Understanding your result
The headline is the money saved per season, with the percentage, the setback and its hours in the sub-line. The bars compare the season cost with and without the setback. The stats give the percentage, the dollars and the thermostat payback. The table shows the degree-hours per day (the setback multiplied by its duration, which is what the saving really depends on), the rule-of-thumb saving, any heat pump recovery penalty, the net saving and the costs.
The degree-hours row explains why a small setback for a long time can beat a deep one for a short time: 4 °F for 16 hours is 64 degree-hours, more than 7 °F for 8 hours at 56. The heat pump recovery penalty appears only when a heat pump with strips is set back more than 3 °F; it is an estimate of the strip heat that recovery would use.
The season cost to enter comes from your bills, or from the heating cost by fuel calculator, the HVAC running cost calculator for any system, or the AC running cost calculator for cooling. The degree-day fuel estimate gives a season cost from the house’s heat loss if you have no bills.
How we calculate this
The 1% per degree for 8 hours is the US Department of Energy’s published rule for setbacks, the basis of its statement that up to 10% a year can be saved with a 7 to 10 °F setback for 8 hours a day; scaling it by hours follows from the same reasoning, since the saving comes from the reduced indoor-outdoor difference over time. The cap and the heat pump penalty are our judgement, set out here so they can be checked: the cap keeps the rule within the range it was derived for, and the penalty approximates strip heat during recovery, which delivers each BTU at about three times the cost of the compressor. The degree-day background is on the BTU per square foot table, and the cost of strip heat per BTU is on the fuel energy content and cost page.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Saving per degree | 1% per °F per 8 hours | US DOE Energy Saver, programmable thermostats |
| Cap | 20% | Judgement: limit of the rule of thumb |
| Heat pump penalty | 0.6% per °F above 3 °F | Judgement: strip heat during recovery at COP 1 |
| Thermostat cost | $130 | Typical smart thermostat price before rebates |
| Season cost | $1,300 | A gas-heated house in a cool climate (see the degree-day calculator) |
Assumptions last reviewed October 8, 2026.
The calculator does not model the house itself: a leaky house in a mild climate saves more than the rule suggests, a tight house in a cold climate less, and slow systems such as radiant floors and high-mass houses less again. It does not account for time-of-use electricity rates, which can make a pre-cooled or pre-heated house cheaper even with a smaller setback. The guide to thermostat setback math explains why setbacks save and when they do not, and the guide to heat pumps in cold climates covers recovery with backup strips.
Two worked examples
An overnight setback with a gas furnace
7 °F for 8 hours a night on a $1,300 heating season, gas furnace, $130 smart thermostat.
- Saving: 7 × (8 ÷ 8) × 1% = 7%
- Money saved: $1,300 × 7% = $91 a season
- Season cost with the setback: $1,209
- Thermostat payback: $130 ÷ $91 = 1.4 seasons
This is the textbook case and the saving is real. Adding a daytime setback while the house is empty for 8 more hours would roughly double it to about $180.
Night and day setback with a heat pump
6 °F for 16 hours a day on a $1,500 season, heat pump with backup strips.
- Rule of thumb: 6 × (16 ÷ 8) × 1% = 12%
- Heat pump penalty: (6 − 3) × 0.6% = 1.8% → net 10.2%
- Money saved: $1,500 × 10.2% = $153 a season
- Thermostat payback: under one season
The penalty is modest here because the estimate assumes some recovery on the compressor. With a thermostat that has adaptive recovery and locks out the strips during recovery, the full 12% is available; with an old thermostat that brings on the strips every morning, the saving can fall below half of that.
Where to find your inputs
Setback and hours. The difference between your normal setting and the setback setting, and the hours it applies each day (for example 10 pm to 6 am is 8 hours).
Season cost. Add up the heating (or cooling) part of the bills for the season: for gas heat, the gas bills from October to April less the summer baseline for water heating and cooking.
Heat pump with strips. Most ducted heat pumps have strips; the thermostat’s “aux” or “emergency heat” indicator shows when they run.
Common mistakes
- Believing recovery wipes out the saving. For furnaces and boilers it does not; the cooler house loses less heat.
- Deep setbacks on a heat pump with a basic thermostat. Recovery on the strips costs more than the setback saves.
- Applying the rule to 16 hours without scaling. Longer setbacks save proportionally more.
- Expecting the full rule with radiant floors. Slow systems drift slowly and save less.
- Setting the air conditioner off in a humid climate. The house gains moisture; a modest set-up works better.
- Overriding the schedule. A setback that is cancelled every evening saves nothing.
Questions people ask
- How much does turning down the thermostat save?
- About 1% of the heating bill for each degree Fahrenheit of setback held for 8 hours, according to the US Department of Energy, which says up to 10% a year is possible with a 7 to 10 °F setback for 8 hours a day. A 7 °F overnight setback on a $1,300 heating season saves about 7%, or $91. Holding it for 16 hours (overnight and a working day) roughly doubles the saving.
- Does it cost more to heat a cold house back up than to keep it warm?
- No. This is the most common myth about setbacks. A house loses heat in proportion to the difference between inside and outside, so a cooler house loses less heat for every hour it is cooler, and the energy to warm it back up is less than the energy saved. The exception is a heat pump whose recovery runs the backup strips, which deliver heat at a third of the efficiency of the compressor.
- Should I set back a heat pump?
- Modestly. A setback of 2 to 3 °F saves money with any heat pump. Deeper setbacks save money only if the thermostat recovers slowly on the compressor alone, which many smart and heat-pump-specific thermostats do with an adaptive or smart recovery setting. A conventional thermostat that calls for a 7 °F rise at 6 am will bring on the strips and can wipe out the saving; the calculator subtracts an estimate of that.
- Does setting up the air conditioner save money too?
- Yes, by the same physics, because a warmer house gains less heat from outside. The DOE gives similar savings per degree for raising the cooling setpoint while away. In humid climates keep the set-up moderate, 4 to 5 °F, because a house left at 85 °F all day gains moisture that the air conditioner then has to remove before the house feels comfortable again.
- Is a smart thermostat worth it?
- A programmable thermostat at $30 to $50 does the setback; a smart one at $100 to $250 adds learning, remote control, run-time reports and often adaptive recovery for heat pumps. On a $1,300 heating season with a 7 °F overnight setback, the $91 saved pays for a $130 smart thermostat in about a season and a half. Many utilities offer rebates that bring the cost down further.
- Why is the saving capped at 20%?
- Because the DOE's figure is a rule of thumb for ordinary setbacks of a few degrees for part of the day. Applied to a 15 °F setback for 16 hours it would claim 30%, which real houses do not achieve, because the house does not drift all the way down to the new setpoint for the whole period and some of the setback hours are mild. Radiant floors and other slow systems save less still.