Your heat pump bill climbs once the cold settles in, and it's enough to make you wonder if something's wrong with the unit. Most homes around you heat with gas, so there's no neighbor's bill to compare it to. Power in Illinois isn't cheap either, which makes the jump feel bigger than it probably should. Before you call anyone out, it helps to know what's normal for this equipment in this kind of winter and what isn't.
A heat pump moves heat from the outside air into your house. That works fine down to a point, but once temperatures drop into the 20s and lower, which happens for stretches most Illinois winters, the outdoor unit can't pull enough heat to keep up. At that point the system leans on electric resistance strips built into the air handler to make up the difference. Those strips run on straight electricity, with none of the efficiency a heat pump normally has, and that's what drives the bill up. A bill that climbs on the coldest weeks is expected. A bill that stays high even on mild days, or a system that feels like it's running on backup heat constantly, points to something not working right, like a stuck defrost cycle or a thermostat calling for heat strips too soon.
That same heat pump earns its keep in July, when the dew point here averages 63°F, according to NOAA climate data. Air that heavy makes a heat pump work through two jobs at once, pulling moisture out as well as cooling the house, and it will run longer stretches to do both. That's normal and it's a big part of why the unit wears the way it does. A heat pump that spent all summer fighting that humidity goes into winter already worked, and anything that was straining quietly in July, a weak capacitor, a coil that never got fully clean, tends to show itself once the system has to run hard for heat instead.
NASA POWER T2MDEW county cells weighted by homes
Go look at the outdoor unit. If the fan is running nonstop and the air coming from your vents feels warm but not hot, it's likely leaning on backup heat to keep up. You might also notice the thermostat never quite satisfies, cycling on again a few minutes after it shuts off, or a burning dust smell the first time backup heat kicks on for the season. None of that means the unit is broken. But if it's leaning on backup heat more than it should, on a system that's losing capacity or low on refrigerant, you'll pay for it every day the cold holds on, not just once.
When frost builds up on the outdoor coil, the unit reverses itself for a few minutes to melt it off, and during that time it often kicks on the electric backup heat so cold air doesn't blow through your vents. On a damp, near-freezing day this can happen several times an hour. Each cycle draws power beyond what the compressor alone would use, and it adds up over a long stretch of January weather.
If your thermostat is set back overnight and then raised a few degrees in the morning, a heat pump can struggle to close that gap on a cold day. Instead of a quick burst of heat, the system may switch to backup heat to recover the temperature faster, drawing far more power than it would holding steady. Keeping the setting level through the day often asks less of the unit than swinging it back and forth.
At that rate, every hour your backup heat runs instead of the compressor alone shows up fast on the bill. It's why the same cold snap that barely touches a gas furnace next door can push your heat pump's number up hard.
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A heat pump doesn't make heat the way a furnace does. It moves heat from outside air into the house, using the same refrigerant loop an air conditioner uses, just run backward. The reversing valve is what flips that direction. In summer it sends refrigerant one way to pull heat out of the house. In winter it sends it the other way to pull heat out of the outdoor air, even when that air is cold, and push it inside. The expansion valve controls how much refrigerant reaches the outdoor coil at a time, which determines how much heat the coil can grab from the air passing over it.
That works well down to a point. Once the outdoor temperature drops far enough, there isn't enough heat left in the air for the compressor to pull out fast enough to keep the house warm, so the system turns on backup heat strips to make up the difference. Those strips are straight electric resistance heat, the same principle as a toaster, and they draw power the whole time they run.
The outdoor coil also frosts up in cold, damp weather. The system clears that frost on a schedule set by a timer and a coil sensor, not by how cold it is outside, and backup heat often covers the house during that cycle so cold air doesn't blow through the vents. A technician checking a high bill starts with the refrigerant charge and the reversing valve, since a problem with either one leans the whole system on backup heat to compensate.
The outdoor unit sits clear and the fins stay open, so air moves through it the way it's built to. The compressor can pull its weight most of the winter without handing the job to backup heat. That keeps your bill tied to what the compressor costs to run, not the backup.
Dirt packed into the fins and grime across the base block the airflow the unit needs to pull heat from cold air. It leans on backup heat sooner and more often just to keep the house warm. A technician can tell from the buildup alone that this unit's been working harder than it should for a while.
The jump in your bill comes down to how much your heat pump leans on backup heat instead of the compressor, and that depends on whether the unit is running the way it's supposed to. A frozen coil, a low charge, a defrost cycle that won't stop cycling, any of these push the system onto backup heat faster and keep it there longer. A technician who works on heat pumps in Illinois winters all season knows which of these tends to show up first and what to check before touching the backup heat strips. Getting matched with one licensed contractor who knows the equipment means someone looks at your unit specifically, not a general number pulled from an average house, and you're not fielding calls from three different companies while you wait. Your bill should track the weather, not a problem sitting in the outdoor unit. Once that's fixed, the swings in your bill should settle back into something that matches what's actually happening outside.
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A technician who services heat pumps all winter can usually tell from the outdoor unit and your thermostat settings whether backup heat is running more than it should. Sending the request puts that answer in front of someone local who sees this same problem on cold Illinois mornings, instead of leaving you to guess at what's driving the bill.
