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What Size Generator to Run a Furnace? Gas and Electric Furnace

In this article, we have discussed what size generator is required to run a gas or electric furnace in your home. We have provided the necessary calculations to determine the generator size on your own.

If one of your primary reasons for purchasing a generator is to operate a furnace, there are a lot of things to be considered including the size of the generator.

Running a furnace on the generator depends on the starting and running watts and size of that capacity, and power consumption of other appliances you are going to run simultaneously.

Typically, an 1800 to 2000 starting watts generator should be enough to run a gas furnace with a small blower fan, control board, and small inducer motor. Large electric furnaces like 8KW or 10KW would need at least a 12KW to 13KW generator to cover the starting watts and amps.

A gas furnace only requires power to run the inducer motor, draft blower fan, and control board whereas an electric furnace needs the power to run the entire unit.

How Many Watts Does a Furnace Use?

Before getting a generator mainly for a furnace, you would need to calculate how many watts your furnace draws.

Typically, most home furnaces are in the ranges of 3000 watts to 5000 watts. It means operating at 20amp at 240V, or 12amps at 120V circuit.

The simplest way to calculate the furnace power requirement is to check the manual, or specification sheet on the furnace.

Suppose the manual is saying your furnace draws 20amps at 240V. Then the formula to calculate the wattage is:

Watt = Volt x Amps

Watts = 240V x 20amp

Watts = 4800watts

That is just a rough calculation. Let’s be precise. For that, we would need separate calculations for gas and electric furnaces because one runs entirely on electricity, and the other needs power to run only the inducer motor, control board, and blower fan.

We would also need to calculate the power draw of the blower draft fan.

Starting and Running Watts of Blower Fan

Fan size  Starting wattsRunning wattsWatt-hour
¼ HP (0.25)1100 to 1200350 to 4000.4kWh
1/3 HP (0.33)1700 to 1800550 to 6000.6kWh
½ HP (0.5)2300 to 2400750 to 8000.8kWh
1 HP4700 to 48001500 to 16001.6kWh

While starting watts of the fans remain the same because they need some extra bit of power to overcome inertia to start, running watts may vary. If the fan operates at low speed, the running watt requirement may drop to 20 to 30%.

What Size Generator Required for a Gas Furnace?

What Size Generator to Run a Furnace

The blower draft fan requires the most electricity in gas furnaces followed by the inducer fan and control board.

Other things like igniter, lights, and sensors require minimum energy to operate. Let’s give you an example.

Q1. Suppose a ¼ (0.25)HP blower fan requires 1200 starting watts and 400 running watts, and the inducer motor requires as low as 100W to operate. What will be the size of the generator for the gas furnace of 0.25HP blower fan?

Ans. Blower starting watts + blower running watts = 1200 + 400 = 1600

Add the power required by the inducer motor to the total= 1600 + 100 = 1700

You would need a 1700+ watts generator for your ¼ HP gas furnace.

Generator SizeBlower Fan  
 2000 watt¼ (0.25) HP
 2500 to 3000 watt1/3 (0.33) HP  
 3500 to 4000 watt½ (0.5) HP
 6500 to 8000 watt1 HP

What Size Generator Required for an Electric Furnace?

From the heating element, blower fan, and transformer, to the control board and electronics, everything in an electric furnace requires electricity to operate.

You would need to see the manual to check how many watts the heating element draws. Don’t worry it doesn’t require starting watts since there is no motor involved in the heating element, but the motor does require starting watts.

Let’s give you an example.

Q1. A heating element of an electric furnace require 5KW to operate, and a blower fan requires 1200 starting watts and 400 running watts to operate. What size generator is required to run the electric furnace?

Ans. Starting watts of blower fan + running watts = 1200 + 400 = 1600

Add the total to the heating element power requirement = 1600 + 5000 = 6600 watts.

You would need a 6600 to a 7000-watt generator to run your electric furnace.

Electric Furnace (20KW) Generator SizeBlower Fan  
22000 watt¼ (0.25) HP
 23000 watt1/3 (0.33) HP  
 24000 watt½ (0.5) HP
 27000 to 28000 watt1 HP

Why Inverter Generators are Preferred for Furnaces?

A generator that provides clean power is required to run gas or electric home furnaces. If your generator’s harmonic distortion is high, the fluctuations in power can damage the furnace.

Some examples of quality inverter generators are Honda EU2200i, Champion 2000W, and Westinghouse 2000W.

Will a Generator Damage My Furnace?

Fluctuations in power can damage the electrical components of a furnace, especially a blower motor. Fluctuations in power also reduce the furnace’s efficiency.

I am a passionate environment engineer, who has been in the solar, and inverter generator industry for years. I have been a part of multiple safety and environment-related courses such as IOSH, Fire Safety, Solid Waste Management, etc. I have been installing solar plants for residential purposes since 2018.

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