What does turning the thermostat down save?

What the number tells you

Heating and air conditioning together are the largest energy use in a typical home, about 32% of the total according to the Department of Energy. A thermostat setback is the cheapest way to cut it: no equipment to buy, only a schedule. DOE's Energy Saver Guide says you can save as much as 10% a year on heating and cooling by turning the thermostat down 7–10°F for 8 hours a day in the fall and winter, and up in the spring and summer. The calculator turns that ceiling into your dollars, and lets you see what a smaller setback, shorter hours or a part-season routine is worth.

The method

Savings % = rule × degrees × (hours ÷ 8) × share of season. The default rule is 1% per degree for an 8-hour setback, which gives 7% for 7°F and 10% for 10°F and agrees with the "up to 10%" ceiling. Hours scale the saving in proportion to the 8-hour base: 4 hours gives half. Savings in dollars = yearly cost × savings %. The model is linear on purpose and is our own derivation; DOE publishes no formula. The 1%-per-degree rule is attributed to DOE by secondary sources, but the DOE pages we could open state only the ceiling of up to 10%, so treat it as a rule of thumb.

Worked example

A $1,200 heating bill. Eight degrees for eight hours every day of the heating season: 8% = $96 a year, $8 a month if spread over twelve months. At 7°F the saving is $84, at 10°F $120, which is DOE's "as much as 10%" ceiling. Only half of the season: $48. Only 4 hours a day (an afternoon away): $48. Going 15°F lower for 8 hours gives 15% in the model, $180, but the calculator flags it as above DOE's ceiling, because the linear rule is not reliable that far and a deep setback may mean a slow, uncomfortable recovery.

Limits and common mistakes

• The ceiling is "as much as". DOE's wording is an upper bound. Your own saving depends on the climate, insulation, thermal mass of the house and how the system recovers.
• Heat pumps need care. DOE says not to do a 7–10°F setback with a heat pump unless the smart or programmable thermostat is designed for heat pumps, and not to set back by hand if it turns on the backup electric-resistance heating, which costs more. This calculator does not model backup heat.
• Radiant floors and slow systems. Systems with a large thermal mass (radiant floors, boilers with large water volume) respond slowly, so a deep setback may not be recovered before you want the heat; use a smaller one. This is general heating practice, not a DOE number.
• Pipes. Do not set the temperature so low that pipes can freeze in a cold snap (common sense, not a DOE figure).
• The percent applies to your heating bill, not the total. If gas also heats water and cooks, enter only the heating share, or the answer is too high.

How much does lowering the thermostat save?

The U.S. Department of Energy's Energy Saver Guide says you can save as much as 10% a year on heating and cooling by turning your thermostat down 7–10°F for 8 hours a day in the fall and winter, and up in the spring and summer. A widely repeated rule of thumb, attributed to DOE in secondary sources, is about 1% for each degree of an 8-hour setback; the calculator uses it as an editable default. On a $1,200 yearly heating bill, a 7°F setback is $84 and a 10°F setback $120.

Does it use more energy to reheat the house than it saves?

Heat leaks out of a house faster the bigger the difference between inside and outside. While the house is cooler, the difference is smaller, so less heat escapes, and that is where the saving comes from. The furnace then has to put back what the house lost while it recovers, but it recovers at the end of the setback with a house that lost less overall. A simple illustration, not a DOE figure: on a 20°F day with the house at 70°F (a 50°F difference), holding the house 8°F cooler for 8 hours cuts the heat loss in those hours by 8 ÷ 50 = 16%, or 5.3% of the day's heat. Real savings are smaller, because the walls and furniture store heat, and larger or smaller depending on the weather, so the calculator's linear rule is only a rough estimate.

What about a heat pump?

DOE is explicit: if you have a heat pump, do not set the temperature back by 7–10°F without a smart or programmable thermostat designed for use with heat pumps. Its heat pump tips add: do not set back the heat pump's thermostat manually if it causes backup electric-resistance heating to come on, because that heating is more expensive. A big overnight setback can make the controls switch on the electric heating strips for the morning recovery and give back the saving. Use a small setback or a thermostat that limits the recovery, and clean or change the filter as DOE advises (once a month or as needed). The calculator's percentage does not model heat pump backup heat, so treat its answer as an upper bound for heat pumps.

Does it work for air conditioning too?

DOE's guidance covers both: turn the thermostat down 7–10°F for 8 hours a day in the fall and winter and turn it up in the spring and summer. Raise it while you are away and the air conditioner runs less. Put your yearly cooling cost in the first field and the number of degrees you raise the setting in the second. Heating and cooling together are the largest energy use in a home, typically about 32% of the household's energy, according to DOE, so the percentage applies to a large part of the bill.

Is a smart thermostat better than a manual setback?

A smart thermostat can do the setback for you, learn your schedule and let you control it from your phone. DOE's guide says smart thermostats that earn the ENERGY STAR label have been independently certified, based on actual field data, to deliver energy savings. The saving still comes from the lower temperatures, not from the device. If you always remember to change the setting by hand, a manual setback saves the same. Use a schedule that matches when you are asleep or away so that the house is comfortable when you return.

Which number goes in 'annual heating cost'?

The part of your bill that heats the house: the heating-season gas, oil or propane cost, or the electricity used for heat. If gas also heats water and cooks, subtract those. Not sure how much heating costs? Use the heating cost calculator on this site to estimate it from fuel and furnace efficiency, then bring the number here.

Sources: U.S. Department of Energy, Energy Saver Guide 2022 (Heat and Cool Your Home Efficiently: Smart and Programmable Thermostats; Heat Pumps; read October 28, 2026). The 1%-per-degree rule of thumb is repeated by secondary sources and is shown here as a rule, not a DOE quotation. Related tools: heating cost calculator (cost by fuel and furnace), heat loss calculator, insulation calculator, window replacement payback calculator. How we check our calculators.