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A generator of 15000-25000 watts is required to run an electric furnace, while 800-2500 watts will be just fine for an oil or gas furnace. It all comes down to how much electricity your fan (or blower) uses. 

You can safely operate your gas furnace on a generator as long as you follow all the safety measures and test it beforehand. You’ll also need a generator that is capable of powering and running the gas furnace, as well as the necessary accessories like power cords and fuel.

In this article, I will cover what size generator you need to run your furnace in case of an emergency and also what is the most practical approach to backing up your furnace.

Will a generator run a furnace?

A generator should be able to power a furnace as long as there is enough output wattage and fuel supply (or battery). If your furnace demands lots of electricity (electric furnace or a heat pump), you may want to use a large standby generator.

A small battery-powered generator with an inverter could be utilized as a power backup for your gas or oil furnace. As you probably know, not all generators can be placed indoors, but if it’s a solar generator or a power station, there are no fumes involved and they could easily be placed next to your furnace to keep it going during the power outage.

When trying to connect your furnace to the generator make sure to follow all of the recommended safety procedures.

Connect generator to the furnace

Before you connect your furnace to your generator, make sure that you have a transfer switch installed if you want it to be turned on from your main power source. A professional will wire it for you and it can be either automatic or manual.

  • If you choose (or can afford) the automatic version, you don’t have to worry, and your furnace will be powered from your generator when the main power source goes out.
  • If you get a manual transfer switch (which is a very popular choice), you will have to flip it before generator power can be used.

You can also use extension cords that could be connected directly to your generator (this is in case you use a dedicated small generator with an inverter and a battery).

** Warning! Even though it looks simple, the generator should be installed by a professional only!

How big of a generator do you need to run a furnace?

If you use an electric furnace, though, you’ll need a considerably more powerful generator. Oil furnaces that use less than 1,000 watts can be powered with 3,500-watt generators, but electric furnaces and heat pumps can quickly utilize over 15,000 watts.

Don’t forget that generators have two types of ratings:

  1. Surge watts – startup watts.
  2. Running watts – rated watts.

Startup (surge) watts usually last for a few seconds to accommodate the initial motor startup, which requires more watts:

Very often generator companies only state their “Starting Watts” and you practically need to read the fine print to find “Running watts”. So, be careful and pay attention to what you are buying.

Furnace starting watts and furnace running watts

You really need to know the starting and running watts of your furnace if you want to size your generator accurately. A blower fan in oil or gas furnaces needs around 1000-2000 watts to get it started and 800-900 watts to keep it running.

Most house furnaces utilize roughly 600-800 watts (depending on their size and brand), your furnace may use more or less. More information on different types of furnaces and their wattage is mentioned below.

Will a 5000-watt generator run a furnace?

Yes, depending on the size of the furnace, a 5000-watt generator can power it. Simply ensure that you have enough capacity to operate other appliances.

Power outages occur frequently, especially during snowstorms or other weather anomalies. If your electricity goes out in the middle of a cold winter, you’ll definitely want your furnace to keep your furnace running and your house warm.

Will a 3500-watt generator run a furnace?

A 3500-watt generator will run your oil or gas furnace without a problem. Electric central air furnaces and heat pumps do require a lot more power.

Consult your papers or labels to find out the exact power usage of your unit and go from there. Make sure you watch your surge watts and get a generator that can handle them.

Can a 2000-watt generator run a furnace?

If you would like to dedicate this generator just for the purpose of running the oil or gas furnace, then yes, a 2000-watt generator can run your furnace. Only make sure it’s running and surge (or startup) watts fit within the generator boundaries.

Also, if you would like, you can keep your 2000-watt solar generator (also known as a power station) indoors. Battery-powered generators don’t produce any fumes and don’t worry, you don’t need actual solar panels to use them.

This neat little device can be plugged into an outlet and get recharged while you have electricity. If you do have solar panels, even better, they will be recharged from them.

Several small power stations around the house and you set yourself up for a warm and cozy winter outage! Use other power stations for your lights and basic appliances.

This may be a good solution if you don’t want to over complicate your life.

Electric Furnace

What size generator do you need to run an electric furnace? A BIG ONE!

You need a generator with the capability of producing at least 10,000 watts to run your electric furnace! A solid, standby natural gas generator will take care of your whole house in those cold winter months during power outages.

Only make sure it produces enough power NOT only for the furnace but for other household appliances that you will highly likely use. For example:

  • Refrigerator
  • Microwave
  • TV
  • Lights
  • Etc.

Another good tip for keeping your home warm and using less electricity is to reverse the direction of your ceiling fans (if you have them). Air circulation will be much better and your home will be much warmer.

