gallons. Failing that, a water budget built from your own ET and a lawn plant factor gets you close, and the hydrozone water budget calculator does that arithmetic. A figure taken from a web page for somewhere else is the one input guaranteed to be wrong.
Why does the payback land in the middle of a year?
Because the calculator interpolates. The cumulative saving crosses the net cost at some point inside a year, and reporting the whole year would overstate the wait by up to twelve months on a short payback. The year-by-year rows show the cumulative figure at the end of each year in whole cents, and the fractional answer is the crossing point between two of them. Both are shown so you can see which year the crossing happens in as well as where inside it.
Should I include the cost of the drip retrofit?
Yes, in the lump-sum field, and it is often a bigger number than people expect because it is labour rather than parts. Whether it belongs there at all depends on the counterfactual: if the sprinkler system was going to need work anyway, only the difference is attributable to the conversion. The same logic applies to anything that was already due — edging that was failing, a section of failed lawn that was going to be relaid. Charging pre-existing maintenance to the conversion makes it look worse than it is.
Does a rebate cap change whether the conversion is worth doing?
It changes the marginal square foot. Below the cap, every additional square foot converted brings its rebate rate with it; above the cap, additional area costs full price and brings only its water saving. That usually means the area right at the cap is the most efficient conversion and anything beyond it has to justify itself on water alone. The calculator shows the capped amount separately for exactly this reason, so you can see how much of the calculated rebate is not being paid.
How reliable is the number if the water price changes?
The saving is directly proportional to the price, so a 30 percent increase cuts the water-driven part of the payback by about 23 percent. Since water prices have generally risen rather than fallen, a payback computed at today's price is on the pessimistic side, which is the right direction for a decision to err in. The way to use that is to run the calculation twice, once at the current price and once at a plausible future one, and see whether the decision is the same in both. If it is, the price uncertainty does not matter.