Calculator
Example data table
| Scenario | Heating cost | Cooling cost | Setback / Setup | Season days | Upfront | Estimated annual savings | Estimated payback |
|---|---|---|---|---|---|---|---|
| Typical household | ₨ 700 | ₨ 450 | 6°F / 4°F (8h/day) | 150 / 120 | ₨ 200 | ₨ 55–₨ 95 | 2.1–3.6 years |
| High HVAC spend | ₨ 1,200 | ₨ 900 | 7°F / 5°F (10h/day) | 170 / 140 | ₨ 260 | ₨ 140–₨ 240 | 1.1–1.9 years |
| Mild climate | ₨ 500 | ₨ 300 | 5°F / 3°F (6h/day) | 90 / 70 | ₨ 180 | ₨ 20–₨ 45 | 4.0–9.0 years |
Formula used
This calculator estimates savings by translating your temperature changes and daily schedule into a seasonal savings percentage, then annualizing it using season length and an adjustment factor.
- Seasonal savings (%) = degrees × (hours per day ÷ 8) × savings rate
- Annualized savings (%) = seasonal savings (%) × (season days ÷ 365)
- Realized savings = baseline cost × annualized savings (%) × behavior factor
- Annual savings total = heating savings + cooling savings + other savings
- Simple payback (years) = upfront cost ÷ annual savings total
- NPV sums discounted annual savings minus upfront cost
Guardrail: seasonal savings is capped at 25% before annualization to avoid unrealistic projections.
How to use this calculator
- Enter your typical annual heating and cooling costs from bills.
- Add the thermostat price and expected installation cost.
- Set your planned setback/setup degrees and hours per day.
- Estimate how many days per year you heat and cool.
- Adjust the behavior factor if schedules may not be followed.
- Choose escalation, discount rate, and analysis years for projections.
- Click “Calculate Savings” to see payback, NPV, and yearly cashflows.
- Download CSV or PDF after the results appear.
Savings drivers you can control
Programmable schedules reduce run time by avoiding unnecessary heating and cooling. Larger setbacks, longer away periods, and consistent compliance increase savings, while comfort limits reduce them. In the calculator, seasonal savings is capped to keep results realistic.
Model inputs and what they represent
Start with annual heating and cooling costs from bills, then enter planned setback and setup degrees, plus the hours each day those settings apply. The savings rate is a rule-of-thumb percent per degree for an eight-hour block; scaling by hours and season days converts it into an annualized rate. The behavior factor reduces savings when schedules are irregular. Optional “other savings” can reflect rebates or fewer service visits.
Interpreting payback and NPV
Simple payback compares upfront cost to first-year savings, helping you see how fast the device recovers its purchase and installation. Net present value discounts future savings using your chosen rate, then subtracts upfront cost. A positive NPV suggests the upgrade is financially attractive under your assumptions. Break-even year uses cumulative, undiscounted savings.
Scenario benchmarks for planning
If HVAC spend is ₨1,150 per year and annual savings is ₨80, payback is about 2.5 years. With ₨2,100 spend and ₨180 savings, payback falls near 1.4 years. In mild climates, smaller season days can push savings below ₨40 annually, extending payback beyond four years. Escalation increases later-year savings when energy prices rise. For example, 3% escalation turns ₨100 in year one into about ₨130 by year ten.
Turning results into action
Use the year-by-year table to confirm cumulative savings crosses zero within your target horizon. If payback is slow, test larger setbacks, longer hours, or improved adherence, but keep comfort and humidity in mind. Compare several thermostat and installation costs, then export CSV or PDF to document the option you plan to implement. For cooling-heavy homes, focus on setup hours during afternoons and weekends; for heating-heavy homes, target overnight and workday setbacks. Recheck results after a month using bills to validate your assumptions.
FAQs
How does the behavior factor change my results?
Behavior factor scales savings to reflect real-world follow-through. If schedules are frequently overridden, set it lower. If your household consistently follows setbacks and setups, set it closer to 100%.
Why is there a seasonal savings cap?
Very large temperature changes can produce unrealistic percentages in simple models. The cap limits seasonal savings before annualizing, keeping estimates conservative and avoiding “too good to be true” paybacks.
Can I model a smart thermostat with learning features?
Yes. Enter the schedule and degrees you expect after learning stabilizes, then raise the behavior factor if the device reliably maintains those patterns. Compare scenarios with different hours to see sensitivity.
What if I only know monthly costs?
Add the last 12 months of heating and cooling spend to estimate annual values. If you have only a few bills, average them and annualize carefully, noting that unusual weather can distort short samples.
What discount rate should I use?
Use a rate that matches your opportunity cost or financing. Many households test 4–10% to see a range. Higher rates reduce the value of future savings and lower NPV.
Does the calculator include maintenance savings or rebates?
You can include them in “other annual savings.” Enter expected rebates spread across a year or estimated avoided service costs. Keep these conservative and document assumptions in the export.