The short answer
A gas or propane furnace uses fuel for heat and electricity only for fans, ignition, and controls. Most can run on a mid-size portable generator.
A heat pump uses electricity for the heat itself. Its compressor is a large motor with a high startup surge, usually on 240 volts, and many systems have electric backup heat strips that draw far more power than most portable generators produce. Running a heat pump on a generator is possible but requires a large generator and careful setup.
Gas furnace electrical needs
The main electrical load is the blower motor, plus the draft inducer, igniter, and control board. Running loads are modest, typically a few hundred to around a thousand watts for the blower, with a brief startup surge. The furnace runs on 120 volts.
What you need:
- A generator sized for the furnace plus other essentials, with headroom.
- Clean power, such as an inverter or a low-THD conventional generator.
- A transfer switch, interlock, or dedicated furnace transfer switch.
See our picks for the best generators for furnaces.
Heat pump electrical needs
Heat pumps have three significant electrical loads:
- Compressor: the largest load. Starting a compressor produces an inrush current well above its running current. The nameplate lists locked rotor amps (LRA), which indicates the startup demand.
- Outdoor fan and indoor air handler blower.
- Electric backup or auxiliary heat strips: resistance heaters that turn on in very cold weather or during defrost. They can draw many kilowatts.
Most whole-house heat pumps run on 240 volts, so the generator must supply 240V.
Sizing comparison
| Factor | Gas furnace | Heat pump |
|---|---|---|
| Voltage | 120V | Usually 240V |
| Main load | Blower motor | Compressor |
| Startup surge | Moderate | High (check LRA) |
| Backup heat strips | None | Often present, very large |
| Typical generator class | Mid-size inverter | Large 240V generator, often with soft start |
| Connection | Transfer switch or interlock | Transfer switch or interlock, 240V capable |
Making a heat pump work on a generator
If you have a heat pump and want generator backup, these steps help:
- Check the nameplate for running amps and locked rotor amps.
- Choose a generator with 240V output and enough surge capacity for the compressor start.
- Consider a soft-start kit. Installed by an HVAC technician, a soft starter reduces the compressor's inrush current, which can let a smaller generator start it. This is widely used for RV air conditioners and home heat pumps.
- Disable or lock out backup heat strips when on generator power, or ensure they are not on a circuit the generator powers. Ask your HVAC technician about the options for your system.
- Use a transfer switch or interlock rated for the 240V circuit.
Even with these steps, the generator must be large. Many households with heat pumps choose to back up only essentials and use a secondary heat source, such as a gas fireplace, wood stove, or pellet stove, during outages.
Alternatives for heat pump homes
- Pellet or wood stove: low electrical needs or none. See our pellet stove backup guide.
- Standby generator: a large whole-home standby unit can run a heat pump with the right sizing.
- Large battery systems: whole-home battery systems can run a heat pump for limited periods, depending on capacity.
- Space heaters on a generator: inefficient but can heat a single room.
Which is easier to back up?
The gas furnace, by a wide margin. If you are choosing a heating system and outage resilience is a priority, it is worth factoring in. Many cold-climate homeowners pair a heat pump with a gas furnace in a dual-fuel system for this and other reasons.
Mid-size dual-fuel inverter generator
$$$Best for Gas furnace homes
Clean power and enough capacity for a furnace plus essentials.
Large 240V generator
$$$Best for Heat pump homes
240V output and high surge capacity for compressors. Pair with a soft starter for easier starts.
Compressor soft-start kit
$$Best for Reducing heat pump startup surge
Professionally installed soft starters reduce inrush current. Confirm compatibility with your specific heat pump.
Run generators outdoors only, far from the house, and install CO alarms inside. Use a licensed electrician for transfer switches and an HVAC technician for soft starters or heat strip lockouts.
For the complete picture, read our winter outage heating guide.
Reading your heat pump's nameplate
The outdoor unit's data plate lists several useful numbers. RLA (rated load amps) or running amps tells you the compressor's normal draw. LRA (locked rotor amps) tells you the brief startup draw. The plate also lists the minimum circuit ampacity and maximum overcurrent protection, which describe the circuit it needs. Take a photo of the plate and share it with your electrician and HVAC technician when planning generator backup.
Dual-fuel heating systems
A dual-fuel system pairs a heat pump with a gas furnace. The heat pump handles mild weather efficiently, and the furnace takes over in very cold conditions. During an outage, you can run only the furnace side on a modest generator, which makes backup much easier. If you are replacing a heating system and outages are common where you live, it is worth discussing with an HVAC contractor.
Frequently asked questions
Can a portable generator run a heat pump?
Sometimes. It needs 240V output, enough surge capacity for the compressor, and backup heat strips disabled. A soft starter often helps.
Why is a heat pump harder to run on a generator than a furnace?
The compressor has a large startup surge and usually runs on 240V, and backup heat strips draw very large amounts of power.
What is a soft-start kit?
A device installed on the compressor that reduces startup inrush current, making it easier for a generator to start it.
What size generator runs a gas furnace?
Usually a mid-size generator, sized for the blower plus other essentials. Check the nameplate.
Should I turn off auxiliary heat on generator power?
In most cases, yes, because heat strips can overload a portable generator. Ask your HVAC technician about safe options for your system.