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You switched from oil to a heat pump expecting lower bills. Then January hit, and your Eversource bill jumped $200-$500. The culprit: auxiliary electric resistance heat running at $0.22-$0.27/kWh during NH's coldest nights. Here is how it works, when it is normal, and how to minimize it.
Most heat pump systems have backup electric resistance heating strips built into the indoor air handler. These activate in two different modes, and the cost difference is enormous.
Activates automatically when the heat pump cannot keep up with demand. The heat pump and electric strips run simultaneously.
Activated manually by setting the thermostat to EM HEAT. The heat pump compressor shuts off completely. Only electric strips provide heat.
NH has four main electric utilities, each with different rates. The rate directly determines how much auxiliary heat costs per hour. Here is what each utility's customers pay when aux heat is running.
| Utility | Rate/kWh | Share | 5 kW Strip/hr | 10 kW Strip/hr | 15 kW Strip/hr |
|---|---|---|---|---|---|
| Eversource NH | $0.25 | ~71% | $1.25/hr | $2.50/hr | $3.75/hr |
| Liberty Utilities | $0.24 | ~6% | $1.20/hr | $2.40/hr | $3.60/hr |
| Unitil | $0.26 | ~11% | $1.30/hr | $2.60/hr | $3.90/hr |
| NH Electric Coop | $0.22 | ~12% | $1.10/hr | $2.20/hr | $3.30/hr |
Look at your indoor air handler. The electric resistance heat strips are labeled with their wattage (typically 5 kW, 10 kW, or 15 kW). A 10 kW strip is most common in 2-3 ton systems for average NH homes. Larger homes may have 15-20 kW. This number directly controls your hourly cost when aux heat runs. If your installer installed oversized strips, ask about downsizing to reduce bill-shock risk.
The "balance point" is the outdoor temperature where your heat pump alone can no longer meet your home's heating demand. Below this temperature, auxiliary heat kicks in. A lower balance point means less aux heat and lower bills.
Good insulation means the heat pump can meet the load down to lower temperatures before needing aux heat.
Balance Point
5F to 15F
Aux Hours/Year
50-120
Moderate insulation. Aux heat kicks in earlier and runs longer during cold snaps.
Balance Point
15F to 25F
Aux Hours/Year
120-250
Poor insulation = high heat loss. The heat pump hits its limit quickly. Aux runs frequently.
Balance Point
25F to 35F
Aux Hours/Year
250-500+
If the heat pump was sized too small, it cannot meet demand even at moderate temperatures. This is the #1 cause of bill shock.
Balance Point
30F to 40F
Aux Hours/Year
400-700+
Manchester, NH averages about 1,100 hours below 20F and 300 hours below 10F per winter. Concord sees about 1,300 hours below 20F and 400 hours below 10F. The North Country (Berlin, Colebrook) endures 1,800+ hours below 20F.
If your balance point is 25F, you will need aux heat for 1,500-2,000+ hours per season in southern NH. If your balance point is 10F (well-insulated home, properly sized cold-climate HP), aux heat runs only 200-400 hours. That difference can mean $300-$600 per season on your electric bill.
Real heating costs in NH vary month to month. These scenarios show what an average 2,000 sq ft home with a 3-ton heat pump system pays during different January weather patterns. All figures assume Eversource rates ($0.25/kWh) and a 10 kW aux strip.
| Scenario | Region | Avg Temp | HP Hours | Aux Hours | HP Cost | Aux Cost | Total | Oil Would Be |
|---|---|---|---|---|---|---|---|---|
| Mild January (Southern NH) | Nashua/Manchester | 22F avg | 600 | 40 | $135 | $50 | $185 | $400-$500 |
| Average January (Southern NH) | Nashua/Manchester | 18F avg | 550 | 80 | $155 | $100 | $255 | $450-$550 |
| Cold January (Southern NH) | Nashua/Manchester | 12F avg | 500 | 140 | $180 | $175 | $355 | $500-$650 |
| Average January (Lakes Region) | Laconia/Wolfeboro | 14F avg | 480 | 120 | $175 | $150 | $325 | $480-$600 |
| Polar Vortex Event | Any NH region | -5F for 5+ days | 200 | 300 | $90 | $375 | $465 | $550-$700 |
| Stuck on EM HEAT (Malfunction) | Any NH region | 20F avg | 0 | 650 | $0 | $813 | $813 | $450-$550 |
The last scenario shows what happens when a system runs in emergency heat for a full month. This is the single most common cause of extreme bill shock. It happens when someone accidentally switches to EM HEAT mode, when the outdoor unit fails and the system defaults to backup heat, or when a power surge resets the thermostat. If your January bill is $600+, check your thermostat immediately — if it shows EM HEAT, switch back to normal HEAT mode.
