Interactive Calculator · 2026
STP Payback Period Calculator
See how long a sewage treatment plant takes to pay for itself through water-reuse savings. Enter the capex and your monthly numbers, and get the payback period — the figure that decides whether recycling water is worth it.
Cumulative net savings cross the capex line at break-even
Your STP numbers
One-time installed cost of the plant — civil, equipment and commissioning.
Freshwater and tanker bills avoided by reusing treated water.
Power, chemicals, sludge disposal, labour and maintenance.
Savings over time
Cumulative net savings against the ₹25,00,000 capex you need to recover.
| Year | Net saving (yr) | Cumulative net | Vs capex |
|---|---|---|---|
| Year 1 | ₹2,04,000 | ₹2,04,000 | In deficit |
| Year 2 | ₹2,04,000 | ₹4,08,000 | In deficit |
| Year 3 | ₹2,04,000 | ₹6,12,000 | In deficit |
| Year 4 | ₹2,04,000 | ₹8,16,000 | In deficit |
| Year 5 | ₹2,04,000 | ₹10,20,000 | In deficit |
Cumulative net savings vs capex
Break-even is where the green savings line meets the capex line — around month 147.
How this is calculated
- Net monthly benefit = monthly saving − operating cost = ₹75,000 − ₹58,000 = ₹17,000.
- Payback (months) = capex ÷ net monthly benefit = ₹25,00,000 ÷ ₹17,000 = 147 months.
- Payback (years) = payback months ÷ 12 = 12.3 years.
- Net annual benefit = net monthly × 12 = ₹2,04,000.
A simple (non-discounted) payback for concept planning. It ignores inflation, tariff hikes, financing and equipment replacement — confirm the full lifecycle economics with a qualified consultant before investing.
Related Guides — Deep-dive reading
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