What Size Generator Do I Need to Run a Gas Furnace?

A gas furnace only needs electricity for the blower and the controls.

Quick answer: Honda lists a furnace fan from 300 running watts and 500 starting watts at 1/8 hp up to 875 running watts and 2,350 starting watts at 1/2 hp, and Champion lists a 1/2 hp furnace fan at 300–800 running watts and 800–1,600 starting watts. Covering the higher 1/2 hp row takes a running rating that clears 875 W and a starting rating that clears 2,350 W. A Honda EU2200i maximum of 2,200 W is just under that 2,350 W start, while the Champion manual's 6,250 W starting rating covers it.

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Running and starting watts

Load	Running watts	Starting watts	Source
Furnace fan, 1/8 hp	300	500	Honda
Furnace fan, 1/6 hp	500	750	Honda
Furnace fan, 1/4 hp	600	1,000	Honda
Furnace fan, 1/3 hp	700	1,400	Honda
Furnace fan, 1/2 hp	875	2,350	Honda
Furnace fan, 1/2 hp	300–800	800–1,600	Champion

Honda's wattage guide lists starting watts, then running watts. Champion's chart lists running watts, then starting watts. This table puts every figure in the Running and Starting columns. Not listed means that source does not give a separate starting figure.

How to size it

Add the running watts of every load that will be on together, then add the single highest starting surge (Generac; Honda). Generac notes that starting watts are often 2–3 times running watts, and that standby units should get a dealer survey.

Read the blower nameplate and use that horsepower's row. A 1/3 hp blower is 700 running watts and 1,400 starting watts, which fits under the EU2200i rated output of 1,800 W and maximum of 2,200 W. The furnace is hardwired, so use a transfer switch or interlock installed by an electrician. See the transfer switch sizing guide. Honda notes that an inverter converts AC to DC and back to AC, which gives lower THD than a conventional generator that runs at full RPM.

Check a full load list on the generator size calculator.

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Run the generator outdoors only, 20 ft from the house (CPSC).

FAQ

Can a Honda EU2200i run a 1/2 hp furnace fan?

The running load can. Honda lists 875 W running, under the 1,800 W rated output. The start is 2,350 W, which is over the 2,200 W maximum.

Can it run a 1/3 hp blower?

Yes on Honda's row: 700 W running and 1,400 W starting, under 1,800 W rated and 2,200 W maximum.

Why is Champion's 1/2 hp start lower?

It is a different published range: 800–1,600 W starting and 300–800 W running. Cover Honda's 2,350 W start if you want both charts.

Does an electric furnace use the same watts?

These rows are blower fans. Honda lists a hot water heater, which is a different resistive load, at 4,500 W running and starting. An electric furnace is not given its own row here.