The mechanism
A heat pump does not make heat, it moves it, which is why it can deliver two to four units of warmth per unit of electricity. But its output falls as the outside air gets colder, exactly when the house needs more. Below a certain outdoor temperature — the balance point — it can no longer keep up on its own, and a bank of resistance elements switches on to make up the difference.
Those elements are ordinary electric heating: one unit of heat per unit of electricity, no better and no worse than a bar fire. Every hour they run is billed at roughly three times the rate the heat pump was managing. Nothing announces this. The house stays warm, the thermostat says the same number, and the bill arrives six weeks later.
At 18.34¢/kWh. The gap is what every hour of unnecessary backup heat is costing.
| Heating a house for a winter | Electricity | Cost |
|---|---|---|
| Heat pump working normally | 2,307 kWh | $423 |
| Resistance elements doing the same job | 9,012 kWh | $1,653 |
Four things that trigger it unnecessarily
- A deep thermostat setback. Recovering four or six degrees quickly looks like an emergency to a thermostat, and many respond by calling for auxiliary heat. On a heat pump a 2°F setback that never triggers the elements beats a 6°F setback that does.
- A thermostat that does not know it is driving a heat pump. A conventional thermostat treats any temperature gap as something to close as fast as possible. A heat-pump-aware one uses adaptive recovery: it starts earlier and climbs gently.
- Emergency heat left switched on. Somebody flips it during a cold snap and forgets. The system then runs on resistance heat exclusively, for months.
- A unit iced up or low on refrigerant. Both reduce capacity, which lowers the balance point at which backup is needed, so the elements engage in weather that should not require them.
How to tell whether it is happening
- Look at the thermostat. Most display “AUX” or “EM HEAT” when the elements are engaged. If you see it on a mild day, that is the answer.
- Compare kilowatt-hours month to month, not dollars. A heating bill that rises in proportion to the cold is normal. One that steps up sharply while the weather only worsens slightly is the signature of a second heat source switching in.
- Watch a whole-home monitor. Resistance strip heat is a large, unmistakable step in draw — typically 5 to 20 kW arriving at once. It is the single easiest thing for a monitor to show you, and often justifies the device by itself.
If you do not have a heat pump
Then the mechanism is different but the arithmetic still applies. Three candidates, in order:
At 18.34¢/kWh, each on this site’s standard usage assumption.
| Suspect | A winter | Why it spikes in winter |
|---|---|---|
| Electric water heater | $499 | Incoming water is far colder in winter, so the same shower takes materially more energy to heat. |
| Space heaters | $182 | One 1,500 W heater per cold room, running unnoticed. Count them; households routinely have three. |
| Baseboard heaters | $218 | Costed one room at a time, billed for all of them at once. |
| Underfloor heating | $124 | Often on a timer that was set once and never revisited. |
Degree days, and why “it was colder” is measurable
Heating energy is close to proportional to how far below a base temperature the outside air sat, and for how long. That quantity has a name — heating degree days — and your utility almost certainly publishes it alongside your usage, precisely so you can separate weather from behaviour.
The usefulness is that it makes a fair comparison possible. If January used 40% more electricity than December and January had 40% more degree days, nothing changed about your house and there is nothing to investigate. If January used twice the electricity for 20% more degree days, something else happened — and that gap is exactly the signature of backup heat engaging.
- Find the degree-day figure on your bill or your utility’s portal, for this month and the same month last year.
- Divide your kilowatt-hours by the degree days for each. That gives kWh per degree day — your home’s heating efficiency, weather removed.
- Compare the two numbers. If they are close, the weather explains your bill. If this year is much worse, your equipment or its settings changed.
Fix the settings before the equipment
The order matters, because the cheap interventions are also the ones most likely to be the actual cause.
| Step | Cost | Typical effect |
|---|---|---|
| Switch off emergency heat if it was left on | Nothing | Can be the entire problem. Check this first, every time. |
| Reduce setback to about 2°F on a heat pump | Nothing | Stops the recovery calling for backup elements. |
| Configure auxiliary heat lockout | A service call, or free if you can access the settings | Prevents strip heat firing in weather the heat pump can handle. |
| Lower the thermostat by 1–2°F | Nothing | Roughly 3–5% of heating energy per degree. |
| Clear snow and ice from the outdoor unit | Nothing | Restores capacity, raising the temperature at which backup is needed. |
| Replace a failed or restricted filter | A few dollars | Restores airflow; a starved coil runs longer for the same heat. |
| Air-seal the obvious leaks | Tens of dollars | Reduces the load itself, which helps every hour of every winter. |
| Replace the equipment | Thousands | Last, and only after the above have been ruled out. |
And check it is not just the rate
Many states price electricity higher outside summer, so a winter bill can rise on rate alone. Measured on the current EIA release, 1 states have a single-month summer figure more than 5% below their year-to-date average — meaning their winter rate is meaningfully dearer. Compare kilowatt-hours with the same month last year before concluding anything about your equipment, and check your own state’s page for the seasonal gap.
Questions
Why did my electric bill double in winter?
Should I turn my heat pump down at night?
Is emergency heat expensive?
Does cold weather make my water heater cost more?
All guides · How the figures are computed · Break down your own bill