Views: 0 Author: Site Editor Publish Time: 2025-08-13 Origin: Site
You need a generator that matches both the running and starting watts of your furnace. Most gas and oil furnace fans use between 300 and 2350 watts when starting, depending on motor size. Check your furnace label or manual for exact numbers. Use the table below for a quick estimate before you begin your cost analysis.
Furnace Fan Motor Horsepower | Starting Watts (Approx.) | Running Watts (Approx.) |
|---|---|---|
1/8 hp | 500 W | 300 W |
1/6 hp | 750 W | 500 W |
1/4 hp | 1000 W | 600 W |
1/3 hp | 1400 W | 700 W |
1/2 hp | 2350 W | 875 W |
Tip: Gather your furnace model details to make your What Size Generator to Run a Furnace: Cost Analysis Breakdown accurate.
Find out your furnace’s running and starting wattage. This helps you choose the right generator size. Add up the running watts for all your appliances. Also, include the highest starting watts in your total. Add 10-20% more to your total wattage. This keeps your generator safe and lets you add more later. Pick the generator type that matches your home and power needs. You can choose portable, standby, or inverter. Think about the cost to buy, install, and use your generator. This helps you know the full price. Use a transfer switch for safe and easy power changes. This lets your furnace and other appliances work safely. Take care of your generator with regular maintenance. This keeps it working well and helps it last longer. Think about what power you might need later. Pick a generator with extra power to avoid spending more money later.
When picking a generator, you need to know how much power your furnace uses. Each furnace type uses electricity in its own way.
Furnace Type | Typical Running Wattage (Watts) | Typical Startup Wattage (Watts) | Notes |
|---|---|---|---|
Gas Furnace | 400 - 800 | 600 - 1,600 | Uses electricity for blower and ignition; startup wattage can be 2-3x running. |
Oil Furnace | 800 - 1,200 | 1,600 - 3,600 | Needs more power for burner and blower; startup wattage can be 2-3x running. |
Electric Furnace | 10,000 - 20,000 | 12,500 - 25,000 | Uses electricity for all heating; startup wattage is about 25-50% higher. |
Gas furnaces heat your home with natural gas. They only need electricity for the blower and to start up. Most gas furnaces use between 400 and 800 watts when running. When starting, they need a bit more power, usually 600 to 1,600 watts.
Oil furnaces use more electricity than gas ones. They need power for both the burner and the blower. These furnaces usually run at 800 to 1,200 watts. When starting, they can use 1,600 to 3,600 watts.
Electric furnaces use the most electricity. They make all the heat using power. Most electric furnaces need 10,000 to 20,000 watts to run. When starting, they can use 25% to 50% more, so sometimes up to 25,000 watts.
You have to think about both running and surge watts. Running watts are what your furnace uses when it is working normally. Surge watts, or startup watts, are the extra power needed when the furnace turns on. Furnaces with induction motors, like blower fans, need much more power to start. The surge can be 3 to 7 times higher than running watts. This extra power only lasts a few seconds, but your generator must handle it.
Tip: Pick a generator that can handle the highest wattage your furnace will use, even during startup.
You can figure out your furnace’s wattage by doing these things:
Find the label or nameplate on your furnace. It should show the highest wattage or amps.
Look in your owner's manual or the spec sheet for wattage details.
If you see only amps and volts, multiply them to get watts. For example, 10 amps × 120 volts = 1,200 watts.
Search online for your furnace model’s specs on the maker’s website or energy sites.
Ask an HVAC expert if you still cannot find the info.
Knowing the exact wattage helps you pick the right generator and keeps you from overloading it.
Picking the right size generator is very important. Your generator must power your furnace and other things you need. You have to check the total watts for everything you want to use during a power outage. Here are some steps to help you figure out the size.
To find out how big your generator should be, do these steps:
List all appliances you want to power. Write down each one and its running watts.
Find the highest starting watts. Some things, like your furnace, need more power to start. Look for the biggest starting watts on your list.
Add up the running watts. This is the total running load.
Add the highest starting watts to the total running watts. This makes sure your generator can handle the extra power needed when things turn on.
