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What Size Generator Do You Need To Run a Furnace?

There are many factors to keep in mind when choosing a generator for a furnace. You’ll have to think about your furnace’s size and capacity, its starting & running watts, and the energy consumption of other appliances you may power at the same time. 

A 2,000 watts generator can run a gas furnace using a small blower fan, draft inducer motor, and control board. A 10-kilowatts electric furnace needs at least a 12-kilowatts generator. The real-time current or amp draw is different for electric & gas furnaces.

Gas furnaces don’t require much electricity. The only components demanding power are the blower fan, draft inducer motor, and control board. However, electric furnaces need electricity to power the entire unit. Read on to determine the right generator size for your furnace. 

How To Choose the Correct Generator Size To Run a Furnace

Salesman in red shirt and baseball cap is showing a couple a new generator in a power tools store

In order to calculate the total required wattage (W) of your furnace, you’ll need to add the starting and running watts of all the electrical and electronic components in your furnace.

After you get that number, the right generator size is ~1.2 x W. W must also include the wattages of the other appliances you may run simultaneously. 

Electricity Requirement of an Electric Furnace

An electric furnace needs power for each of these parts: 

  • Heating elements 
  • Blower fan 
  • Transformer 
  • Control board 
  • Electronics 

The heating elements have the highest wattage requirement, followed by the blower fan. The control board, transformer, and electronics have regular power needs. 

The wattage requirement of the heating elements is equivalent to their combined capacity. For example, a 5 kW heating element requires 5 kilowatts of power, and a 20 kW electric furnace needs a generator with more than 20,000 watts capacity. After getting that number, add the fan’s power needs. 

The heating elements in an electric furnace don’t have a distinct starting wattage. It’s enough to account for the running watts. However, the blower fan does have starting wattage distinct from the running watts. Blower fans furnaces are similar to those in HVAC units, whether they’re electric or gas-powered. 

Electricity Requirement of a Gas Furnace

Unlike an electric furnace, a furnace running on natural gas or propane doesn’t need the generator to account for the burners and heat exchangers. However, gas furnaces do need electricity to power the following parts: 

  • Blower fan 
  • Draft inducer motor 
  • Control board 
  • Igniter 
  • Sensors 
  • Lights 

The blower fan in a gas furnace requires the most electricity, followed by the draft inducer motor and control board. The igniter is engaged only when the furnace starts, so there is no continuous power requirement. Meanwhile, the sensors and lights need very little electricity. 

The most significant calculation for a gas furnace is determining the wattage requirement of the blower fan. This real-time energy requirement depends on its horsepower, which could be around 0.5 to 1 or higher for larger gas furnaces. Variable speed blowers that need much less power than regular fan motors. 

Calculate the Starting and Running Watts of the Blower Fan

The starting wattage of induction motors is generally 2 to 3 times its running watts. You can usually check the running watts of your furnace and blower fan on their manual or on the pieces themselves.

For example: if you have a 0.25 horsepower or 186 watts motor, the fan’s running wattage is up to 400 watts. Its starting wattage will sit between 800 to 1200 watts, depending on the model.

Here are the estimated starting and running watts for standard furnace blower fans: 

Blower Motor SizeFan’s Running Watts*Fan’s Starting WattsHourly Requirement
0.25 or ¼ HPUp to 400 WattsUp to 1200 Watts~0.4 kWh
0.33 or ⅓ HPUp to 600 WattsUp to 1800 Watts~0.6 kWh
0.5 or ½ HPUp to 800 WattsUp to 2400 Watts~0.8 kWh
1 HPUp to 1,600 WattsUp to 4,800 Watts~1.6 kWh

Note: Furnaces with variable speed fans don’t require their maximum running watts all the time. The running wattage may drop to as low as 30% to 20% when the fan operates at its slowest speed. 

