Analysis · rainwater
Rainwater tank or borehole: which saves more in your metro?
A tank is the cheap alternative everyone mentions. We ran five years of daily rain and evaporation through a roof, a tank and a lawn in each metro to see how much municipal water it really replaces, and compared it with a borehole.
By Borehole Payback · updated 24 October 2026
Will a borehole pay for itself?
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Garden water from each metro's average 2021–2025 weather; 2026/27 water and sewage tariffs incl. VAT; pump electricity at R3.50 per kWh. Drilling at R1 000 per metre incl. casing.
What our numbers show
Our day-by-day simulation, ERA5 rain and evapotranspiration 2021–2025, 2026/27 tariffs
- With a 150 m² roof and a 5 000 L tank, rainwater covers 44 % of a 250 m² garden's water in Durban but only 17 % in Cape Town, 19 % in Johannesburg and 17 % in Pretoria.
- Cape Town's problem is timing: 54 % of its rain falls in June to August, when a lawn needs no watering, while December to February bring 52 % of the garden's yearly need. The tank overflows in winter and is empty by summer.
- A bigger tank helps less than you would think. Going from 2 000 L to 20 000 L, ten times the storage, only multiplies the yearly saving by 2.1 in Cape Town and 2.4 in Durban.
- In Durban a 5 000 L tank saves R2 936 a year, about three fifths of the R4 769 a 60 m borehole would save for the same garden. In Cape Town it saves R2 508 against R12 830: there, the borehole is the tool for the garden and the tank is a top-up.
| eThekwini (Durban) | 44 % of garden water | |
| Johannesburg | 19 % of garden water | |
| Cape Town | 17 % of garden water | |
| Tshwane (Pretoria) | 17 % of garden water |
150 m² roof, 80 % of roof rain collected, lawn need from daily evapotranspiration with 40 mm soil storage, average of 2021–2025.
What each tank size covers
Share of a 250 m² lawn's yearly water that comes from the tank, and the municipal water and sewage charges it avoids at 2026/27 tariffs, with 15 kl a month of indoor use on top:
| Metro | Garden need a year | 2 000 L tank | 5 000 L tank | 10 000 L tank | 20 000 L tank |
|---|---|---|---|---|---|
| eThekwini (Durban) | 81 kl | 29 % · R1 914 | 44 % · R2 936 | 57 % · R3 772 | 70 % · R4 606 |
| Johannesburg | 165 kl | 14 % · R1 856 | 19 % · R2 455 | 24 % · R2 955 | 30 % · R3 651 |
| Cape Town | 145 kl | 13 % · R1 918 | 17 % · R2 508 | 22 % · R3 092 | 29 % · R3 933 |
| Tshwane (Pretoria) | 181 kl | 12 % · R1 516 | 17 % · R2 103 | 21 % · R2 662 | 27 % · R3 328 |
Roof 150 m², 80 % of the rain on it reaches the tank. The lawn uses 0.7 × reference evapotranspiration a day, first from rain stored in the soil (up to 40 mm), then from the tank, then from the tap. Average of the years 2021–2025.
Tank against borehole
The same 250 m² garden, watered fully from a 60 m borehole, against what a 5 000 L tank saves. The borehole figure is after pump electricity and R1 000 a year of upkeep:
| Metro | 5 000 L tank: saved a year | 60 m borehole: net saved a year | Tank share of the borehole's saving |
|---|---|---|---|
| eThekwini (Durban) | R2 936 | R4 769 | 62 % |
| Johannesburg | R2 455 | R10 630 | 23 % |
| Cape Town | R2 508 | R12 830 | 20 % |
| Tshwane (Pretoria) | R2 103 | R11 163 | 19 % |
In Durban the summer rain arrives when the garden needs it, so a tank does most of a borehole's job, and Durban is also where a borehole pays back slowest. In Cape Town, Johannesburg and Pretoria a tank replaces a fifth of the garden's water at best; the rest still comes through the meter at the top-block price.
Why bigger tanks disappoint
A tank only earns money when it holds water on a day the garden is thirsty. Small tanks fill and empty with each summer storm and are used again and again. Every extra thousand litres is filled less often: in a dry spell a large tank empties once and then stands empty like a small one, and in the wet season it overflows like a small one. That is why ten times the storage only about doubles the saving.
Before you buy
- Divide the quote by the yearly saving. The table above gives the saving; your installer gives the price, including the stand, gutters, first-flush diverter and pump.
- Roof size matters as much as tank size. Only the part of the roof whose gutters feed the tank counts.
- Tank and borehole together. A borehole needs a storage tank anyway; connecting the gutters to it captures summer rain for free.
See how much water a garden needs in each metro and what a borehole costs.
This week
Metro of the week: eThekwini (Durban)
A household in eThekwini (Durban) using 30 kl of municipal water a month pays about R2 253 a month for water and sewage under the 2026/27 tariffs. A 60 m borehole watering a 250 m² garden would save about R4 769 a year after pump electricity and upkeep, and pay back in 19.8 years.