Key Takeaways
- COP drops as temperature drops: Heat pumps deliver COP 3.0-4.0 at 47°F but decline to 1.75-2.5 at 5°F for cold climate units
- Balance point determines when bills spike: Modern inverter systems have lower balance points than textbook calculations suggest
- Regional economics vary wildly: Heat pumps beat gas in cheap electricity regions, but struggle where gas is under $1.00-1.25/therm, depending on local electricity rates and actual seasonal COP achieved
- Auxiliary heat is the bill killer: Running on backup heat costs 3-5× more per hour than compressor operation
You’ve probably gotten that service call: Customer bought a heat pump last year, loved it through fall, and now they’re staring at a heating bill that makes their mortgage payment look reasonable. “This thing is broken,” they say. “It has to be.”
Most of the time, it’s not broken. It’s doing exactly what physics says it should do. The problem is nobody explained what that means for their wallet.
Here’s how to have that conversation without losing a customer or making promises you can’t keep.
The COP Reality Check
Coefficient of Performance (COP) is heat delivered divided by electricity consumed. COP 3.0 means 3 units of heat for every 1 unit of electricity, or 300% “efficiency” compared to resistance heat at COP 1.0.¹
But your customer isn’t calling at 47°F. They’re calling at 5°F with “AUX” lit up. Here’s what actually happens:
The Math Part (Don’t Skip It)
A heat pump’s COP depends entirely on the temperature difference (ΔT) between outdoor and indoor. The bigger the gap, the harder the compressor works, and efficiency drops. We’ve got:
- 47°F outside (fall): ΔT = 22°, COP = 3.5-4.0
- 32°F (early winter): ΔT = 37°, COP = 2.5-3.0
- 5°F (deep winter): ΔT = 64°, COP = 1.75-2.5
This isn’t broken equipment. This is thermodynamics. The same compressor in a walk-in cooler delivering minus 10°F handles higher head pressure and does way less work per degree of cooling. Your heat pump working backward (heating when it’s freezing outside) is the hardest job it can do.
What the Customer Sees When COP Tanks
At 5°F with a COP of 2.0:
- 1 kWh of electricity = 2 kWh of heat delivered
- Resistance heating at an electric baseboard = 1 kWh of electricity = 1 kWh of heat
- So at low temperatures, the heat pump is only twice as efficient as backup heat, not three to four times
And then there’s the balance point.
Balance Point: When Your Heat Pump Asks for Help
Your system has a balance point—the outdoor temperature where the heat pump alone can no longer meet the heating load, even running continuously. Below that point, backup heat (resistive elements or gas) kicks in.
The balance point is not magic. It’s simple math: Heating load needed ÷ COP available = electricity required from compressor. If that exceeds max compressor power, AUX turns on.
Modern inverter-driven heat pumps have lower balance points than old fixed-speed units. A 3-ton compressor running at reduced speed can hit a balance point of 20-30°F instead of the old 35-40°F. But it’s still a real point. It exists. When temperature drops below it, the system runs the compressor and auxiliary heat. Both. At the same time.
Here’s Where the Bill Gets Ugly
Resistance heat elements are expensive to run – usually 3-5 times the per-kWh cost of compressor operation, once you factor in power draw and efficiency loss. A 5 kW resistance heater running 24/7 in a 5°F day will use 120 kWh and cost around $15-20 on most residential rates.
Your customer’s house doesn’t need 120 kWh. It might only need 50 kWh to maintain 68°F. But if the COP is down to 1.8 and the balance point is 25°F, the system might use 25 kWh from the compressor and 10-15 kWh from resistance, costing $5-9 for the day. That’s not broken.
But spread across January, February, and March in a climate that hits 5°F multiple days, and you’re looking at an extra $200-500 over the winter versus milder climates. That feels broken to the homeowner.
Regional Economics: Where Gas Still Wins
Here’s the honest part: In some regions, gas is still cheaper than heat pump electricity, especially when temperatures drop.
Do the math for your area:
- Gas cost per therm: Check your customer’s bill. If it says $0.80/therm, great. If it says $1.50/therm, heat pumps win. (US average is around $1.00-1.10.)
- Electricity cost per kWh: Residential rates vary wildly. $0.12/kWh in Louisiana. $0.25/kWh in Massachusetts.
- Seasonal COP in your climate: Not the manufacturer spec. The actual cop your system achieves. NEEP and Energy Vanguard have tables for cold climates.
- Do the comparison: Cost to heat 1 million BTU with gas vs. electricity using your local COP.
