HVACDiscoverable field guide

Why Your Heat Pump's Auxiliary Heat Is Eating Your Electric Bill

Auxiliary heat is the electric resistance backup that switches on when your heat pump cannot keep up, usually below about 40 degrees outside, after a thermostat jump of 3 degrees or more, or during a defrost cycle. It runs at 100 percent efficiency while the heat pump delivers two to three units of heat per unit of electricity, so an hour of aux heat costs roughly three times an hour of heat pump heat.

January bills arrive and the number jumps, sometimes by a hundred dollars or more, with no change in how warm the house feels. The thermostat still says the same temperature. The heat pump still hums outside. What changed is the little AUX light, and how many hours it spent lit.

The Department of Energy puts it plainly: a heat pump moves heat instead of making it, delivering up to three times more heat than the energy it uses, and cutting electricity use for heating by about half compared with electric furnaces or baseboard heaters. Auxiliary heat gives all of that back. It is a set of resistance strips, the same principle as a space heater, making one unit of heat for every unit of electricity. The question is never whether aux heat costs more. It is how many hours it runs.

AUX and EM HEAT are not the same setting

Two labels on the thermostat confuse almost everyone, and the difference matters more than any other fact in this post. AUX is automatic. The system brings the resistance strips on to help the heat pump, and the heat pump keeps running alongside them. EM HEAT is a manual setting you switch on yourself, and it shuts the heat pump off entirely, running on strips alone.

Trane puts it plainly: emergency heat is a setting to use only in a true emergency, when the heat pump has failed and it is really cold outside. Rheem's owner manual says the same thing in plainer language, that emergency heat is a backup mode for a suspected problem with the outdoor unit. If you switched EM HEAT on because the house felt cold, switch it back. You threw away the efficient half of your system for the expensive half.

When aux heat is supposed to run

Three triggers are normal. Outdoor temperature drops below your system's balance point, typically 35 to 40 degrees for most systems. You raise the thermostat more than 3 or 4 degrees at once, and the thermostat calls in the strips to close the gap fast. Or the outdoor unit runs a defrost cycle, reversing briefly to melt ice off the coil, and the strips temper the cold air that blows inside while it does.

At the 2026 national average of about 18.4 cents per kilowatt-hour, the arithmetic is blunt. A 4 kW heat pump alone runs about 74 cents an hour. Add a 10 kW strip bank and the same hour costs about $2.58. A hundred hours of aux runtime in a cold month, which is not unusual during a long freeze, adds roughly $184 to the bill. That is the whole story of the January spike: not the cold, but the hours.

What a high aux bill is usually telling you

Aux heat running above 45 degrees outside, or staying lit for hours in mild weather, is not normal operation. The usual culprits are cheap ones. A dirty filter is the first check, and it is the one homeowners skip most. The DOE's Energy Saver guidance warns against setting a heat pump back manually if it causes backup resistance heat to come on, which is exactly what a deep overnight setback does: the thermostat wakes up, sees the house 8 degrees below setpoint, and calls in the strips to catch up.

Keep setbacks shallow, a degree or two, and let the thermostat ramp slowly. Ecobee, Nest, and most smart thermostats report aux runtime directly in their monthly runtime reports, so you can see the number instead of guessing. If aux runs constantly above 40 degrees after you have changed the filter and flattened the setbacks, call a technician. A faulty sensor, a wiring fault, or a thermostat misreading the temperature is cheaper to fix in November than the electric bill it writes in January.

Frequently asked questions

What is auxiliary heat on a heat pump?

Auxiliary heat is a bank of electric resistance strips in the indoor air handler that switches on automatically to help the heat pump when it cannot keep up. It runs alongside the heat pump, unlike emergency heat, which shuts the heat pump off entirely.

Why does auxiliary heat cost so much?

Because resistance heat makes one unit of heat per unit of electricity, while the heat pump moves two to three units of heat per unit of electricity. At the 2026 average of about 18.4 cents per kilowatt-hour, an hour with the strips on costs roughly three times an hour of heat pump heat alone.

When is it normal for aux heat to come on?

When it is below about 35 to 40 degrees outside, when you raise the thermostat more than 3 or 4 degrees at once, or during a defrost cycle. Aux heat running constantly above 45 degrees, or for hours in mild weather, is a sign something is wrong.

Should I ever switch on emergency heat?

Only when the outdoor unit has failed, is damaged, or is frozen solid, and you are waiting for a technician. Emergency heat runs on the resistance strips alone with the heat pump off, so it is the most expensive way to heat the house and should never be an everyday setting.