Size a generator for the largest heat-pump operating mode you intend to support, including any electric auxiliary heat that can run with the compressor and indoor blower. Compressor starting demand and sustained resistance-heater demand require separate checks, especially during a winter outage.
Open the Heat Pump Generator Size Calculator.
Inventory the entire heating system
The outdoor unit is only part of a ducted heat-pump system. The indoor blower, controls, and supplemental heater can be supplied through different circuits and appear on different equipment labels. Collect the outdoor-unit model, indoor air-handler model, installed heater-kit identification, and the system’s control sequence.
The electrical heater installed in your air handler may differ from the largest kit that the air-handler cabinet accepts. A brochure listing several optional heater sizes does not establish which one your home uses. Ask the HVAC installer to verify the installed stages and their actual ratings.
Also identify whether supplemental heating is electric resistance or a fuel-fired furnace. ASHRAE describes systems pairing a heat pump with a furnace or boiler in its residential space-conditioning handbook. Their electrical load schedules differ substantially from a system using resistance strips.
Write down what the backup plan promises. “Support compressor heating when conditions allow” and “maintain heating with all installed auxiliary stages” are different requirements. The generator estimate should match the stated promise.
Separate electrical input from heating capacity
Heat-pump capacity is commonly stated in Btu/h or nominal tons. Those units describe heat transfer, not the electrical watts the generator supplies. Do not convert heating output directly to generator kW and assume the resulting number is the electrical load.
Use the equipment’s electrical input data at relevant operating conditions. A variable-speed system may draw different power as compressor speed and outdoor temperature change. A single mild-weather measurement is not necessarily the upper electrical demand during a cold outage.
The indoor blower also needs an entry unless it is already included in the manufacturer’s system input figure. The same rule applies to controls or other accessories: include them once, with a clear boundary showing what each number covers.
For electric auxiliary heat, begin with the installed electrical heater ratings. Resistance heat can dominate a backup load schedule even if it operates infrequently in normal weather. Its contribution is sustained real power while the stage remains energized, so a generator’s brief starting rating cannot substitute for adequate running capacity.
Model the modes that the controls can actually request
A useful worksheet includes compressor-only operation, compressor plus permitted auxiliary stages, any emergency-heat mode, and the largest likely household overlap. Ask the installer how defrost, thermostat recovery, and outdoor-temperature controls affect stage operation on your system.
Do not assume the word “emergency” guarantees the compressor remains on or off in every system. The applicable sequence depends on the equipment and controls. Model the actual documented sequence rather than an interpretation of the thermostat label.
Controls can also permit staged auxiliary operation. If only one heater stage is supported during generator operation, that limitation must be enforced by an appropriate designed arrangement. A reminder to avoid a thermostat button is weaker than a verified control sequence, especially when the house cools during a long outage.
Any proposal to disable or restrict heat needs an HVAC assessment of available heating capacity. Electrical load reduction can be mathematically effective while leaving too little heat for the building under severe conditions.
An illustrative winter load schedule
Consider a system with an illustrative compressor and outdoor-fan running input of 3.2 kW, a separate 0.6 kW indoor blower, and two 5 kW resistance-heater stages. Suppose other essential household loads contribute 1.2 kW.
For illustration, assume the compressor start requires 8.0 kW total for the event being modeled. That is a supplied example value, not a generic prediction for a 3.2 kW heat pump.
| Supported operating mode | Running demand including other essentials | Compressor-start event if applicable |
|---|---|---|
| Compressor, blower, essentials | 3.2 + 0.6 + 1.2 = 5.0 kW | 8.0 + 0.6 + 1.2 = 9.8 kW |
| Same loads plus one heater stage | 10.0 kW | 14.8 kW |
| Same loads plus both heater stages | 15.0 kW | 19.8 kW |
| Both heater stages, blower, essentials; compressor off | 11.8 kW | Not a compressor start |
With an illustrative 20% allowance, the all-stage running comparison is 18.0 kW and the corresponding compressor-start comparison is 23.76 kW. These comparisons do not establish that a generator labeled 24 kW will perform the event; starting kVA and actual transient response still need review.
