What Size Generator to Run a Furnace? Watts for Gas, Oil and Electric Heat

A gas or propane furnace needs only enough generator power for its blower, inducer fan, igniter and controls: about 300 to 875 running watts, plus a short surge to 1,600–2,350 watts as the blower starts, according to Champion’s and Honda’s wattage charts.^1,2 The furnace alone fits a 2,000-watt generator whose starting rating covers that surge, and a 3,000-watt unit runs it with a fridge, lights and a TV at sea level. An oil furnace adds its burner. An electric furnace or a heat pump is a different class of load: about 4,300 to 25,000 running watts, on 240 volts.

Below, we check those chart numbers against furnace makers’ own blower data and add our NOAA altitude analysis: in the coldest third of US ZIP codes, about 1 in 7 lose 10% or more of a portable generator’s output.

How many watts each type of furnace draws

The fuel makes the heat. Electricity runs what moves the air and lights the burner. In a gas furnace, that means the circulating blower, the inducer fan that pulls combustion gases through, a hot-surface igniter and the control board.^8,9 Propane furnaces are the same machines: Carrier approves its 58SC models for natural gas or propane, converted with a gas kit.⁸

For a gas furnace the blower is nearly the whole load; an oil burner adds up to 5.8 amps.¹¹ Honda’s wattage estimation guide lists furnace fans for gas or fuel oil by size, from 300 running and 500 starting watts for a 1/8 hp fan up to 875 and 2,350 for a 1/2 hp fan.² Champion’s wattage chart gives a 1/2 hp furnace fan 300–800 running and 800–1,600 starting watts.¹ Our generator size calculator uses Champion’s top figures. If you don’t know your blower’s size, Honda’s 2,350 is the safer starting number.

Even Honda’s own pages differ: one lists a 1/4 hp furnace fan at 1,600 starting watts, another at 1,000.^2,3 Plan on the top of a range.

How to read your own furnace’s numbers

Spec sheets and rating labels give volts and amps. Multiplied, they give a ceiling, not the running draw. Carrier’s 58SC gas furnaces list maximum unit amps of 5.6 to 13.4 at 115 volts, depending on size: about 640 to 1,540 watts.⁸ Thermo Pride’s OL6 oil furnace lists a total current of 8.7 amps with its 1/2 hp PSC blower, about 1,040 watts at 120 volts. Its own blower table shows 512 to 682 watts at the heating speeds.¹⁰

Skip the “minimum circuit ampacity” line. Goodman defines it as 1.25 times the blower amps plus the inducer amps, a wire-sizing figure.⁹ And none of the Carrier, Goodman, Lennox or Thermo Pride documents we read gives a starting figure for the blower, so the generator makers’ charts are the only published surge numbers.

Older PSC blowers and newer ECM blowers

What a blower really draws depends on its motor. Thermo Pride publishes watts for two versions of its OL6 oil furnace. At 0.5 inch of static pressure, the 1/2 hp PSC (permanent split capacitor) blower draws 512 to 682 watts at the heating speeds, and the 1 hp ECM version draws 215 to 309 watts.¹⁰ On the top (cooling) speed the two are close: 869 and 855 watts.

Federal rules push the same way. Furnace fans made on or after July 3, 2019 must meet DOE’s fan energy rating limits.¹⁴ For the 3-ton gas furnace in DOE’s 2012 analysis, which moves 1,158 cubic feet of air a minute, the limit works out to about 233 watts per 1,000 cfm, against 380 for the old PSC baseline. DOE put that baseline blower at 382 watts in heating.^14,15

A furnace made since 2019 may run well under the chart figures, but no document we read says how much less it needs to start, so keep the chart’s starting watts.

Worked examples: gas and oil furnaces

Each example uses our calculator’s rule: running watts plus 5%, divided by the altitude factor (Honda: about 3.5% less power per 1,000 feet),⁴ and a starting rating of at least the starting total divided by the same factor. Elevations are for the NOAA station nearest each city.¹⁷

Staggering loads shrinks these numbers. Honda’s guide puts it plainly:³

it is not necessary to power everything at the same time.