Here is an easy formula:
Total Generator Size (Watts) = Total Running Watts + Highest Starting Watts
Let’s look at an example. You want to run a refrigerator (700W running, 220W starting), a television (500W), a furnace fan (800W running, 2350W starting), and some lights (450W). Add the running watts:
Appliance | Running Watts | Additional Starting Watts |
|---|---|---|
Refrigerator | 700 | 220 |
Television | 500 | 0 |
Furnace Fan (1/2 HP) | 800 | 2350 |
Lights (6x75W) | 450 | 0 |
Total running watts = 700 + 500 + 800 + 450 = 2,450W
Highest starting watts = 2,350W (Furnace Fan)
So, you need 2,450 + 2,350 = 4,800W.
Tip: You can use a generator sizing calculator online to help with these numbers.
You should add a buffer margin to your total wattage. This helps your generator handle sudden power jumps. It also keeps your generator from working too hard. Experts say to add 10-20% more to your total.
If your total is 4,800W, add 10% (480W) to get 5,280W.
For a 20% buffer, add 960W for a total of 5,760W.
Running your generator below its top power helps it last longer. It also lets you add more things later if you need to.
Note: Try to keep your generator running at no more than 80% of its rated power for best results.
If you want to use many things at once, you must count all of them. Write down every appliance you want to use, like your furnace, fridge, and lights. Add up all the running watts, then find the highest starting watts. This way, your generator can handle everything, even if many things start at the same time.
If you have an automatic transfer switch, your generator will power all the things you picked when the power goes out. You cannot choose which ones to run at that moment. That is why you must size your generator for the biggest load you think you will use.
Here are some examples:
For a few important things (like a fridge, small furnace, and a window AC), a 4,500-watt portable generator usually works.
For most things in a small home (without central air), an 8,000-watt portable generator is a good choice.
For whole-house power (with central air), you may need a standby generator between 15,000 and 18,000 watts.
Tip: Always include the highest starting wattage in your calculation, especially for things with motors like furnaces and air conditioners.
If you follow these steps, you can pick the right size generator for your furnace and your home.
Picking the right home generator size keeps your furnace working in a blackout. Think about how big your house is. Also, think about what kind of furnace you have. Count how many things you want to power. The table below shows how much generator power different home sizes need. This helps you know if you want backup for just your furnace or your whole house.
Home Size (sq ft) | Recommended Generator Wattage Range | Coverage Description |
|---|---|---|
Under 2,000 | 10,000 to 15,000 watts | Covers important things like furnace, lights, fridge, and medical devices |
2,000 to 3,000 | 15,000 to 22,000 watts | Runs most things like A/C, sump pumps, and home systems |
3,000 to 5,000 | 22,000 to 32,000 watts | Can handle ovens, water heaters, more HVAC, and big appliances |
Over 5,000 | 36,000 watts or more | Good for big homes with pools, hot tubs, and EV charging |
Portable generators give you power for a short time. You can move them around your house. They are good for small homes or if you only need to run a furnace and a few things.
Pros:
Easy to move and put away
Costs less than other types
Good for short use or emergencies
Cons:
You must start and fill them by hand
Not much power (2,000-10,000 watts)
Makes a lot of noise
Not good for powering a whole house
Portable generators work best for homes under 2,000 square feet. You can use them for your furnace, lights, and fridge. You need to watch the fuel and start them yourself.
A home standby generator connects to your house’s power system. It turns on by itself when the power goes out. Most use natural gas or propane. This type is the most common for furnace backup in homes.
Pros:
Starts by itself with a transfer switch
Gives lots of power (7,000 to 50,000+ watts)
Uses natural gas or propane, no need to fill it
Runs quietly with sound covers
Cons:
Costs a lot to buy and set up
Needs a pro to keep it working well
Stays in one place, not easy to move
A home standby generator is best for big homes and whole-house power. You can run your furnace, air conditioning, and all big appliances. You do not need to worry about filling it during long blackouts.
Inverter generators make clean and steady power. They are quiet and save fuel. You can use them for computers and small things.
Pros:
Runs quietly
Saves fuel
Safe for computers and electronics
Cons:
Not much power (usually under 7,000 watts)
You must start it by hand
Not good for powering a whole house
Inverter generators are good for small homes or apartments. You can use them for your furnace, computers, and lights. They do not work for big homes or electric furnaces.
Tip: Always pick a home generator size that fits your house and furnace. For whole-house power, choose a home standby generator with enough watts for everything.
When you start looking for backup power, you need to think about upfront costs. These costs are the price of the generator and any extra parts you might need. The kind and size of generator you pick will change how much you spend.
