What Size Generator Is Needed to Run a Furnace?

A generator can power a furnace and comes in handy when you have power blackouts for extended periods, especially during the cold season.

Generators are rated according to the amount of wattage they produce when running. Therefore, the size and power consumption of the furnace determines the size of the generator to pick.

Besides, the type of generator you pick also plays a role when determining the size. In this article, I will discuss a few steps to determining the size of the generator you need to power your furnace.

Steps for Determining the Size of the Generator to Pick

What Kind of Furnace Do You Have?

An electric furnace requires between 15,000 and 25,000 watts to run. However, a gas or oil furnace requires 800 to 2500 watts to run. The primary determinant of energy consumption is the amount of power that a blower or fan uses.

If you run a central HVAC system, it is good to determine the maximum power consumption when all vents or outlets are working so that you have enough power even when multiple rooms are being warmed.

Determine the Startup and Running Watts

There are two types of generator ratings: surge wattage and running wattage. The surge watts are also startup power and last a few seconds to power the motor when it starts. At startup, motors consume a lot of power and, at times, half of it to run the appliance in question.

Running watts are the amount of power the generator will provide to the motor after starting. Most generator companies hide the running watts in the fine print, and you need to check carefully when making a choice.

On average, an electric furnace running a ¼ hp fan requires about 1600 watts of surge power and about 700 watts of running load wattage.

On the other hand, you would need about 1000 watts of surge wattage and about 700 watts of running load wattage. This may vary according to the size of the furnace.

Using the above example, you would need a generator that produces at least 2500 watts to cover the power needs effectively.

Determine the Type of Generator to Pick

You have a choice of petrol or solar generators for your furnace. A solar generator, also called a power station, can be kept indoors as it does not produce any fumes. You can also have a battery-operated power backup system to use with your furnace.

These do not have start-up and running loads. Instead, the maximum capacity should cater to these two load requirements.

If you pick the power backup option, ensure that the total wattage is above the maximum start-up and load wattages and can sustain the furnace for a considerable period before power resumes.

In most cases, you would need a power station that could sustain loads for at least three to five days.

Do You Need to Power Other Appliances?

Practically, you have a generator that powers other appliances. In that case, go for a generator that can power all the other appliances together with the furnace.

You will use the same calculation as discussed above to determine the maximum amount of wattage you need.

Here is an example: Suppose you have a refrigerator, a furnace, five lights, and a TV. A refrigerator has a starting wattage of 1600 and a running one of 200.

Five lights have a surge power requirement of 300 watts and a similar requirement for the running load. A TV has a similar power requirement to five lights.

The total power requirement is 3800 watts of surge wattage and 1400 watts of running wattage. In this case, a 3800 generator will do.

In some cases, you can get by with a smaller generator of as low as 2000 watts if you do not start all the appliances at the same time.

The key to getting a generator that runs your furnace is making accurate calculations of the start-up and running wattages. Consider hiring a technician if you have problems making the correct calculations.

The connection of the generator to the circuit just requires a changeover switch, which you can use conveniently. It is inexpensive to install and does not require any modifications to your circuit.


The size of the generator you can use to power your furnace depends on the power requirements of the appliance. Here is how to calculate it.

  1. What type of furnace do you have?
  2. Determine the surge and run a load of the furnace.
  3. Determine the type of generator to pick, whether a petrol-fuelled or battery-powered generator.
  4. If you wish to power other appliances alongside the furnace, calculate their total power requirements and add them to that of the furnace.

You can lower the power requirement if you do not start all the appliances at the same time.