Power outages can leave homes without heat during cold weather, making it essential to know what size generator is needed to run a furnace. This guide helps homeowners in the United States determine the appropriate generator size for their furnace and essential loads. It covers gas and electric furnaces, how to calculate running and starting watts, and practical sizing strategies for common home setups.
Understanding Furnace Power Requirements
Furnaces rely on several electrical components that determine its power needs. The most significant loads are the blower motor, inducer, control board, and ignition system. Gas furnaces typically require modest running power but can have noticeable startup surges, while electric furnaces demand considerably more energy at runtime.
Key points to consider include the furnace’s data plate, which lists voltage, current (amps), and sometimes running watts (W) or kilowatts (kW). If a manual is available, it often provides the expected running and startup loads. For gas furnaces, typical running power ranges from a few hundred watts for the blower to under a kilowatt total, with startup surges that can exceed 1,000–2,000 watts. Electric furnaces, by contrast, use heating elements that draw many kilowatts continuously.
Practical takeaway: Always check the furnace data plate or manual for running watts and startup watts, and consider any additional loads tied to the heating system, such as central air and humidifiers.
How To Calculate Running And Starting Watts
Calculating the correct generator size involves two numbers: running watts (continuous power) and starting watts (surge power). Running watts cover the furnace’s blower, control circuit, ignition, and any fans or pumps that run continuously. Starting watts account for motor inrush when the furnace starts up.
Steps to calculate:
- Identify the furnace’s running wattage from the data plate or manual. If only amperage is listed, multiply volts by amps to get watts (Watts = Volts × Amps).
- Estimate starting watts for motors. A common rule is to multiply the running wattage of a motor by 2–3 for the startup surge, though some motors surge higher. Add other startup loads as needed.
- Sum running watts for all essential loads you want to power (furnace, thermostat, lights, refrigerator, sump pump, etc.).
- Choose a generator with nominal (running) watts at least equal to the total running watts, and with surge (starting) watts at least equal to the highest startup surge you calculated.
Example:
| Load | Running Watts (W) | Starting Watts (W) |
|---|---|---|
| Furnace Blower Motor | 350–600 | 1,000–1,800 |
| Inducer Fan | 80–150 | 350–500 |
| Furnace Control Board & Ignition | 20–100 | — |
| Thermostat & Small Electronics | 5–20 | — |
If the home’s essential loads include a sump pump (600–1,000 W running; 1,200–2,000 W starting) and a refrigerator (running 100–250 W; startup 600–800 W), factor these into the total. For gas furnaces with electric components, expect the running wattage to be on the lower end, but do not neglect startup surges. If the furnace uses electric heating elements, plan for substantially higher running watts, which may require a larger generator.
Generator Size Scenarios For Different Furnaces
Choosing the right generator size depends on furnace type, home size, and desired comfort level during outages. The following scenarios reflect common American setups and provide practical guidance.
Gas furnace with basic essential loads (small to average home): A portable generator in the 8–12 kW range is often sufficient to run the furnace, a few lights, a refrigerator, and a sump pump or well pump. For homes with central air during summer outages, a slightly larger unit may be needed.
Gas furnace with additional essentials (medium home): A 12–16 kW portable or standby generator typically covers furnace operation plus multiple essential circuits (lights, fridge, well pump, modem/router, and water heater on demand, if applicable).
Whole-house standby system (standard American home, larger loads): A 16–20 kW standby generator provides reliable coverage for furnace operation plus several high-demand circuits, with automatic transfer switching for seamless operation during an outage.
Electric furnace or heavy heating loads: Electric furnaces demand substantial power. In these cases, a generator in the 20–25 kW range (or larger) is often necessary to maintain heat while powering other essential loads. For new builds or homes with aggressive heating needs, consult a licensed electrician and consider a dedicated, appropriately sized generator and transfer system.
Note: These ranges are general guidelines. The exact size depends on the furnace’s watts (running and startup), the number of other critical circuits, and how long outages last. When in doubt, size up to provide headroom and prevent overloading the generator during startup surges.
Practical Sizing Tips And Best Practices
To optimize performance and safety, follow these practical tips for sizing and using a generator to run a furnace.
- Check the transfer method: Use a manual or automatic transfer switch to avoid backfeeding and to safely isolate the house from the grid.
- Prioritize loads: Prioritize furnace operation, refrigeration, sump pump, and lighting. If needed, run a staged load approach to reduce the required generator size.
- Evaluate headroom: Choose a generator with 20–25% extra running wattage capability to accommodate unexpected surges or additional loads.
- Consider fuel and efficiency: Standby generators are typically wired to run on natural gas or propane and maintain power automatically, but portable units rely on gasoline. Evaluate fuel availability and the generator’s runtime at 50–100% load.
- Plan for 240V needs: If your furnace or central air is 240V, ensure the generator can supply 240V or use an appropriate transfer switch and wiring configuration.
- Documentation matters: Keep the furnace’s data plate, installation manual, and the transfer switch specifications available for reference when sizing the system or performing maintenance.
- Consult a professional: An electrician or HVAC technician can verify wattage calculations, verify wiring safety, and ensure code compliance for installation and transfer switches.
Safety, Installation, And Compliance
Proper installation and safe operation are essential when using a generator to power a furnace. Outdoor placement, CO safety, and correct wiring are non-negotiable considerations.
Key safety and compliance guidelines include:
- Operate generators outdoors, well away from doors, windows, and vents to avoid carbon monoxide buildup.
- Use carbon monoxide detectors in living spaces as a precaution when a generator is in use.
- Run appropriate extension cords rated for outdoor use, or better, install a transfer switch linked to a dedicated circuit panel.
- Ensure all connections are compatible with the generator’s voltage and amperage to prevent electrical fires or equipment damage.
- Schedule professional installation and periodic maintenance to ensure reliable performance during emergencies.
In summary, choosing the right generator size to run a furnace hinges on accurately assessing running and starting watts, adding essential loads, and selecting a model with adequate headroom. For most homes with gas furnaces and moderate extra loads, a robust 8–16 kW generator—paired with a transfer switch—provides dependable heat during outages. For homes with electric heating or larger power demands, a larger generator or a standby system may be necessary. Always prioritize safety and professional guidance when planning installation and operation.