Calculator inputs
Example data table
| Scenario | Day / Night | Hours | Heating savings | Cooling savings | Estimated annual savings |
|---|---|---|---|---|---|
| Typical heating season | 70 / 62 °F | 8 | 8.00% | 0.00% | $ 72.00 |
| Cooling-focused schedule | 74 / 78 °F | 7 | 0.00% | 1.75% | $ 9.10 |
| Balanced year | 71 / 64 °F | 8 | 6.22% | 0.00% | $ 49.78 |
Examples use default factors and the same formulas as the calculator. Replace with your own bills and schedule for accuracy.
Formula used
The calculator estimates seasonal HVAC savings using a normalized “degree-change” approach. It assumes each sustained setpoint change yields a small percent reduction in seasonal energy.
- Degree change (°F): heating Δ = max(0, day − night); cooling Δ = max(0, night − day).
- Duration scaling: duration ratio = (hours per night) / 8.
- Season scaling: season ratio = min(1, nights used / season days).
- Savings %: savings% = Δ × duration ratio × season ratio × factor.
- Cost savings: annual cost saved = baseline annual cost × (savings% / 100).
- Net savings: net annual savings = total cost saved − annual maintenance.
Results are estimates. Real-world savings vary with insulation, equipment efficiency, humidity, occupancy, and how quickly your system recovers each morning.
How to use this calculator
- Choose Heating, Cooling, or Both, then pick °F or °C.
- Enter your day and night thermostat setpoints and setback hours.
- Fill in baseline annual costs for heating and/or cooling from past bills.
- Optionally enter energy use and rates to derive baseline costs automatically.
- Add an upgrade cost to estimate simple payback and 5-year ROI.
- Press Calculate savings, then export with CSV or PDF.
Typical setback ranges and comfort
Practical night setbacks are often 3–8°F for heating and 2–5°F higher for cooling, limited by comfort and morning recovery. With the default heating factor of 1% per °F for an 8‑hour night, a 6°F setback used every night in a 180‑day heating season estimates about 6% seasonal savings. For cooling, the default factor is 0.5% per °F, so a 4°F higher night setpoint suggests roughly 2% savings across a full season.
Degree change and hours matter
Savings scale with temperature difference and setback duration. The calculator applies a ratio of setback hours divided by 8. A 4°F heating setback for 6 hours gives 4 × (6/8) ≈ 3% before season scaling. Extending to 9 hours raises the same case to about 4.5%. Cooling behaves similarly: 3°F for 7 hours yields 3 × (7/8) × 0.5 ≈ 1.3% when applied nightly.
Season coverage and consistency
If you apply setbacks on only some nights, the model multiplies by nights used divided by total season days. Using a 6°F setback for 8 hours on 120 of 180 heating days yields 6% × (120/180) = 4% estimated savings. This approach rewards consistent schedules and avoids overstating occasional manual changes.
Rates and baseline bill inputs
Dollar savings come from baseline heating and cooling costs (or energy use times rate). With a $900 annual heating baseline, a 6% estimate equals about $54 per year. If you enter 600 kWh and $0.12/kWh, the tool derives $72 and applies the same percentage. Bill totals reduce sensitivity to unit conversions, while energy-and-rate inputs help validate assumptions. Many households target 5–12% total HVAC savings, which can translate to one to three months of typical annual bills.
Payback and five-year planning
Automation costs are compared to net savings after any maintenance. A $150 thermostat with $60 net annual savings implies a 2.5‑year simple payback. Over five years, that’s roughly $150 net benefit and about 100% ROI. The built‑in cap (default 25%) prevents extreme setpoints from inflating business cases.
FAQs
Does this work for heat pumps or furnaces?
Yes. Enter your heating and cooling baselines from bills, or supply energy use and rates. The percentage method is technology‑agnostic, but real savings vary with equipment type, climate, and how quickly your system recovers each morning.
Why is there a savings cap?
The cap limits the savings percentage to keep extreme setpoints from producing unrealistic results. It’s a guardrail for modeling, especially when large temperature differences or long hours would otherwise compound into overly optimistic savings.
Should I enter costs, or energy use and rates?
Use costs if you know your annual spend. Use energy plus rate if you want the calculator to derive cost from kWh, therms, or other units. If both are entered, the derived cost is used for that season.
What if I only follow setbacks on weekends?
Set nights used to the number of nights you actually follow the setback schedule. The calculator scales savings by nights used divided by season days, so partial or weekend-only behavior produces proportionally smaller estimates.
How do I choose the heating and cooling factors?
Start with defaults, then adjust using local utility studies, building audits, or your own before/after bills. If your home is leaky or recovery is slow, reduce the factor. If bills are stable and setbacks are consistent, keep it near default.
Do bigger setbacks always save more?
Not always. Larger setbacks can increase morning recovery load or reduce comfort. The calculator estimates savings from reduced overnight demand, but your best setting balances comfort, humidity control, and how your system performs during recovery.