Worked comparison: what does another dollar of storage buy?
One design idea taken through all five sites. The same thousand dollars of rainwater storage does five different things, and at three sites it does nothing at all.
Starting from the excellent reference design, what does another dollar of rainwater storage actually buy at each site?
It is worth working through because the intuition is so strong and so often wrong. A site that imports water is short of water; a tank holds water; therefore a bigger tank fixes it. That reasoning is correct at exactly one of these five sites.
Method
New-storage spend is stepped from $2,000 to $10,000 at $130/kL for the shell. Each step re-runs the monthly tank balance with a three-year spin-up and re-prices the whole bill of materials from the course schedule, including the required 10 % contingency and the per-tank establishment charge, which steps up every 25 kL.
Everything else in the excellent reference design is held fixed. The only variable is the money spent on tank shells, and the only outputs that move are the ones a larger tank can move.
The result
| Storage spend | Canberra | Alice Springs | Brisbane | Adelaide | Darwin |
|---|---|---|---|---|---|
| $2,000 | 92 %20 kL | 96 %37 kL | 88 %18 kL | 70 %20 kL | 82 %60 kL |
| $3,000 | 92 %28 kL | 96 %45 kL | 91 %26 kL | 70 %28 kL | 82 %68 kL |
| $4,000reference | 92 %36 kL | 96 %53 kL | 95 %34 kL | 70 %36 kL | 83 %76 kL |
| $5,000 | 92 %43 kL | 96 %60 kL | 96 %41 kL | 70 %43 kL | 83 %83 kL |
| $6,000 | 92 %51 kL | 96 %68 kL | 96 %49 kL | 70 %51 kL | 84 %91 kL |
| $7,000 | 92 %59 kL · over budget | 96 %76 kL · over budget | 96 %57 kL | 70 %59 kL | 84 %99 kL · over budget |
| $8,000 | 92 %67 kL · over budget | 96 %84 kL · over budget | 96 %65 kL | 70 %67 kL · over budget | 86 %107 kL · over budget |
| $9,000 | 92 %74 kL · over budget | 96 %91 kL · over budget | 96 %72 kL · over budget | 70 %74 kL · over budget | 89 %114 kL · over budget |
| $10,000 | 92 %82 kL · over budget | 96 %99 kL · over budget | 96 %80 kL · over budget | 70 %82 kL · over budget | 89 %122 kL · over budget |
Five sites, five different answers
Brisbane: storage is the right purchase, and it has a knee
The townhouse is the one site where the intuition holds. Going from $2,000 to $5,000 lifts water closure from 88 % to 96 %, cuts imports from 27 kL to 10 kL, and eliminates overflow entirely. Brisbane's roof out-collects its household over the year, so its shortfall really is a timing problem, and timing problems are what tanks solve.
Then it stops. Above $5,000 the curve is flat: there is no more surplus left to catch. Spending past that point buys kilolitres that are never full.
Canberra, Alice Springs and Adelaide: storage buys nothing
At three sites the water closure column does not move at all across the entire range — $2,000 to $10,000, and at Canberra that is 20 kL against 82 kL of tank, for an identical 92 %.
The reason is that these sites are supply-limited, not storage-limited. Canberra's roof yields about 142 kL a year and the household alone wants more than that once the garden is ignored entirely. Adelaide collects 92 kL against a 219 kL domestic demand. Alice Springs collects 73 kL and its bore is capped at 100 kL. A tank cannot hold water that never fell on the roof, and every one of these sites empties its tank every year no matter how large it is.
What would work at these three is different in each case: more catchment area, lower domestic demand, or — at Adelaide, where rain and demand are out of phase by six months — accepting that some imports are structural and designing the garden around them.
Darwin: storage keeps paying, and the budget stops you
Darwin is the only site where the curve is still climbing at $10,000: water closure rises from 82 % to 89 %, and garden water satisfaction from 27 % to 39 %. With 411 kL landing on the roof there is always more to catch.
But look at the overflow column: even at 122 kL of tank, Darwin still throws away 180 kL and still imports 36 kL. And the budget runs out first — $6,000 is the most Darwin can afford before the total passes $30,000.
Darwin's problem is not solvable by storage at this budget, and a design that spends its whole allowance trying is worse than one that spends a third of it and puts the rest into demand reduction.
| Site | Largest affordable spend | Extra cost from the reference | Water closure gained | Garden gained | Verdict |
|---|---|---|---|---|---|
| Canberra | $6,000 | $2,200 | none | none | Do not buy it; spend elsewhere |
| Alice Springs | $6,000 | $2,200 | none | none | Do not buy it; spend elsewhere |
| Brisbane | $8,000 | $5,500 | +1 pp | +2 pp | Buy it, up to the knee |
| Adelaide | $7,000 | $4,400 | none | none | Do not buy it; spend elsewhere |
| Darwin | $6,000 | $2,200 | +1 pp | +3 pp | Buy some; the budget binds before the benefit does |
Why this comparison is on the site at all
Because the course got it wrong. The reference designs used to spend $9,000 on storage at every site, which is past the knee at four of the five and unaffordable at all of them once establishment, first flush, treatment, irrigation and contingency were priced. Costed against the full schedule, those designs were $3,704 to $9,545 over the allowance.
The fix was to spend where the curve is still rising and stop where it is not, which is the same judgement this page is asking you to make. The bands on the method page were recomputed afterwards.
What to take from it
- Find your own curve before you spend. Step your tank size and plot the indicator against capacity. The shape tells you whether your site is storage-limited or supply-limited, and they need opposite responses.
- A flat curve is information, not a failure. Three of these sites say plainly that the money belongs somewhere else.
- Watch both water indicators. At Darwin, storage moves garden water satisfaction more than it moves water closure. A site can be getting meaningfully better at something the headline number barely registers.