Calculate thermostat setback savings in percent and dollars per year from your setback degrees, hours, and energy cost, plus smart thermostat payback time.
Thermostat Setback Savings Formula
The default quick method uses this calculator’s linear planning heuristic: one percentage point of affected HVAC energy-cost savings per °F held for 8 hours daily. It is not an exact DOE formula. DOE guidance describes potential annual heating and cooling savings up to about 10% from a 7–10°F setback for 8 hours per day; that broad guidance does not establish a universal linear coefficient.
Apply the modeled percentage only to the variable heating or cooling energy cost affected by the plan, excluding fixed charges and unaffected energy. An optional installed thermostat cost gives a simple estimated payback. Blank omits payback, zero means no purchase cost to recover, and positive cost with zero modeled annual savings has no finite payback:
Payback Years = P / Annual Dollar Savings
The optional climate method is an idealized proportional sensible-load calculation, not a balance-point, weather or building simulation. Let G be the positive indoor-minus-outdoor gap for heating, or outdoor-minus-indoor gap for cooling, using the same difference unit as setback D:
Variables:
- S is a modeled percentage of the affected variable heating or cooling energy cost, not a guaranteed reduction of the entire utility bill (%)
- D is the setback depth, meaning how many degrees you turn the thermostat down in winter or up in summer (°F; Celsius entries are converted automatically)
- h is the hours per day the setback is held
- d is the days per week the setback is used
- C is the affected variable HVAC energy cost for the entered period (USD), excluding fixed charges and unaffected energy
- P is the installed price of a smart or programmable thermostat ($)
- Tin and Tout are the normal indoor and average outdoor temperatures. G = Tin − Tout for heating, or Tout − Tin for cooling. Temperature differences and setback depths are converted consistently.
Savings mode sums one or two disjoint periods and applies a chosen 20% modeling cap, while also showing the uncapped estimate. The cap is not an empirically established maximum. Both second-period fields for time and its depth are required when any is entered; all three blank omit that period. Collapsing optional settings does not deactivate entered values. Capacity checks reject more than 168 contributing hours per week and forced shared days above 24 hours, but start times are not entered, so actual overlap cannot be proved. Target mode inverts only the quick heuristic for a positive USD or percentage goal. The chosen model limits are depth up to 30°F and, for climate estimates, a correctly directed gap of at least 3°F; climate depth cannot cross the no-load point. These physical limits are converted consistently for Celsius.
Setback Savings by Depth, Duration, and Climate
The first table shows arithmetic from the quick linear heuristic for schedules used 7 days per week. Its percentages apply to the affected HVAC energy-cost component. They are illustrative model results, not measured savings or guarantees.
| Setback depth | 8 h/day (night or workday) | 12 h/day | 16 h/day (night + workday) |
|---|---|---|---|
| 2°F | 2% | 3% | 4% |
| 4°F | 4% | 6% | 8% |
| 6°F | 6% | 9% | 12% |
| 8°F | 8% | 12% | 16% |
| 10°F | 10% | 15% | 20% (cap) |
In the idealized sensible-load model, reducing a smaller indoor/outdoor gap removes a larger fraction of that modeled load. The second table applies this arithmetic to an 8°F setback for 8 hours nightly with a 70°F indoor setpoint. Its regional labels and outdoor temperatures are illustrative cases, not local forecasts or measured field data; equipment, recovery, latent loads and unaffected energy are not represented.
| Winter climate | Average outdoor temp | Indoor-outdoor gap | Bill saved by 8°F x 8 h nightly |
|---|---|---|---|
| Mild (Gulf South, coastal CA) | 50°F | 20°F | 13.3% |
| Moderate (Mid-Atlantic, Southwest) | 40°F | 30°F | 8.9% |
| Cool (Ohio Valley, Pacific NW inland) | 30°F | 40°F | 6.7% |
| Cold (Upper Midwest, New England) | 20°F | 50°F | 5.3% |
| Very cold (northern MN, ND, interior AK) | 10°F | 60°F | 4.4% |
Absolute dollar savings depend on the affected energy cost as well as the modeled percentage; a climate label alone does not determine which home saves more dollars. Neither table establishes whether the quick heuristic overstates or understates the savings of a particular building.
Example Problems
Example 1: The affected variable winter heating cost is $180 per month, excluding fixed charges, over a 6-month season. The plan uses an 8°F setback for 8 hours nightly. An installed thermostat price of $130 is included for an illustrative simple payback estimate.
Under the quick heuristic, S = 8 * (8/8) * (7/7) = 8%. Modeled savings = 0.08 * $180 = $14.40 per in-season month, or $86.40 annually assuming one 6-month season per year. Simple payback = $130 / $86.40 ≈ 1.5 years, or about 18 months. This omits financing, maintenance, changing tariffs and differences between actual and modeled savings.
Example 2: The affected HVAC energy cost is $1,000 per year and the target is $100 per year for a schedule of 10 hours/day, 7 days/week. The target is 10% of that cost, so the quick heuristic gives D = 10 / ((10/8) * (7/7)) = 8°F. This is a modeled depth, not a guarantee of $100 savings or a comfort recommendation. DOE’s broader potential-savings guidance does not validate this exact linear schedule.
Thermostat Setback FAQs
Does a setback save money after recovery? A smaller indoor/outdoor gap can reduce heating load during a setback, but actual bill savings also depend on equipment efficiency, recovery controls, auxiliary heat and building conditions. Recovery is not always cost-free, and the calculator’s linear time scaling does not simulate it. Treat the result as a planning estimate rather than proof of savings.
What matters for a heat pump? Recovery can trigger auxiliary resistance heat in some systems and reduce or erase expected savings. PNNL guidance discusses recovery controls that can help limit unnecessary auxiliary heat. Equipment settings and manufacturer guidance matter; this calculator does not select a universally suitable setback depth or assume every recovery strategy is efficient.
How is cooling represented? A cooling setup raises the indoor setpoint toward a warmer outdoor temperature. Select cooling for the climate model and use only the affected cooling energy cost. Humidity, latent loads, recovery, equipment controls and comfort can change the outcome; the calculation does not determine a universally appropriate summer setup.