Add the Wattage of Other Components in the Furnace

The second major component in a gas furnace that requires electricity is the draft inducer motor. This fan does not require as much energy as the blower, but you still have to account for at least 100 watts. Specific wattage will depend on the size of the furnace. Keep in mind the draft inducer fan needs a starting wattage of 2-3 times its running watts. 

In the end, you’re looking at at least 300 watts of starting wattage for the draft inducer motor. Add 100 watts more for the igniter, control board, sensors, lights, and transformer. Generally, most transformers will lose or waste some energy, but that gets accounted for in the total wattage. 

Overall, you need around 500 watts to start and approximately 200 watts to run the rest of the components on a gas furnace. Electric furnaces don’t have draft inducer motors, so you can exclude it from its power requirement.

The draft inducer motor in a gas furnace starts before the blower fan, so you may exclude the starting wattage of the draft inducer fan. 

The above chart already accounts for the blower fan’s starting watts. Since the blower fan has a much greater power requirement, the draft inducer motor’s starting watts are easily met within the selected capacity of the generator.

After that, you only need to add the watts of the additional parts to the starting and running wattages of the furnace blower motor. 

Total Electricity Requirement of a Gas Furnace

Before we get to the math, let’s define a few terms: 

  • Blower Fan: B (B1 + B2. B1 accounts for the starting watts, B2 for running watts). 
  • Draft Inducer Motor: D (you may consider only the rated or running watts).
  • Heating Elements: E (only counts in electric furnaces).
  • Others: O (this is also optional). 

For a gas furnace, the total required wattage is W = B + D + O. However, O does not need a separate calculation. The additional starting wattage for the blower fan is more than sufficient for all the smaller components. The net total wattage is W = B + D

Example X1 – Gas Furnace

Here’s example X1 for a gas furnace: 

  • A 0.25 or ¼ HP blower fan needs up to 1,200 watts (B1) to start and 400 watts (B2) to run. Thus, B (B1 + B2) is 1600 watts. 
  • A standard draft inducer motor in this furnace should not require more than 100 watts to run, so D is 100 watts. The starting wattage of the draft inducer is provided by B1. 
  • The result: W = B + D becomes 1600 + 100 = 1700 watts

Note: The W value in X1 is not the running wattage of a gas furnace. It is the total wattage you need to start all the electrical components. The running wattage in example X1 would be ~600 watts

Example X2 – Electric Furnace

For an electric furnace, the total required wattage is W = B + E.

The E value is the combined kW of the heating elements. You don’t need to account for D since an electric furnace does not have a draft inducer motor.

Here’s the example X2 for an electric furnace: 

  • You have a 10 kW electric furnace. Thus, E is 10 kW. 
  • A 0.25 or ¼ HP blower fan’s B is 1600 watts (from example X1). 
  • The result: W = B + E becomes 1.6 kW + 10 kW = 11.6 kW (~12 kW). 

The O value can be the same as in example X1. You can also exclude it since the blower fan’s starting watts will more than compensate for whatever energy the other components consume. This principle applies to all furnaces, whether they’re electric or gas-powered.

Choose the Appropriate Generator Size

An electric generator in a metal cupboard

The gas furnace in example X1 needs a 2,000 watts generator. However, a larger gas furnace using a 0.33 or ⅓ HP blower fan needs up to 1800 watts to start and 600 watts to run. Such a furnace may have a larger draft inducer motor, too.

In that case, the value of D is higher. However, you are already accounting for a much greater wattage requirement of the blower, so D gets factored in.

W in the case of a 0.33 or ⅓ HP blower motor is 1800 + 600 = 2,400 watts. You would need a 2,500 to 3,000 watts generator for such a gas furnace. In the same way, the generator capacity should be 4,000 watts for a 0.5 or ½ HP blower motor and 7,000 to 8,000 watts for a 1 HP fan. 

