What Size Generator You Need to Run a Furnace and Refrigerator Together

Start with the sum of both loads, then add 50 percent

A furnace and refrigerator running at the same time need a generator rated for roughly 1.5 times their combined wattage. A typical furnace draws 3,000 to 5,000 watts when the burner ignites, and a refrigerator draws 600 to 800 watts running steady but 1,200 to 2,000 watts when the compressor first kicks in. That means you are looking at a minimum 7,000-watt generator, and 10,000 watts is safer.

The reason for the cushion is that both appliances have inrush current — a spike when they start. If your furnace and refrigerator compressor both start within a few seconds of each other, the demand can briefly exceed the sum of their individual ratings. A generator undersized for that moment will shut down or damage itself trying to meet the load.

The exact size depends on your furnace type and your refrigerator age. Older refrigerators and those with ice makers draw more. Gas furnaces are lighter loads than electric ones. The steps below show you how to find your actual numbers and size correctly.

Key Takeaways

• Your furnace nameplate lists watts or amps; multiply amps by 120 to get watts, then assume the compressor and burner do not start at the exact same moment and add 50 percent to the total.
• A refrigerator's running wattage is usually 600 to 800 watts, but the compressor inrush can be 1,200 to 2,000 watts — check the manual or the label inside the fridge for the actual figure.
• A 7,000-watt generator is the practical floor; 10,000 watts removes most risk of shutdown or damage during simultaneous startup.
• Furnace type matters: gas furnaces need far less power than electric ones, so a gas furnace with a refrigerator may fit in a 7,000-watt unit, while an electric furnace almost certainly needs 10,000 or more.
• Test the setup before you rely on it — run both appliances together for at least 30 minutes to confirm the generator holds steady.

Find your furnace wattage from the nameplate

Open your furnace cabinet and look for a metal label on the unit itself — not the thermostat or ductwork. The label will list electrical specifications. You are looking for either watts or amps.

If the label shows watts, write that number down. If it shows amps, multiply by 120 to convert to watts. For example, a furnace rated at 15 amps × 120 = 1,800 watts. Most gas furnaces fall between 600 and 1,500 watts; electric furnaces often run 5,000 to 15,000 watts or higher. If you cannot find the label, the furnace manual is usually in a folder taped inside the cabinet door, or you can search the model number online.

Write down the furnace wattage. You will need it in the next step.

Find your refrigerator's running and inrush wattage

The refrigerator manual or the label inside the fridge (usually on the back wall or under the crisper drawer) will list both running watts and compressor inrush watts. If the label is missing or illegible, check the manufacturer's website by model number, or call their customer service line — they can tell you in a minute.

Write down the compressor inrush wattage, not the running wattage. That is the number that matters for generator sizing. If the label only shows running watts, assume the inrush is roughly double that figure. A typical refrigerator compressor inrush is 1,200 to 2,000 watts.

Older refrigerators and those with ice makers or water dispensers draw more. If your refrigerator is over 15 years old, assume the high end of the range — 2,000 watts — unless you can confirm otherwise.

Add the loads and multiply by 1.5

Take your furnace wattage and your refrigerator compressor inrush wattage. Add them together. Then multiply the total by 1.5 to account for the possibility that both appliances start within a few seconds of each other.

Example: A gas furnace at 1,200 watts plus a refrigerator compressor inrush of 1,800 watts = 3,000 watts. Multiply by 1.5 = 4,500 watts minimum. A 5,000-watt generator would work, but a 7,000-watt unit is safer.

Another example: An electric furnace at 5,000 watts plus a refrigerator at 1,800 watts = 6,800 watts. Multiply by 1.5 = 10,200 watts. You need a 10,000-watt or larger generator.

If your calculation lands between standard generator sizes, round up. Generators come in 2,000-watt increments: 5,000, 7,000, 10,000, 12,000, 15,000 watts and higher. Do not buy the smaller size hoping it will work.

Account for furnace type and startup sequence

Gas furnaces are the lightest electrical load — they use electricity only for the blower motor and control circuits, not for heat. Electric furnaces use resistance heating and draw far more. If you have a gas furnace, you can often get away with a smaller generator than the calculation suggests. If you have an electric furnace, you may need to go larger.

Some generators have a soft-start feature that staggers the inrush current when a compressor starts. If your generator has this feature, you can subtract 10 to 15 percent from your calculated size. Check the generator's manual or product page to see if it includes soft-start technology.

In practice, the furnace and refrigerator rarely start at the exact same moment. The furnace cycles on and off based on temperature; the refrigerator compressor cycles based on internal temperature. If you run the generator at full load for other reasons — space heaters, well pumps, or other appliances — the risk of simultaneous startup rises, and you should stick to the larger size.

Test the setup before you depend on it

Once you have the generator, connect it to your furnace and refrigerator using a transfer switch or heavy-duty extension cords rated for the wattage. Never plug a generator directly into a wall outlet — that creates a fire and electrocution hazard. A licensed electrician can install a transfer switch; it costs $500 to $2,000 but is the safe, legal way to connect a generator to your home.

Run both appliances together for at least 30 minutes. Watch the generator's load meter or listen for any change in engine speed or strain. The generator should run smoothly without fluctuation. If it shuts down, surges, or sounds labored, the unit is undersized and you need a larger one.

Check the furnace and refrigerator after the test. Both should operate normally — no unusual noise, no tripping of breakers, no dimming of lights if you have other loads on the generator. If everything runs steady, you have the right size.

Frequently Asked Questions

Can I run a furnace and refrigerator on a 5,000-watt generator?

Only if your furnace is very small (under 1,000 watts) and your refrigerator is efficient (under 1,500 watts inrush). Most combinations exceed 5,000 watts when both start. A 7,000-watt generator is the practical minimum for most homes.

What if I add other appliances like lights or a TV?

Add their wattage to your total. A typical LED light uses 10 to 15 watts; a TV uses 50 to 200 watts. These are small compared to furnace and refrigerator, but they add up. If you plan to run multiple appliances, increase your generator size by another 1,000 to 2,000 watts.

Does a portable generator work the same way as a standby generator?

Yes, the wattage calculation is identical. Portable generators are smaller and less expensive but require manual setup and fuel management. Standby generators are permanently installed and start automatically during an outage. Both need the same sizing logic based on your appliance loads.

What happens if the generator is too small?

The generator will either shut down automatically to protect itself, or it will strain and overheat, shortening its lifespan. In rare cases, an undersized generator can damage the appliances it is powering. Oversizing is always safer than undersizing.

Should I buy a generator larger than my calculation suggests?

Going one size up (from 7,000 to 10,000 watts, for example) is reasonable insurance and costs only a few hundred dollars more. A generator running at 50 to 70 percent of its rated capacity is more efficient and lasts longer than one running at 90 percent capacity all the time.