Last updated: 2026-07-13

Turning Down the Heat Pays Off

Heating is the largest energy use in most cold-climate homes, so the thermostat is one of the biggest levers on the winter bill. Every hour the house is held a little cooler, it loses heat more slowly and the heating system runs less. A programmable or smart thermostat that drops the temperature while everyone is asleep or out of the house captures that saving automatically. This calculator turns three numbers — the annual heating cost, how far the thermostat is set back, and how long it stays there — into a rough estimate of the money saved.

How It Works

The estimate rests on a well-known US Department of Energy rule of thumb: turning the thermostat back by one degree Celsius for eight hours a day saves about one percent of heating energy (roughly 0.55% per °F). The saving scales with both the depth of the setback and how long it lasts:

where $s$ is the share of heating saved, $\Delta T$ is the setback in degrees Celsius, $t$ is the hours per day at the lower setting, $C$ is the annual heating cost, and $S$ is the money saved. Dividing the hours by eight scales the rule to setbacks that are shorter or longer than the eight-hour reference period.

The physics behind it is simple: a home loses heat roughly in proportion to the gap between the indoor and outdoor temperature. Holding a lower indoor temperature shrinks that gap for the hours it lasts, so less heat escapes and the furnace or heat pump has less to replace.

Worked Example

Suppose a home spends $1,200 a year on heating and the thermostat is set back 3 °C every night for 8 hours.

First, the share of heating saved:

Then the money saved:

Deepening the overnight setback to 6 °C would roughly double the saving to about $72 a year, while a shallower 2 °C setback would cut it to about $24. Adding a second setback period — for example while the house is empty during the workday — stacks on top, because the hours at the lower temperature simply add up.

Common Setbacks

The Heat-Pump Caveat

The rule of thumb was built around furnaces and boilers, which reheat a home efficiently on demand. Heat pumps are different. They run most efficiently at a steady, moderate output, and recovering from a deep setback can push them into an inefficient high-output mode or switch on electric backup heat — which can cancel much of the saving. If the home is heated by a heat pump, keep setbacks shallow, or use a thermostat designed to manage the recovery ramp gradually rather than blasting heat all at once.

Does Reheating Cancel the Saving?

A persistent myth says a home burns more energy warming back up than it saved while cool, so it is cheaper to hold a steady temperature. It is not true for conventional heating. The energy needed to reheat only replaces the heat that was not lost during the cooler hours; it can never exceed it. As long as the setback lasts more than an hour or two, turning the thermostat down always uses less energy than leaving it up.

Reading the Result

The figures here are a first-order estimate, not a building-energy model. Real savings depend on how well the home is insulated and sealed, the local climate, the thermostat schedule, and the type of heating system. Use the result to judge whether a setback is worth the small loss of comfort and to compare shallow versus deep schedules — not as a precise forecast of next year's bill.