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Sizing a Backup Generator for the Homestead

Where to put it, why a transfer switch is not optional, and the starting-watt problem that strands well pumps.

A well it house

The Consumer Product Safety Commission counted 789 deaths from portable generator carbon monoxide between 2012 and 2022. The number that should change where you set the machine is buried deeper in that report. Only 17 percent of those incidents happened during a weather-caused power outage. Most generator CO deaths are not storm deaths.

Placement comes before sizing, because a correctly sized generator still kills people when the exhaust finds a way inside. CPSC and CDC both say at least 20 feet from the house, exhaust pointed away, and clear of windows, doors and vents. Purdue adds that the machine needs 3 to 4 feet of open space on every side and overhead for its own cooling, which rules out running it under a tarp on the porch.

Twenty feet is a minimum and not a guarantee. CPSC’s own report documents a 2021 case in which a generator sat outdoors under the eaves, beneath soffit vents that fed the attic, with the windows and doors shut. The unit had a CO shutoff. It never tripped, it ran until the tank was dry, and three people died.

Enclosed spaces come with no exception language attached. CDC and CPSC both say never in a home or garage even with the doors and windows open, and CPSC’s prohibited list names porches. The fatality breakdown shows why the open-door argument keeps failing. Of 553 deaths in fixed-structure homes, 99 happened in an attached garage, enclosed carport or attached barn, and in roughly a third of the cases where information existed, someone had attempted ventilation and died anyway.

Put CO alarms on every level and outside each sleeping area. Test them monthly. Treat an alarm as an evacuate-and-call-911 event rather than something to go investigate.

Never energize the house without a transfer switch

Backfeeding through a dryer outlet or a double-ended cord puts generator voltage onto the utility drop, steps it up through the pole transformer, and can kill a lineman working a span they believe is dead. The University of Kentucky and NDSU both require a double-throw transfer switch for any connection to premises wiring. Cornell adds that it has to be rated for the full service, so a 400-amp service takes a 400-amp switch. No source in the extension literature describes a safe way to do this without one.

Running appliances directly on cords needs no transfer switch, and for most small homesteads that is the honest answer.

Size it by load list, not by house

Square footage tells you nothing. The propane industry’s own figures make the point by accident. A 1,500 square foot home is estimated at about 14 kW if it heats with propane and 22 kW if it is all electric. Same house.

The method is a load list. Add up running watts for what you actually intend to run, then add the starting surge of the largest single motor on that list. Motors are the whole problem. Mississippi State says a motor draws about three times its running current to start, Cornell says three to five times, and NDSU says four. Sizing to the bottom of a range that reaches five is how people end up with a generator that runs lights and a freezer and cannot start the well pump.

NDSU gives the rule that makes pumps tractable: roughly 1,000 watts to run and 4,000 watts to start, per horsepower of motor. A three-quarter horse pump is then about 750 watts running and 3,000 starting. Their worked whole-house example, covering a water pump, a refrigerator, a freezer, a gas furnace blower and some lights, comes to about 2,000 watts continuous against about 5,000 watts peak. That two-and-a-half-to-one gap is the entire lesson.

Cornell brackets it for planning. Four thousand to 6,000 watts covers basic needs and 12,000 to 15,000 covers something like normal living. Kentucky’s advice is to buy above your calculation rather than at it. If voltage sags below 200 on a 240-volt circuit or below 100 on a 120-volt circuit, shed load. Those thresholds are NDSU’s, and Alabama sets them a little tighter at 210 and 105.

Fuel

Gasoline is the weak point in any plan that sits idle for a year. Oklahoma State says straight gasoline should not be stored beyond 60 days without stabilizer, and Wisconsin Extension says it begins to degrade in as little as 30 days. Those are two different thresholds rather than a contradiction, and both are far shorter than the year or two most people assume.

Mississippi State’s consumption figure is coarse but useful. A large generator can burn a gallon an hour even at light load, and several days of running after an ice storm can take 50 to 100 gallons. Shut down and let it cool completely before refueling. Cornell adds a schedule the machine itself cares about, shutting down at least once every 24 hours during continuous operation and two or three times a day in a long outage to check fluids.

Propane does not go stale, which is why people recommend it, and the cold-weather problem with propane is not the fuel. It is vaporization. Propane industry data for a 150-gallon tank gives a maximum withdrawal rate of 214,900 BTU per hour at 40 degrees, 134,700 at zero degrees and 107,100 at 30 below. Frost forming on the tank insulates it and makes the rate worse as you draw. A 30 kW generator wants 493,000 BTU per hour at full load, which a single 150-gallon tank cannot supply at zero degrees at any load setting. Size the tank against your coldest night rather than against the generator’s label.

Cords

Undersized cords damage motors, not just cords. Low voltage at the motor pulls high current, which overheats the windings. Eastern Washington University’s table for a 1.5 to 2 horsepower motor at 115 volts allows 12 gauge out to 25 feet, 10 gauge to 50 and 8 gauge to 75. At 3 horsepower that same 10 gauge is good for only 25 feet. UC ANR gives a usable rule of thumb, which is to go one wire gauge heavier for every additional 100 feet of length. If a cord feels hot in your hand or the plastic softens, it is finished.

Two claims would not confirm against any government or extension source, so treat them as marketing until they do. Nothing published quantifies the power derate from running an engine on propane instead of gasoline, and nothing published gives total harmonic distortion figures for inverter versus conventional generators. Both numbers circulate widely and both trace back to manufacturers.

Run it monthly. Cornell says start it and put it under load for at least 30 minutes, while Mississippi State says every two to four weeks in one publication and every couple of months in another. Under load is the part that matters. A generator that starts and idles has told you almost nothing about whether it will carry the well pump on the coldest night of the year, which is the same night the water lines are working against you too.

Sources

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