Thermostat Savings Calculator

Heating and cooling are usually the largest line on a household energy bill, commonly 45 to 55% of it in a temperate climate. That is why a couple of degrees on the thermostat is worth more than most of the other savings people chase.

Energy Savings formula (approximate): Savings per Degree per Day ≈ 1% of heating/cooling cost per degree per 8 hours

More precise formula: Heating Energy = (Indoor Temp − Outdoor Temp) × Hours × Building Heat Loss Rate / System Efficiency

What each variable means:

• Setpoint: the target temperature you program the thermostat to maintain
• Setback: the reduced temperature when away or sleeping (a lower heating setback saves money; a higher cooling setback saves money in summer)
• Building Heat Loss Rate: measured in BTU/hour per °F of temperature difference; depends on insulation, windows, and square footage; a typical 2,000 ft² home loses 15,000–25,000 BTU/h per 20°F difference
• System Efficiency (AFUE/SEER): a 95 AFUE furnace converts 95% of fuel to heat; a SEER 18 AC unit produces 18 BTU of cooling per watt-hour consumed
• Degree Days: a climate data metric; 1 heating degree day = 1 day where the average outdoor temperature is 1°F below 65°F

The rule this page uses

The US Department of Energy’s rule of thumb is about 1% off your heating or cooling bill for each degree of setback, held for eight hours a day. Double the hours and you roughly double the saving.

Saving % = degrees of setback × (hours at setback ÷ 8)

Worked example: a 7°F setback held for 8 hours a night gives about 7% off the heating portion of the bill. On $1,200 a year of heating, that is roughly $84.

Hold the same setback for 16 hours a day, overnight plus while everyone is out, and it is nearer 14%.

US DOE recommended settings:

• Winter: 68°F (20°C) when home, 60°F (15.5°C) asleep or away
• Summer: 78°F (25.5°C) when home, 85°F (29.5°C) when away

Three things the rule of thumb does not know

Your insulation. A well-insulated house saves less from a setback, because it was losing less heat to begin with. A draughty one saves more. The rule is calibrated on an average house, and there is no average house.

Heat pumps behave differently. A deep setback can trigger the electric resistance backup during recovery, which is expensive enough to wipe out the saving and then some. With a heat pump, use a shallow setback of two or three degrees, or let the thermostat’s own adaptive recovery handle it.

The myth about leaving it constant. It is not true that reheating a cold house costs more than keeping it warm. Heat loss is proportional to the temperature difference with outside, so a cooler house always loses less. The recovery does use a burst of energy, but never more than was saved. The heat pump case above is the only real exception.

Smart thermostats earn their keep less through cleverness than through consistency: they apply the setback every single day, including the ones where you would have forgotten. If you already run a reliable schedule on a programmable thermostat, the upgrade saves much less than the marketing suggests.