Thermostat Setback Savings Calculator
A setback saves energy in proportion to how far the house drifts and for how long. The widely quoted 1% per degree rule is a special case, not a law.
Thermostat setback savings calculator
Setback amounts, at your hours
| Setback | Energy saved | Money saved | Average indoor |
|---|
Why setback works at all
Heat loss through a building shell is proportional to the temperature difference across it. Hold a house at 70°F on a 35°F night and the difference is 35 degrees; let it drift to 62 and the difference is 27. The furnace has less work to do for every hour the house sits lower, and the saving is roughly the ratio of the two differences, weighted by how long each applies.
That is the whole mechanism, and it is also the limit. Setback does not make the equipment more efficient and it does not change the house. It simply asks for less heat for part of the day, which is why the saving is bounded by the temperature difference you create and the hours you hold it.
Where the 1 percent per degree rule comes from
The familiar rule — about 1 percent of the heating bill for each degree of setback over eight hours — falls out of that arithmetic at a particular set of conditions. At a 40 degree indoor-outdoor difference, one degree held for eight hours out of twenty-four reduces the average difference by about 0.8 percent. Round it to 1 percent and you have the rule.
The rule then gets quoted in climates where it does not hold. In a mild climate with a 20 degree difference, one degree for eight hours is worth about 1.7 percent, so the rule understates. In a cold climate with a 55 degree difference, the same degree is worth 0.6 percent, so the rule overstates. The arithmetic in this calculator does not use the rule at all — it uses your temperatures, which is why the answer changes when you change the outdoor temperature.
Why a heat pump changes the answer
A gas furnace recovering from a setback is doing what it always does: burning fuel at a fixed efficiency. A heat pump recovering from a setback may do something different — it may call for electric resistance backup, because the recovery load is larger than the heat pump's capacity at that outdoor temperature, and the thermostat is not patient. Eight degrees of recovery on a cold morning can put the strips on for twenty minutes, and strips at a COP of 1.0 cost about two and a half times what the heat pump costs to run.
That is why heat pump manufacturers and the DOE recommend gentler setbacks — 5 to 8 degrees rather than 10 or 12 — and a gradual recovery that starts before anyone wakes up. A smart thermostat set to "gradual" or "economy" recovery does this by ramping the setpoint rather than stepping it, which keeps the strips off and lets the heat pump do the work at its own pace.
Cooling setback is weaker, and for a different reason
In cooling, the same degree-difference logic applies, but three things work against it. First, the indoor-outdoor difference in summer is smaller than in winter — 75 against 90 is 15 degrees, where winter might be 45 — so each degree of setback is a larger share of a smaller number, which sounds good until you notice that the absolute saving is small because the load itself is small.
Second, a house that warms up during a setback has to have that heat removed again, and the removal happens at the hottest part of the day when the equipment is least efficient. Third, and most important in humid climates, a warmer house holds more moisture. The coil removes less water while the system is off, the indoor humidity climbs, and the house feels worse at the same temperature when the system comes back on. A cooling setback of 3 to 5 degrees for the hours nobody is home is a reasonable compromise; 10 degrees is not.
Thermal mass, and why the first hour is free
A house does not drop to the new setpoint the moment the thermostat changes. Furniture, drywall, slab and contents hold heat, and in a masonry or slab-on-grade house that mass can carry the indoor temperature for an hour or two before the equipment notices. That lag is a free saving at the start of every setback — and a cost at the end, because the same mass has to be brought back up.
The net effect is that short setbacks save less than the arithmetic predicts and long ones save close to it. A four hour setback is mostly recovery; a ten hour overnight setback is mostly saving. If you are choosing between a deep short setback and a shallow long one, take the long one.
What this calculator does not model
It assumes steady state, which real buildings never are. It does not model the recovery spike, the strip-heat penalty beyond a crude multiplier, or the humidity effect in summer. It also cannot see whether your house has a zoned system, in which case the setback applies to one zone and the equipment may still run for another. The numbers here are the ceiling of what a setback can do, not a promise of the bill.
Frequently asked questions
How much does an 8 degree setback save?
At a 35 degree outdoor average and 70 indoors, holding 8 degrees lower for nine hours a day is worth about 8.5 percent on paper and roughly 7 percent once thermal mass and recovery are allowed for. In a colder climate the same setback saves less, because the baseline temperature difference is larger.
Is the 1 percent per degree rule accurate?
It is accurate near a 40 degree indoor-outdoor difference over eight hours. In mild climates it understates the saving and in cold climates it overstates it. Using your actual temperatures, as this calculator does, is more reliable than the rule.
Should I use a bigger setback with a heat pump?
Usually not. A large setback can trigger electric resistance backup during recovery, which costs two to three times as much per BTU as the heat pump. Five to eight degrees with a gradual recovery keeps the strips off.
Does setback help in summer?
Less than in winter, and it can hurt comfort. The absolute load is smaller, recovery happens during the hottest hours, and a warmer house holds more humidity. Three to five degrees while you are out is the sensible range.
Is it cheaper to leave the thermostat alone all day?
No. That advice dates from the era of oversised equipment and poor insulation. Every hour at a lower indoor temperature is an hour of reduced heat loss, and the recovery cost is smaller than the saving for any setback longer than a couple of hours.
Do smart thermostats save more than a schedule?
They can, mostly through occupancy detection — not heating an empty house on an unexpected schedule. Their gradual recovery also matters for heat pumps, because it avoids the strip-heat spike a stepped setpoint causes.
Why did my bill not drop after setting a schedule?
Three common reasons: the schedule was overridden by someone at home, the setback was short enough that recovery ate the saving, or the weather changed between the two billing periods. Compare degree days, not months.
Disclaimer: This tool estimates setback savings for planning purposes. It uses a steady-state degree-difference model and does not simulate building thermal mass, equipment staging, or heat pump recovery behaviour, all of which reduce the saving in practice.
Related: All HVAC calculators · Heating cost per MMBtu · Heat pump vs furnace · Materials calculators