The formula:
Cost to heat 1 MMBtu with gas = (1 MMBtu ÷ 100,000 BTU per therm) × $/therm = cost per therm × 10
Cost to heat 1 MMBtu with heat pump = (1 MMBtu ÷ 3,412 BTU/kWh) ÷ (actual seasonal COP) × $/kWh
If gas cost is $1.25/therm and electricity is $0.14/kWh:
- Gas: $1.25 × 10 = $12.50 per MMBtu
- Heat pump (COP 2.5): (1,000,000 ÷ 3,412) ÷ 2.5 × $0.14 = $16.38 per MMBtu
Gas wins by $4 per MMBtu. Over 100 MMBtu for a winter, that’s $400. Not huge, but real.
Swap the rates to $0.80/therm and $0.14/kWh:
- Gas: $0.80 × 10 = $8.00 per MMBtu
- Heat pump (COP 2.5): $16.38 per MMBtu (unchanged)
Now heat pump still loses.
But flip it to $1.50/therm and $0.12/kWh:
- Gas: $1.50 × 10 = $15.00 per MMBtu
- Heat pump (COP 2.5): (1,000,000 ÷ 3,412) ÷ 2.5 × $0.12 = $14.04 per MMBtu
Heat pump wins by $1.
The point: Don’t pretend regional economics don’t matter. They do. If gas is under $1.00-1.25/therm in your service area, many customers will look at the electric heating bills and think the system is broken. It’s not. The region is just expensive to heat with electricity.
How to Talk to Customers About This
When they call and say “my bill is $400 for December,” you don’t say “that’s normal.” You say:
1. “Let’s check what the system is actually doing.”
Pull the runtime data. How many hours is the compressor running? How many hours is the auxiliary heat running? If AUX is running 60% of the time at 5°F, the system is doing exactly what it should – compressor is running flat out, can’t keep up, so aux is helping. Not broken.
If AUX is barely running but the bill is still high, pull superheat, subcooling, and pressures. There might actually be a problem.
2. “Let’s check your balance point.”
Look at the thermostat settings. Some units let you see or manually set the balance point. If it’s set to 32°F or lower, the system is already optimized. If it’s set to 45°F and the customer lives in a cold climate, you could lower it and reduce AUX run time (slightly – thermodynamics is immovable).
3. “Let’s check your rates and compare.”
Ask for their electricity bill. Ask what they paid for gas if they’ve got it elsewhere. Do the MMBtu calculation. Sometimes the news is “your heating bills are high because electricity is expensive in your area, not because the heat pump is broken.”
That’s not a popular answer, but it’s honest.
4. “Here’s what we can do.”
If there are no mechanical issues:
- Lower the thermostat by 2-3 degrees and expect proportional energy savings (roughly 2-3% per degree)
- Set the heat pump to not run below the balance point if the customer has gas as backup (so AUX doesn’t run continuously)
- Recommend a setback to 62-65°F overnight and when away – this often saves $20-30 per month in cold climates
- Check ductwork for leaks, insulation for gaps, and air sealing around the house. This impacts balance point more than anything else
But don’t promise savings that physics and regional economics don’t allow. You’ll lose the customer and your reputation.
The Reality
Heat pumps work great in mild climates where the COP stays above 3.0 most of the season. In cold climates, they’re still more efficient than resistance heat and comparable to or cheaper than gas – if electricity rates are favorable. If gas is cheap where you work, heat pump heating bills can legitimately feel high.
The customer isn’t wrong. The system isn’t broken. The region is expensive to heat with electricity.
Knowing the math and the COP reality means you can close the sale on a heat pump without having an angry customer in January, or you can fix their expectations upfront so they don’t blame you for basic thermodynamics.
References and Further Reading
- “Heat Pump Efficiency in Cold Climates”, Northeast Energy Efficiency Partnerships (NEEP), 2024
- “Residential Heat Pump Specification and Installation Guide”, AHRI, 2024
- “Cold Climate Air Source Heat Pumps for Single-Family Homes”, NEEP, 2024
- “Heat Pump COP at Minimum Capacity”, Green Building Advisor, 2023
- “Simple Way to Calculate Heat Pump Balance Point”, Energy Vanguard, 2023
- “Cold Climate Ductless Heat Pump Specification and Recommendations”, NEEA, 2025
- “Electric Power Monthly”, US Energy Information Administration, 2025
- “Heat Pump Cost Savings”, CBC News, 2023
- “Why Your Heat Pump’s Auxiliary Heat Runs Too Much”, T. Byrd HVAC, 2024