Duct booster fans (paid link), like the one in a picture below, can be found on Amazon if anyone is interested:

All you do is place fans inside your ducts and they start blowing air through your home. Many of them come with thermostats and humidity control.

Electric furnace wattage

As you see from the table below, electric furnaces are quite power-consuming! The generator you will need in this case is well above 20,000 watts (even if your electric furnace is 15kw).

In the table below, you will see how many watts electric furnace uses:

Devices Watts (running) Watts (starting)
Central Electric Furnace (15 kW) 15,350
Central Electric Furnace (20 kW) 20,490
Central Electric Furnace (25 kW) 25,670

Oil furnace

Even though oil heaters take longer to heat a space, they deliver a more pleasurable heating experience. Furthermore, the size of the room affects cost efficiency.

And yes, oil furnaces do need electricity!

Even though they don’t use nearly as much as an electric furnace, they are reliant on electricity. Same with natural gas furnaces that use gas as the main fuel source.

Oil and Gas furnaces still need electricity, but only to get them going and after this, they start burning the fuel.

Generator for oil furnace

A typical oil-fired furnace will require more than 800 watts to operate. This means you will need a generator larger than 800-watt to run an oil furnace.

This is due to the fact that the burner and the blower will both be operating at the same time. It draws oil from the tank using a motor and then pumps it through the ignitor with a tiny pump.

Then, the oil is ignited by the ignitor, and the warm air is forced into the home by the blower. Don’t forget that the startup wattage will be a lot more than running wattage.

Oil furnace wattage (or watts)

Approximate oil furnace wattage goes as follows:

Devices Watts (running) Watts (starting)
Oil Furnace Fan Blower (1/8 HP) 300 500
Oil Furnace Fan Blower (1/6 HP) 500 750
Oil Furnace Fan Blower (1/4 HP) 600 1,000
Oil Furnace Fan Blower (2/5 HP) 700 1,400
Oil Furnace Fan Blower (3/5 HP) 875 2,350

** Note. The above numbers are approximate. The only way to find out how many watts does an oil furnace use is to get the actual numbers from the nameplate.

It is also a good idea to invest in a wattmeter (paid link) to find out exactly how many watts you are using to run an oil furnace. You can use this handy device to check how much energy your furnace really requires:

The wattmeter is also called an energy meter and it can be plugged in between your power outlet and furnace to give you real numbers. This is in case you just DON’T FEEL LIKE looking for that wattage on the nameplate 😃!

Gas furnace

What size generator to run a gas furnace? The amount of power required by your generator is determined by the power needs of your furnace’s fan!

  • ¼ HP Fan. If you have a gas furnace with a quarter-horsepower fan, your furnace will require between 1000 and 1600 watts to begin operating. The furnace will require 600 watts to keep operating once it is started.
  • ½ HP Fan. A gas furnace with a half-horsepower fan will require more than 2,000 watts to start and around 900 watts to stay running.

Gas furnace wattage (starting watts & running watts)

How many watts does a gas furnace use generally comes down to the size of the blower fan. Here are some numbers:

Devices Watts (running) Watts (starting)
Gas Furnace Fan Blower (1/8 HP) 300 500
Gas Furnace Fan Blower (1/6 HP) 500 750
Gas Furnace Fan Blower (1/4 HP) 600 1,000
Gas Furnace Fan Blower (2/5 HP) 700 1,400
Gas Furnace Fan Blower (3/5 HP) 875 2,350

Of course, the numbers above are approximate, but I am sure you’ve got the picture. You can also plug your furnace into a wattmeter and get the most accurate reading:

What size generator do you need for a gas furnace?

Most household generators have a power output of roughly 5,000 watts, which is more than enough to operate your gas heater and other essential appliances during a power failure. You can acquire a smaller generator if you only need to run your gas furnace only.

If your gas furnace is a primary concern, you can simply power it with a power station, which can be conveniently placed indoors. Good power stations (also called solar generators) are also available from Amazon.

Since this type of generator is battery-powered (it’s pretty much an inverter with a battery), you don’t have to deal with fumes and can keep a dedicated generator just for your furnace.

Here is a 1000-watt power station that should give you plenty of power to run a gas furnace from Jackery (paid link):

Power stations like these can be charged from either solar or regular outlets. Due to their battery-powered design and noiseless output, these generators are commonly kept indoors (next to a furnace) for this purpose.

They will be replenishing their battery from the outlet when you have electricity and power your gas or oil furnace during cold winter power outages. In my opinion, this is a very convenient generator for furnaces.

If your furnace is rather tiny, you can use a small generator like this as a dedicated power supply.

Heat pump

Heat pumps, just like electric furnaces, also like to utilize a lot of electricity. If you have a heat pump, you will need a much larger generator than you would for a gas or oil furnace.