Here is a table that shows the average upfront costs for generators that can run a furnace in a normal home:
Generator Type | Power Output (kW) | Unit Cost Range (USD) | Installation Cost Range (USD) | Total Estimated Cost (USD) | Notes |
|---|---|---|---|---|---|
Portable Generators | N/A | $500 - $2,500 | None | $500 - $2,500 | Not good for long-term or whole-home use |
Standby Generators | N/A | $3,000 - $10,000 | $1,000 - $5,000 | $4,000 - $15,000 | Stays in place, powers whole house and furnace |
7-10 kW Generators | 7-10 | $2,000 - $3,500 | Included in standby range | N/A | Good for small homes |
20-22 kW Generators | 20-22 | $4,000 - $5,500 | Included in standby range | N/A | Good for medium or big homes |
Generac Guardian 22kW | 22 | ~$4,500 | ~$2,000 | ~$6,500 | Example of a whole-home standby generator |
You can see portable generators cost less at first. Standby generators cost more, but they give you more power and are easier to use. For a 2,000 square-foot house, a standby generator (16-18 kW) usually costs $8,500 to $12,000 with installation. This price covers electrical work, gas lines, permits, and licenses.
A standby generator can also raise your home’s value by about 60% of what you pay to install it. This is a smart choice if you lose power a lot.
When you figure out what size generator to run a furnace, always count both the price of the unit and the cost to install it. This helps you know your total cost.
Installation costs can change a lot depending on the generator, your house size, and what fuel you use. You need to plan for these extra costs when you decide what size generator to get for your furnace.
Here are some main things that change installation costs:
Generator size and power: Bigger homes need bigger generators, which cost more to put in.
Generator type: Standby generators cost more to install than portable or inverter ones because they need to be wired in and set up for good.
Fuel type: If you already have a natural gas line, it costs less. If you use propane, you might need a tank, which costs more.
Location and access: Hard-to-reach places make the job harder and cost more.
Electrical panel upgrades: Old panels may need to be changed to work with the new generator.
Permits and inspections: Local rules can add extra fees.
Site prep: You might need a concrete pad or yard work.
Transfer switch installation: Needed for safe power, and the price can change.
Fuel line installation: New gas lines or tanks add to the bill if you do not have them.
Labor costs: These change based on where you live and how hard the job is.
Brand and model: Fancy brands usually cost more to install.
Factor | Impact on Installation Cost |
|---|---|
Generator Size & Power Output | Bigger homes need higher kW generators, so installation costs go up (for example, 20kW for big homes costs more than 7kW for small homes). |
Generator Type | Standby generators cost more to install than portable or inverter types because they stay in place and turn on by themselves. |
Fuel Type | Natural gas generators cost less if you already have a gas line; propane or diesel need extra fuel storage, which costs more. |
Location & Accessibility | Hard-to-reach spots make labor and materials cost more. |
Electrical Panel Upgrades | Old or small panels may need upgrades, which adds to the price. |
Permits & Inspections | Local permit fees are different everywhere and can add to the total cost. |
Site Preparation | Concrete pads, yard work, or grading may be needed, which costs more. |
Transfer Switch Installation | Price changes based on generator type and size; needed for safe power. |
Fuel Line Installation | New gas lines or propane tanks add to the cost if you do not have them. |
Labor Costs | Prices change by area and how hard the job is; cities may cost more. |
Brand & Model | Fancy brands and models usually cost more to install. |
When you look at what size generator to run a furnace, remember that installation costs can be as high as the generator itself. Always ask for a full quote before you begin.
After your generator is set up, you need to think about how much it costs to use. The cost to run it depends on the fuel, the size of the generator, and how much power you use.
Natural gas is usually the cheapest fuel. It costs about $0.02 to $0.05 for each kilowatt-hour (kWh). A 20kW generator running at full power uses about $3 to $6 of natural gas every hour. Propane costs more, about $0.08 to $0.12 per kWh, so a 20kW generator can use $5 to $10 per hour. Diesel generators cost even more but are good for big jobs.
