Generator size by appliance / furnace
What size generator runs a gas furnace
Furnace fan blower, 1/3 hp: 700 W running plus 1,400 W more for a few seconds as it starts, so a generator needs at least 2,100 W of starting capacity to run it on its own. The smallest of the six portables this site covers that can do it is the Honda EU2200i (1,800 W running, 2,200 W max).
A gas or oil furnace burns fuel for heat, but it cannot run without electricity for its blower, igniter and controls. Its electrical load is the blower motor, which is why the tables list furnaces by fan horsepower.
The 1/3 hp row comes from EWEB and the 1/2 hp row from Generac. The blower's horsepower is on the motor label inside the furnace cabinet.
A furnace is hard-wired, so a generator can only feed it through a transfer switch or interlock installed by an electrician; OSHA warns that connecting a generator to house wiring any other way endangers utility workers.
The figures and where they come from
| Device | Source | Running W | Additional starting W | Starting need alone |
|---|---|---|---|---|
| Furnace fan blower, 1/3 hp | EWEB (2019) | 700 | 1,400 | 2,100 |
| Furnace fan blower, 1/2 hp | Generac GP brochure (2023) | 875 | 2,300 | 3,175 |
Highlighted rows are the figures the calculator uses. Every table here prints additional starting watts: the surge on top of running watts.
Which portables can run it
| Generator | Running / peak W | Alone | With fridge, 6 LED bulbs, 2 chargers |
|---|---|---|---|
| Honda EU2200i | 1,800 / 2,200 | Yes | No |
| Honda EU3000iS | 2,800 / 3,000 | Yes | No |
| Westinghouse iGen5000 | 3,900 / 5,000 | Yes | Yes |
| Honda EU7000iS | 5,500 / 7,000 | Yes | Yes |
| Champion 201040 | 7,500 / 9,375 | Yes | Yes |
| Westinghouse WGen9500DF | 9,500 / 12,500 | Yes | Yes |
A generator fits when its running rating covers every running watt and its peak (or "max") rating covers the running total plus the single biggest starting surge. Adding the basics — a refrigerator (700 W running, 2,200 W to start), six 15 W LED bulbs and two phone chargers — raises the running load to 1,530 W and the starting need to 3,730 W, because the refrigerator's surge is now the biggest one. The smallest portable here that covers that is the Westinghouse iGen5000.
On a standby generator
A home standby unit is rated in kilowatts and has no overload capacity: Generac's rating definition says "No overload capability is available for this rating." Using Generac's own formula — sum of running watts plus the highest single starting burst — the furnace with the basics needs 3.7 kW. The smallest Guardian model that covers it on natural gas is the 10 kW, rated 9 kW on natural gas and 10 kW on propane, which leaves 5.3 kW of starting room for everything else. Whole-home lists are on the standby generator pages.
Furnace fan blower, 1/2 hp
The smaller size needs 875 W running and 2,300 W more to start, 3,175 W in all; the Westinghouse iGen5000 is the smallest portable here that starts it alone. With the basics it needs 4,005 W to start.
Before running anything, read the carbon monoxide distance rules and how a transfer switch keeps power off the utility lines. Other appliances: refrigerator · sump pump · well pump · window AC · central air · heat pump · space heater · water heater · washing machine · garage door opener · CPAP · oxygen concentrator.