Rainwater tank or borehole - when does it pay for itself?

The install cost against the municipal water it replaces, year by year, at published South African water tariffs.

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Example data - to be completed. These numbers belong to a sample household. Move the sliders to your own household →

Breaks even in year 10

A R 28 000 rainwater tank saves about R 1 792 in year 1 and is R 31 309 ahead after 15 years.

Breaks even
year 10
The year the savings have covered the cost
Payback
9.6 years
Counting part of the last year
Net gain at 15 years
R 31 309
Savings less the cost and interest
Saved in year 1
R 1 792
Water not bought, less running costs

What this means

  • The savings cover the R 28 000 rainwater tank in about 9.6 years.
  • Year 1 saves about R 1 792 (R 149 a month) against buying the water; price increases make every later year worth more.
  • After 15 years you are R 31 309 ahead, in rand as spent.
  • Figures are calculations, not advice - no product, driller or installer is recommended.

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Assumptions

  • A rainwater tank at R 28 000, paid in cash.
  • The roof (150 m2, 700 mm of rain a year, 50% caught) gives about 4.4 kL a month; the tank replaces 4.4 kL of the 5 kL wanted.
  • Municipal water at R 41.00 a kL in year 1, rising 9.3% a year.
  • Running costs R 30 a month, not increased.
  • Rand as spent (not adjusted for inflation); sanitation charged on water used, repairs and a new pump are left out.

Where the typical figures come from

  • Water prices: Johannesburg Water 2026/27, per kL a month - 0-6 kL free, 6-10 R28.91, 10-15 R29.84, 15-20 R35.65, 20-30 R64.53, 30-40 R69.45, 40-50 R86.81, over 50 R94.92 (12.5% up on 2025/26). A tank or borehole takes kL off the top of your bill, so each preset uses the band its saved kL fall in, plus 15% VAT: R33.25 (6-10), R41.00 (15-20), R74.21 (20-30), R89.85 (30-50, the average of two bands). City of Johannesburg, Water Tariffs 2026/27.
  • Cape Town 2026/27 (domestic, excl. VAT): R22.10 a kL to 6 kL, R30.37 to 10.5 kL, R47.48 to 35 kL, R95.63 above. Cape Town also charges sanitation on the water you use, so a saved kL is worth more there - add it to the price slider. City of Cape Town, 2026/27 water tariffs.
  • Price increases (Low 4.5%, Moderate 9.3%, High 12.5%): Cape Town's 2026/27 increase on its first two steps, on its third step, and Johannesburg's increase on every band. Same sources as above.
  • Borehole R70 000-R100 000: inside OurPower's 2026 Johannesburg range of R55 000-R130 000 for a complete domestic borehole (drilling, casing, pump, yield and quality test, electrics). OurPower borehole costs, 2026.
  • Rainwater tank R22 000-R48 000: a 2 400 L JoJo tank fully installed (plumbing, electrics, pump, labour) was R20 000 in 2024; a 5 000 L tank alone is about R5 000-R5 500. The Post / IOL, 2024.
  • Rainfall 700 mm a year: Johannesburg's average (South Africa as a whole: about 464 mm). Climate of South Africa.
  • Estimates, not sourced: each household's monthly use and the kL it would replace, the 5 000-15 000 L tank prices (scaled from the 2 400 L figure), where each borehole sits in its range, the running costs, and the 50% of roof rain a tank catches before it overflows. Use your own bill and your own quote.

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