Here is a table that shows how much fuel different generator sizes use and what it costs to run them on natural gas:
Generator Size (W) | Fuel Consumption at 25% Load (ft⊃3;/hr) | Cost at 25% Load ($/hr) | Fuel Consumption at 50% Load (ft⊃3;/hr) | Cost at 50% Load ($/hr) | Fuel Consumption at 75% Load (ft⊃3;/hr) | Cost at 75% Load ($/hr) | Fuel Consumption at 100% Load (ft⊃3;/hr) | Cost at 100% Load ($/hr) |
|---|---|---|---|---|---|---|---|---|
1,000 | 1.86 | 0.02 | 3.72 | 0.04 | 5.57 | 0.06 | 7.43 | 0.08 |
2,000 | 3.72 | 0.04 | 7.43 | 0.08 | 11.15 | 0.12 | 14.86 | 0.16 |
3,000 | 5.57 | 0.06 | 11.15 | 0.12 | 16.72 | 0.18 | 22.29 | 0.24 |
4,000 | 7.43 | 0.08 | 14.86 | 0.16 | 22.29 | 0.24 | 29.72 | 0.32 |
5,000 | 9.29 | 0.10 | 18.58 | 0.20 | 27.86 | 0.30 | 37.15 | 0.40 |
A 5,000-watt generator running at 60% power uses about 22.29 cubic feet of natural gas each hour. This costs about $0.24 per hour. If you only run your furnace and a few lights, you can keep your fuel costs low.
A 20kW natural gas generator uses about 204 cubic feet of gas per hour at half power. This costs about $2.50 per hour. At full power, it uses about 301 cubic feet per hour, costing around $3.65 per hour. If you use a bigger generator, your fuel costs will go up.
Tip: Only run the things you really need during a power outage. This helps you save fuel and keeps your costs down.
When you plan what size generator to run a furnace, always count fuel and maintenance costs in your budget. Maintenance, like oil changes and new batteries, can add $500 each year for home standby generators. If you have a commercial generator, you might spend $1,000 to $2,000 or more every year.
You can see that the price of the generator is only part of the total cost. When you look at what size generator to run a furnace, you need to add up the upfront, installation, and running costs to see the whole picture.
Taking care of your generator helps it work when you need it. You need to do regular maintenance so it does not break down. Each generator type needs its own kind of care. Some jobs you can do yourself. Other jobs need a professional.
Common Generator Maintenance Tasks:
Change the oil and oil filter
Replace air and fuel filters
Inspect and clean spark plugs
Check battery health and charge
Test the transfer switch
Run the generator for short periods to keep parts moving
Inspect fuel lines and connections
Clean cooling vents and remove dust
Check for leaks or corrosion
Follow the schedule in your owner's manual. Most companies say to check your generator every month. You should do a full service once or twice a year.
Average Annual Maintenance Costs by Generator Type
Generator Type | DIY Maintenance Cost (USD) | Professional Service Cost (USD) | Typical Tasks Included |
|---|---|---|---|
Portable Generator | $50 - $150 | $150 - $300 | Oil change, filter replacement, spark plug cleaning |
Standby Generator | $100 - $250 | $300 - $700 | Full inspection, battery test, transfer switch check, fuel system cleaning |
Inverter Generator | $40 - $120 | $120 - $250 | Oil change, air filter cleaning, spark plug check |
Tip: If you use your generator a lot, do extra maintenance. Keep a log to remember service dates and repairs.
You can save money by doing easy jobs yourself. Hire a professional for electrical checks and fuel system work. If you skip maintenance, your generator might stop working during a power outage. You could also lose your warranty.
Plan for maintenance costs when you pick your generator. Regular care helps your generator last longer and stay safe. If you plan ahead, you will not be surprised. Your furnace will work when you need it most.
Picking the right fuel type helps your furnace work well. You can choose gasoline, diesel, propane, or natural gas. Each fuel type has good and bad points.
Gasoline is easy to buy. It works for small portable generators. It is best for short emergencies. Gasoline costs more to use. It does not last long in storage. Diesel fuel is more efficient. It lasts longer than gasoline. Diesel generators are good for big homes and long emergencies. They need less maintenance. They give steady emergency power. Diesel generators are noisy and cost more at first.
Propane burns cleaner than gasoline or diesel. It stores well for a long time. You can use propane with portable or standby generators. Natural gas is the cleanest fuel. It connects right to your home’s supply. You do not need to refill tanks. This makes it great for backup power all the time. Natural gas may not be in rural areas. Local rules and space for fuel storage matter too. Think about how easy it is to get fuel, the cost, and how often you will use your generator.
Tip: Always check local rules before picking a fuel type. Do a life cycle cost analysis. This helps you find the safest and cheapest way to power your furnace in an emergency.
A transfer switch is very important for your generator setup. It lets you safely switch your furnace and other appliances to generator power. Without a transfer switch, you can only use devices with regular plugs. You would need extension cords. This is not safe and can be a hassle.
