Why lowering the heat saves money
A house loses heat in proportion to the difference between the indoor and outdoor temperatures. Lowering the thermostat reduces that difference, so the house loses less heat and the heating system works less. The common claim that “the furnace has to work harder to warm the house back up” is a myth: the energy needed to reheat is less than the energy saved while the house was cooler.
The formula
The calculator uses the average temperature difference over the heating season:
Savings ≈ (permanent reduction + hours lowered ÷ 24 × degrees lowered) ÷ (indoor − average outdoor temperature)
Example
A household spends $1,200 a year on heating and keeps the house at 68 °F. The heating season averages 40 °F outside, a difference of 28 °F. Setting back to 62 °F for 8 hours at night saves 8 ÷ 24 × 6 ÷ 28 = 7.1%, or about $86 a year. Each degree lowered all day saves about 3.6%.
This matches the US Department of Energy’s estimate that setting the thermostat back 7 to 10 °F for 8 hours a day can save up to 10% a year on heating.
| Climate (heating season average) | Savings per °F lowered all day |
|---|---|
| Mild, 50 °F | about 5.6% |
| Temperate, 40 °F | about 3.6% |
| Cold, 25 °F | about 2.3% |
When savings are smaller
- Heat pumps: a deep setback can trigger the electric backup heat during recovery, which may cancel the savings. Smart thermostats with adaptive recovery limit this; a setback of 2 to 4 °F is safer.
- Radiant floors and heavy masonry houses respond slowly; long setbacks are less practical.
- Very well insulated homes cool down slowly, so the indoor temperature may not reach the setback value at night.
Practical tips
- A programmable or smart thermostat makes setbacks automatic.
- Lower the heat when you are away for more than a few hours.
- Keep bedrooms cooler; 60 to 67 °F is comfortable for sleep for most people.
- Do not let any room drop below about 55 °F in freezing weather, to protect pipes.