What size generator will run a heat pump?

A good standby or whole-house generator of at least 15,000 watts should be able to power a heat pump. Always look at the surge rating of your generator, as it should be able to handle the initial start-up surge of your pump.

Don’t’ forget to add all of your appliances running wattages (that you will be using at the same time to the heat pump Running watts:

Devices Watts (running) Watts (starting)
Heat Pump 4,800 10,800

Add your TOTAL running watts (appliances + heat pump) to your heat pump starting watts and you will know what size generator you need to run the heat pump.

Portable generator

Electric furnaces and heat pumps require a lot of power to operate (15,000 watts or more) and as a result, a portable generator is insufficient to power these devices. On the other hand, it’s a perfect solution for oil and gas furnaces!

Portable generators (especially those that use inverter technology) will also be quieter than bigger non-inverter generators. The main advantage of having a compact generator is that it is lighter and easier to transport.

You are spending so much money anyway, so might as well make it useful for purposes other than a power failure! It can also be used for other purposes like hosting a party or supplying your shop with extra power.

Keep in mind that you’ll need a generator LARGER than your calculated power needs. The reason for this is that you never know the real power output (which can be influenced by weather and location conditions) and what if you want to add something to it at the last minute?

In the event of a power outage, a LARGER portable generator will allow you to run additional appliances along with the gas/oil furnace.

If you simply want a dedicated generator to power your furnace, you can use a power station (also called a solar generator) or any other battery+inverter combination to do so.

Power stations are safe to keep indoors since they produce no fumes. A transfer switch may be needed if you would like to have your generator as a permanent set up in your house

** Warning! Transfer switch should be installed by a professional only!

Should you use a space heater instead?

This is actually NOT a bad idea! Power outages usually do not last for a long time and why should you invest in a huge generator for that purpose. Small space heaters during those times are the perfect solution to keep yourself and your family warm.

Of course, I will not take all your power consumption down to candles, but if you have an electric furnace in your house, just having a HUGE generator to power it for a couple of hours, just may not make much sense…

What size generator do I need to run a space heater?

The size of the generator comes down to your total power consumption and the type of space heater you want to use. If you look at the heater’s label, you will be able to see how much wattage your particular model requires and size your generator accordingly.

In my opinion, having a couple of space heaters, along with heated blankets is a good (and a warm) way to go. Here are some wattages for you:

Devices Watts (running) Watts (starting)
Electric Heater 1,000 2,000
Space Heater (small) 800
Space Heater (large) 1,800
Oil Filled Radiator (750w) 750
Electric Thermal Radiator 750
Electric Fireplace (heating mode) 1,500
Electric blanket 200

How to run furnace when power is out

If your generator is powered by gasoline (like in most cases), the safest place for it is OUTSIDE. You can also dedicate a specific shelter for your generator with good ventilation and fire-resistant coating.

A small power station (also called a solar generator) could be placed indoors and be dedicated to power your furnace only. These are generally not very big units, and they get recharged from the outlet or solar panels.

If you want to use a regular generator for your furnace, make sure it’s large enough to accommodate your furnace and other power needs. Powering a whole house during the power outage usually requires a solid, stand-by generator (very commonly – natural gas).

Standby generators are kept outdoors, so no worries there, and with proper maintenance, it will serve you for a long, long time. It is safe to use a portable generator to power your gas furnace as long as you take the usual precautions when doing so.

Here are some tips on how to use your generator to run the furnace and keep your family away from harm:

  1. Stay clear of the gas. Generators can emit carbon monoxide gas that is dangerous if inhaled. They are not safe to use even when a room (or garage) is thoroughly vented by opening windows and doors.
  2. Use a carbon monoxide detector. Using a carbon monoxide monitor when running a generator will ensure that nothing gets into your living areas! Also, place your generator as far from the home as possible.
  3. Get extra fuel. If power outages are common in your area and predictable, get some extra gasoline or propane tanks to be ready for one.
  4. Charge batteries. If you’re using a battery-powered (solar) generator, make sure it’s always completely charged and stored safely so it’s ready to go when you need it.
  5. Properly sized cord. Use an outdoor-safe (good quality) cord that is capable of carrying all the wattage you need to operate your gas heater when connecting it to your generator.
  6. Parallel generators for more power. Inverter generators could be paralleled to give you a power boost during the outage (in case you need more appliances than just a furnace). You can use one to run your appliances and the other to run your furnace. Consult manufacturer’s instructions for more info.

Attention! This article is for informational purposes ONLY and is NOT a replacement for a professional advice! You will need to visit school's website for details and updates, as well as consult your local HVAC specialist for appropriate solution to your problem.


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