Similarly, a 40 kW electric furnace needs that much electricity to operate the heating elements. Plus, extra energy is needed to operate its larger blower motor. You can calculate this by multiplying that number by 1.2. The result is a 48 kW generator for the 40 kW electric furnace in this example. A 48 kW generator can efficiently operate a 1 HP blower motor and the 40 kW heating elements. 

Note: These generator sizes are exclusively for the furnace. You need a more powerful generator if you intend to run other household appliances at the same time. 

Here are the estimated generator sizes to sum up our previous examples: 

Blower Fan MotorSize of FurnaceGas Furnace Generator SizeElectric Furnace (20 kW) Generator SizeElectric Furnace (30 kW) Generator Size
0.25 or ¼ HP2 Kilowatts22 Kilowatts32 Kilowatts
0.33 or ⅓ HP2.5 to 3 Kilowatts23 Kilowatts33 Kilowatts
0.5 or ½ HP3.5 to 4 Kilowatts24 Kilowatts34 Kilowatts
1 HP7 to 8 Kilowatts28 Kilowatts38 Kilowatts

How To Calculate the Electricity Requirement of a Furnace

To start, you’ll need to know the volt, amp, wattage, or horsepower of the parts of your furnace. Check the owner’s manual, labels on the electrical components or the stickers on your furnace. Then you can calculate its power requirement using this watt-volt-amp calculator

For a gas furnace, your priority is the blower fan motor. Multiply the voltage and amperage to get the wattage. The value you’ll get is the rated or running watts.

The starting watts will be 2 to 3 times this value. Similarly, if you have the voltage and wattage information, you can divide the watts by volts to calculate the amperage. 

Should You Run a Furnace on a Generator?

You can run a gas furnace on a generator since its electricity requirement is regular when compared to many other appliances. Calculate the wattage correctly, and you should have no issue running a gas furnace. However, a generator may be unviable for electric furnaces. 

Will a Generator Damage My Furnace?

A generator can damage your furnace’s electrical components if it cannot provide the necessary power, especially when it comes to the blower motor. Also, fluctuations in a generator’s power supply reduce the efficiency of a furnace, affecting the longevity of a few components. 

Wattage Needs of Other Common Household Appliances

You could have a generator to be used exclusively for your furnace or a larger one for multiple household appliances. Having one generator for multiple purposes will need you to balance the entire real-time load, especially when you operate a large appliance demanding significant starting watts. 

Here are the typical wattage needs of other common household appliances: 

ApplianceRunning WattsStarting WattsComments
Light Bulb60 to 75 WattsIndoor lights
Television40 to 200 WattsPer screen type, size.
Refrigerator & Freezer700 Watts2100 WattsSubject to capacity and energy rating
Microwave600 to 1,500 WattsPer the wattage
Window AC~3,300 Watts~10,000 Watts1 Ton (12,000 BTU)
Central AC~4,000 Watts~12,000 Watts2 Ton (24,000 BTU)
Heat Pump~5,000 Watts~15,000 WattsSubject to size
Sump Pump800 to 1,100 Watts2,400 to 3,300 Watts⅓ to ½ HP sizes
Water Pump1,000 Watts3,000 Watts⅓ HP
Electric Water Heater4,000 WattsSubject to efficiency
Vacuum Cleaner700 to 1,500 Watts
Washing Machine1,000 Watts2,000Subject to capacity
Iron1,200 Watts
Hair Dryer1,250 to 1,500 WattsPer model & wattage
Dishwasher200 to 1,000 Watts500 to 2,000 WattsSubject to modes

Bottom Line

Add the running watts of the electrical components you’ll need to power with your generator. Then add the highest starting wattage among them to the total running watts. This total wattage is your minimum generator size. 

Total running watts may reach 4,000. Suppose the highest starting wattage among these components is 3,500 watts. In that case, you’d need 7,500 watts.

Multiplied by 1.2, this value is 9,000 watts. This calculation gives you a buffer, but you may be alright with an 8 to 8.5 kW generator.

For more, check out How Often To Run Generator To Keep the Refrigerator Cold?