A licensed electrician puts the transfer switch near your main electrical panel. The switch lets your generator power your furnace, water heater, and well pump. It stops backfeeding. This keeps utility workers and your home safe. Installing a transfer switch costs $500 to $1,500. It usually takes less than one day. Plan early to get an electrician before storms or emergencies.
Benefits of a Transfer Switch:
Powers hardwired appliances safely
Stops dangerous backfeeding
Makes switching power easy
Keeps your furnace running in a power outage
Note: Test and maintain your transfer switch and generator often. This makes sure you have emergency power when you need it.
Professional installation keeps your furnace and generator safe. Experts follow local codes and use the right tools. This helps avoid problems like gas leaks or carbon monoxide. Hiring a licensed installer protects your warranty. It also helps you avoid costly mistakes.
Professionals know where to put your generator. They make sure it has good ventilation and safe fuel storage. They help you pick the best system for your home. Whole-house standby generators connect to natural gas lines. They start by themselves during an emergency. You do not need to start or refill them by hand.
Routine professional maintenance keeps your generator ready. Local rules need permits and inspections. This makes sure your system is safe. Always follow the manufacturer’s instructions and local codes. This helps you avoid fines and safety problems.
Tip: Ask your installer about adding more circuits later. You may want to upgrade your generator as your needs change.
When you pick a generator for your furnace, think about what you might need later. Your house could change as time goes on. You might build new rooms or get more family members. You could also buy new appliances. All these things can make you use more electricity.
A lot of people forget to plan for bigger power needs. If your generator only fits what you need now, you might have to buy a new one soon. You can stop this problem if you plan ahead.
Here are some reasons your power use might go up:
You add a new room or finish your basement.
You get big appliances like a hot tub, electric car charger, or a new HVAC system.
Your family gets bigger, so you use more lights and electronics.
You upgrade your furnace or add air conditioning.
Each of these things can make your total wattage higher. If your generator is too small, it might not work for everything. You could lose power to important things during an outage.
Experts say you should pick a generator with extra power. Add 20-30% more than what you use now. This extra space helps your generator handle new things and surprises. For example, if you need 8,000 watts now, look for a generator that gives at least 9,600 to 10,400 watts.
Tip: Always think about the future when you choose a generator. A bigger generator costs more at first, but it saves you money and trouble later.
Here are some good things about planning for more power:
You do not have to buy a new generator soon.
You can add new appliances without worrying about too much power.
Your generator works better with some extra space.
You are ready for changes in your house or family.
Big houses need generators with more watts. If you want to make your house bigger, pick a generator that can run more things. This way, your furnace keeps working and you stay comfortable during blackouts.
You should talk to a pro installer about your plans. Tell them if you want to add new rooms or devices. The installer can help you pick a generator that works for you now and later.
If you plan for more power, you will feel better. You know your generator will work as your house grows. You protect your money and keep your furnace running when you need it most.
When you pick a generator for your furnace, first write down all the important things you want to power and how many watts they use. Figure out how much power each one needs to run and to start. Add at least 20% more power as a safety margin. This keeps your generator from getting overloaded and lets you add more things later. Think about how much the generator costs to buy, set up, and keep working. If you are not sure what to do or your setup is tricky, ask an expert for help. Before you start, find your furnace details and make a list of everything you need to power.
You can check the label on your furnace or look in the owner’s manual. If you see amps and volts, multiply them to get watts. For example, 10 amps × 120 volts = 1,200 watts.
Yes, a portable generator can run most gas or oil furnaces. Make sure the generator’s wattage matches your furnace’s starting and running needs. Always use a transfer switch for safety.
A generator that is too small may overload and shut off. Your furnace and other appliances may not work. You could damage your generator or connected devices.
Yes, you need a transfer switch to safely connect your furnace to a generator. It prevents backfeeding, which can harm utility workers and your home’s wiring.
You should check your generator every month. Change the oil and filters at least once a year. If you use it often, follow the maintenance schedule in your manual.
Yes, you can run other appliances if your generator has enough wattage. Add up the running and starting watts for all devices. Do not exceed your generator’s rated power.
Natural gas works best for most homes. It connects to your home’s supply and does not need refilling. Propane is a good backup if natural gas is not available.
Add a buffer of 10% to 20% above your total wattage needs. This helps your generator handle power surges and keeps it from running at full load all the time.