The table also shows why a generator chosen from compressor-only data can be inadequate when auxiliary heat engages. The household has not added a new appliance, but its heating system has entered a different mode.
Enter auxiliary heat without multiplying it as compressor surge
The calculator has fields for heat-pump running watts, a startup multiplier, other simultaneous loads, a sizing factor, and a percentage margin. Its formula multiplies the heat-pump field by the chosen startup factor before adding other loads.
Therefore, do not combine 10 kW of resistance strips with 3.2 kW of compressor power in the first field. Doing so would apply a compressor startup multiplier to the resistance heat too. Instead, enter the compressor load in the first field and the supported heater stages, separate blower, and household loads in the other-load field.
For the example’s all-stage start, the unrestricted arithmetic would use 3,200 W, the 2.5× startup option, and 11,800 W of other load. With a sizing factor of 1 and 20% margin, the result is (3,200 × 2.5 + 11,800) ÷ 1,000 × 1.20 = 23.76 kW.
Here, the chosen multiplier reproduces the illustrative 8 kW event. It is not evidence that every standard compressor starts at 2.5 times running watts. Likewise, the inverter or soft-start option is an estimate rather than a verified performance value.
The calculator caps other simultaneous loads at 10,000 W. Consequently, this 11,800 W example cannot be entered fully within the displayed input range. Use the hand calculation and a complete load schedule for that case rather than omitting 1,800 W to make the form accept it.
For a case within the tool’s range, model one heater stage. Other loads become 5,000 + 600 + 1,200 = 6,800 W. The same compressor start and 20% margin give (8,000 + 6,800) ÷ 1,000 × 1.20 = 17.76 kW. That is the one-stage event, so label the saved result accordingly.
Understand the tool’s extra sizing factor
The calculator offers factors of 1, 1.1, and 1.2 for broad load categories. These are planning multipliers; they are not measured load power factors and do not calculate actual apparent power from kW.
If you choose a 1.1 factor and a 20% margin, the combined multiplier is 1.32. Keep both assumptions visible so a supplier does not apply another allowance without realizing what is already included.
For real power-factor analysis, use actual electrical data. A compressor may place a substantial current requirement on the alternator during starting even when its real-power estimate looks acceptable. Cummins discusses the importance of voltage recovery and the complete starting event in its motor-starting paper.
Ask for performance at the expected pre-existing load. Starting the compressor on an otherwise unloaded generator is different from starting it while a heater stage and household essentials are already running.
Check the practical limits of a reduced-load plan
If the full heating schedule produces an impractical generator requirement, compare specific alternatives with the installer. Possibilities to evaluate include supporting fewer approved heater stages, using documented load management, or supplying a different planned heating mode.
Each alternative needs both an electrical and a heating-capacity check. Removing 5 kW from the electrical worksheet is easy; establishing whether the remaining system can protect the home through the design outage is a separate task.
Record the loads that are shed before a compressor start and when they may return. Make sure the sequence addresses restoration after a complete outage, when thermostats and other automatic loads may call at the same time.
Portable generators must operate outdoors, at least 20 feet from homes with exhaust directed away, according to CPSC guidance. Use an appropriate installed transfer arrangement for fixed household loads rather than improvised backfeeding.
What should the equipment supplier verify?
Provide the outdoor and indoor model numbers, installed heater stages, voltage and phase, electrical input data, manufacturer starting information, and the intended winter operating modes. Include the other essential loads and any documented shedding sequence.
Ask the generator supplier to verify continuous demand, compressor acceleration, current capacity, and acceptable voltage and frequency recovery. Request confirmation for the actual fuel and expected site conditions, not merely the largest output number in the product name.
Retain the final control settings and permitted heating modes with the load schedule. Review them when an air handler, heater kit, thermostat, or compressor is replaced; a system with the same nominal heating capacity can impose a different electrical load.
References
- ASHRAE: residential heat-pump and dual-fuel arrangements.
- Cummins: motor-starting capability and generator response.
- CPSC: portable-generator placement and carbon monoxide.