You cannot eliminate auxiliary heat in NH winters entirely, but you can dramatically reduce it. These strategies are listed in order of impact.
The number one cause of excessive aux heat is an undersized heat pump. In NH, Manual J calculations must account for -5F to -15F design temperatures. If your contractor sized for 10F, the system will struggle. A properly sized cold-climate unit can handle 90%+ of NH heating hours without aux assistance.
NHSaves offers subsidized home energy audits ($100-$200 after rebate) and insulation rebates covering 50-75% of costs. Adding attic insulation and sealing air leaks lowers the balance point by 5-10 degrees, meaning less aux heat. A home that triggers aux at 25F might only need it at 15F after weatherization.
With oil or gas, setting the thermostat back 5 degrees at night saved money. With heat pumps, the opposite is true. When the thermostat calls for a 5-degree recovery, the heat pump cannot respond fast enough, triggering aux heat strips. Keep the temperature steady (within 1-2 degrees). Use a heat-pump-aware smart thermostat (Ecobee, Honeywell T10 Pro) that manages setbacks intelligently.
Older or standard heat pumps lose capacity rapidly below 20F. Cold-climate models (Mitsubishi Hyper-Heat, Fujitsu XLTH, Daikin Aurora) maintain 75-100% rated capacity down to -5F. This pushes the balance point far lower, reducing aux hours from 300+ to under 100 in southern NH.
Dirty outdoor coils, clogged filters, and low refrigerant all reduce heat pump capacity, forcing earlier aux activation. Annual fall maintenance ($150-$250) includes coil cleaning, filter replacement, refrigerant check, and defrost verification. A clean system operates at full capacity longer into cold weather.
If your system has 15 kW of backup strips but a 10 kW or even 5 kW strip would suffice for short-duration backup, ask your installer about downsizing. Smaller strips cost less per hour and prevent the dramatic $800+ monthly bills. The tradeoff: slower recovery during extreme cold.
Modern smart thermostats track how many hours aux heat ran each day. If you see aux running at 30F+ outdoor temperature, something is wrong. Ecobee and Honeywell apps show detailed aux heat runtime. Set alerts for excessive aux usage so you catch problems early.
Even with auxiliary heat running 100-200+ hours per season, a heat pump in New Hampshire costs significantly less than oil or propane over a full heating season.
The most common cause is auxiliary (backup) electric resistance heat running more than expected. During cold snaps below your system's balance point (typically 15-25F for average NH homes), the heat pump cannot meet the full heating load and activates electric resistance strips that consume 5-15 kW. At NH rates of $0.22-$0.27/kWh, a 10 kW strip running 8 hours per day adds $52-$65 to your weekly bill.
Auxiliary (AUX) heat activates automatically when the heat pump needs help meeting demand — the heat pump and electric strips run simultaneously. Emergency (EM) heat shuts down the heat pump compressor entirely and runs only electric resistance strips. EM heat should only be used when the heat pump is broken. Running in EM heat mode continuously costs 2-3 times more than normal heat pump operation.
The balance point is the outdoor temperature where the heat pump alone can no longer meet your home's heating demand. Below this temperature, auxiliary heat engages. For a well-insulated NH home with a properly sized cold-climate heat pump, the balance point is typically 5-15 degrees Fahrenheit. For a drafty older home or undersized system, it can be 25-35 degrees, meaning aux heat runs much more often.
At the NH average electric rate of $0.25/kWh: a 5 kW aux strip costs $1.25/hour, a 10 kW strip costs $2.50/hour, and a 15 kW strip costs $3.75/hour. During a polar vortex, aux heat may run 12-16 hours per day, adding $15-$60 per day depending on strip size and duration.
Not necessarily. During extended periods below your balance point (common in NH January/February), auxiliary heat running is normal and expected. However, if AUX heat activates at temperatures above 30-35F, the system may be undersized, low on refrigerant, or have a dirty outdoor coil. Call a technician if aux runs above 35F consistently.
Yes. Five proven strategies: (1) weatherize and insulate to lower the balance point, (2) ensure the heat pump is properly sized — undersized systems use more aux, (3) do not set the thermostat back and forth — this triggers aux heat during recovery, (4) clean or replace air filters monthly in winter, and (5) upgrade to a cold-climate model like Mitsubishi Hyper-Heat or Fujitsu XLTH that operates efficiently down to -13F to -15F.
NH rates range from $0.22/kWh (NH Electric Coop) to $0.26/kWh (Unitil). While not the highest in New England (MA is $0.28-$0.32), NH's colder temperatures mean more heating hours. Even with defrost and aux heat costs, a heat pump in southern NH saves 35-50% compared to oil heating at $3.60/gallon.
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