How Big Of A Generator To Run Furnace? | Real Wattage Guide

You lose power in the middle of a winter storm, and the house starts getting cold fast. The furnace won’t run, and neither will anything else tied to the breaker panel. Your first thought is probably about grabbing a generator — but guessing the size can leave you with either a machine that won’t start the blower or one that’s way more expensive than needed.

The generator size you need for a furnace depends on two factors: whether it’s a gas or electric unit, and the starting surge required by the blower motor. This article walks through the wattage ranges for each furnace type, explains how to calculate surge vs. running watts, and gives practical examples so you can pick a generator that actually works when you need it.

Knowing Your Furnace Type Matters Most

Gas furnaces and electric furnaces have drastically different power appetites. A standard gas furnace with a blower motor typically needs 1,600 to 5,000 starting watts, according to Honda Power Equipment’s generator guide. That’s low enough that many portable generators can handle it alongside a refrigerator, as long as you’re smart about turning other high-draw appliances off.

Electric furnaces are a different story. They rely on resistance heating elements that draw massive current. Home Depot’s wattage chart lists the range for an electric furnace at 5,000 to 25,000 watts, with many standard residential units falling around 20,000 watts. That means you’re likely looking at a larger standby generator or a very powerful portable unit to keep electric heat running.

A radiant space heater, by contrast, uses roughly 1,300 watts — a much smaller load that a small generator can handle easily. That option won’t heat your whole house, but it could keep one room livable during an outage.

Why Surge Wattage Catches People Off Guard

The trap most homeowners hit is buying a generator based on the furnace’s running watts alone. Electric motors in blowers and compressors need 2 to 3 times their running wattage just to get spinning. That initial spike is called surge or starting wattage, and it can stall a generator that’s sized too close to the running load.

Here’s how different furnace types compare on surge versus running demands:

• Gas furnace blower: Running watts typically 600–800; surge watts 1,600–2,400. A 3,000-watt portable generator usually handles this easily, with room for lights and a fridge.
• Larger gas furnace with powerful blower: Running watts around 1,200; surge watts up to 4,000–5,000. Some sources suggest a 6,500-watt generator covers this comfortably.
• Electric furnace (resistance heat): Running watts 10,000–20,000; surge is minimal because resistance elements don’t have motors, but the blower adds a small surge on top. The total starting demand still often exceeds 20,000 watts.
• Radiant heater (space heater): Running watts ~1,300; surge is negligible. A small inverter generator can power one.
• Central air conditioner: Running watts 2,000–4,000; surge can hit 6,000–8,000 due to the compressor. Not directly furnace-related, but useful if you’re planning a whole-home generator.

The key takeaway: always size your generator for the surge, not the run. A generator that barely covers the running watts will likely trip its breaker the moment the blower motor kicks on.

Calculating Your Total Load

To figure out the actual generator size you need, start by listing everything you plan to power during an outage. Home Depot’s recommendation is straightforward: add up the running watts of all appliances, then add the single highest starting wattage among them. That total is your minimum generator capacity.

For example, if you want to run a gas furnace (600 running, 2,000 surge), a refrigerator (700 running, 2,200 surge), and a few LED lights (100 watts total), your calculation looks like this: running total of 1,400 watts, plus the highest surge of 2,200 watts, gives you a need for at least 3,600 starting watts. A 4,000-watt generator would work, and some homeowners report running that exact setup on a 4,000-watt unit without issues. For reference, professional HVAC forums like surge wattage for motors discuss real-world cases where larger blower motors push those numbers higher.

Electric furnaces are simpler in one way — their surge is modest relative to the running load. But the running load itself is so high that you’ll still need a generator in the 10,000–25,000 watt range. That typically means a standby unit wired directly into your panel, not a portable you roll out during a storm.

How To Match Generator Size To Furnace In Practice

Once you know the wattage numbers, the actual buying decision comes down to a few practical steps. Here’s a logical sequence that avoids common mistakes:

1. Check your furnace’s nameplate: The data sticker on the side of your furnace lists the blower motor’s amps and volts. Multiply amps by volts to get running watts, then multiply that number by 2.5 to estimate surge. That’s a rough but useful rule of thumb.
2. Decide what else needs power: If you’re only running the furnace, a 3,000-watt portable generator may be enough for a gas unit. If you’re adding a fridge, sump pump, or well pump, add their running and surge numbers to the total.
3. Choose between portable and standby: Portable generators work fine for gas furnaces in the 3,000–7,000 watt range. For electric furnaces, you’re usually looking at a hardwired standby generator that connects to your panel and starts automatically.
4. Allow headroom for future loads: Generators run best at 75–80% of their rated capacity. Buying one that’s slightly oversized gives you room to add a space heater, lights, or a coffee maker without pushing the unit to its limit.
5. Consider fuel type: Gasoline generators need regular refueling during long outages. Propane or natural gas models run longer unattended, which matters if you’re powering heating for days.

The connection method also matters. Plugging a generator into your furnace requires a transfer switch or a proper inlet — never backfeed through a dryer outlet or extension cord. That’s a safety issue, not just a convenience one.

Real-World Sizing Examples And Common Pitfalls

A homeowner with a standard gas furnace and a refrigerator might find a 4,000-watt generator works perfectly in practice. Some users on home improvement forums report running that exact setup plus lights and a sump pump without tripping breakers. The Honda guide confirms a gas furnace can run on as little as 1,600 watts if you turn off other heavy loads.

But a common mistake is overlooking the well pump or the furnace’s own control transformer. Gas furnaces also need power for their electronic ignition and control board, which is negligible (about 5 watts) but still a factor. If you have a heat pump instead of a furnace, the outdoor compressor adds a significant surge that many homeowners don’t account for until the generator stalls on startup.

For electric furnaces, some sources recommend a minimum of 10,000 to 15,000 watts for a smaller unit, but standard residential electric furnaces often push closer to 20,000 watts. A home with a 15-kW electric furnace would need a generator capable of at least 18,000 starting watts. Guidelines from hardwired generator size suggest a minimum of 5,000 watts for a gas furnace and 10,000–15,000 for electric — though those numbers assume a fairly modest setup and may need adjustment for larger homes.

The Bottom Line

For most gas furnace setups, a 4,000 to 5,000 starting watt generator will handle the furnace, refrigerator, and a few lights during an outage. Electric furnaces need a much larger investment — typically a 15,000-watt plus unit, often a hardwired standby generator. Always size for surge watts, not just running watts, and check the nameplate on your furnace before buying anything.

A licensed electrician can confirm your furnace’s exact power requirements and help you choose the right generator and transfer switch for your home’s setup.