In Honda’s example, a 3,000-watt generator runs a fridge, furnace, lights, TV and microwave easily, and as little as 1,600 watts can do if the furnace and fridge run at separate times and the lights and TV go off whenever one of them starts.³

Altitude: cold places lose more generator power

Engines make less power in thin air, and many places that lean hardest on a furnace are high. We applied Honda’s 3.5%-per-1,000-ft rule to the NOAA station nearest each of 40,789 ZIP codes with both an elevation and a heating degree day normal.^4,17 Nationally, 8.6% lose 10% or more of a portable’s power. In the coldest 31% (6,000 heating degree days or more), 13.6% do, about 1 in 7; in the mildest band, under 2,000 degree days, 0.6%. About half of all the ZIP codes losing 10% or more are in that cold group.

A 3,000-watt generator covers the furnace, fridge, lights and TV in every city; only the starting rating climbs, from 2,720 watts at sea level to 3,329 in Denver. Add a sump pump and a microwave, as in our calculator’s default list, and Denver needs a 5,500-watt generator and Flagstaff a 6,500.

Heat doesn’t trim a generator in winter: Generac’s temperature derate starts above 77°F,⁵ and the warmest January average high in Table 2 is 48.5°F (Denver). It matters in summer, as the Denver example below shows. The calculator applies both rules to your ZIP code.

Electric furnaces and heat pumps are a different class of load

An electric furnace makes its heat with resistance elements. Goodman’s AMST air handler takes heat kits from 3 to 25 kW; its “10 kW” kit is rated 9.6 kW and draws 40 amps at 240 volts.¹² Resistance heat has no starting surge (Champion lists none for an electric water heater or space heater),¹ but it carries the whole house’s heat.

A heat pump’s compressor is the same machine that cools in summer. Goodman’s 3-ton GSZ14 heat pump is rated 36,000 Btu/h for both cooling and heating. Its compressor’s rated load is 15.4 amps at 208–230 volts, about 3,540 watts at 230 volts, close to Champion’s 3,000–3,500 running watts for a 3-ton central AC. As it starts, it draws 83.9 amps (locked-rotor current), more than five times that.^13,1 In deep cold, backup electric strips can add their kilowatts on top; our guide to heat pumps in cold weather explains when they come on.

Sizes come from the calculator’s standard list, with its 5% margin; check each unit’s starting rating too. Both systems run on 208/240 volts,^12,13 so the generator must supply 240 volts at those wattages.

For backup power, a gas furnace is the far smaller load; our heat pump vs furnace comparison covers what each costs to run by city.

Inverter or conventional generator for furnace electronics

Furnaces run on control boards, and Lennox sets generator limits in its SL280DFNV gas furnace instructions: 120 volts (102 to 132), 60 Hz within 5% (57 to 63 Hz), correct polarity and a proper ground, both checked before the furnace runs on temporary power.⁷ They add:

Generator should have a wave form distortion of less than 5% THD

THD (total harmonic distortion) measures how far power departs from a clean wave. Generac says standard portable generators typically run 9% to 15% THD, some up to 30%, while several of its non-inverter lines (XD, XG, XC, XT-EFI and GP-EFI) stay under 6%.⁶

Generac inverter generators produce ≤3% THD, making them ideal for use with sensitive electronics.

So on paper an inverter generator meets Lennox’s limit and a typical conventional portable doesn’t. “Under 6%” doesn’t prove “under 5%”, so compare the spec sheet’s THD figure with your furnace manual. Not every manual sets one: the Goodman and Carrier sheets we read say nothing about generators.^8,9

Polarity and ground matter too. Goodman’s wiring diagram has the control board flash rapidly for reversed polarity and tells the technician to verify the ground.⁹ If a furnace won’t light on generator power, that is a job for the electrician who made the connection, not for an adapter.

Getting generator power to the furnace

The Goodman and Carrier spec sheets show a furnace wired to a 115-volt supply with its own fuse or breaker,^8,9 so it doesn’t plug into a generator. Honda’s manual says connections to a building’s electrical system must be made by a qualified electrician, through a transfer switch that isolates generator power from utility power. Improper connections can backfeed the utility lines and electrocute line workers.⁴ Search results for this question include videos of homemade cords and breaker-box hookups; Honda’s warning is why we don’t repeat them.

Carbon monoxide: where the generator goes

Our portable generator safety guide covers placement, alarms, cords and refueling, with CPSC’s records of generator deaths.

Next step

Note your furnace’s volts, amps and blower size, list what else must run, and enter it with your ZIP code in the generator size calculator. Its rules are on our methodology page; the generators section has our other backup-power guides.