Start with the load, not the generator label
Generator sizing has two jobs: cover the watts that run continuously and survive the brief surge when the largest motor or compressor starts. The safest planning number comes from the actual nameplates/manuals for your equipment. Typical tables are useful for screening, but they are not a substitute for the equipment data in your house.
| Load | Typical running range | Possible starting surge | What changes it |
|---|---|---|---|
| Refrigerator/freezer | 150–400W | Up to ~1,200W | Compressor size and age |
| Gas furnace blower | 400–800W | Often 1,000–1,600W+ | Blower motor type/HP |
| Sump pump 1/2 HP | ~800W | Around 2,000W | Pump design and head pressure |
| Lights/router/chargers | 100–400W | Minimal | How many are on |
Use the site's Wattage Lab to build the simultaneous load, then leave headroom rather than planning to run a generator continuously at its ceiling.
Sizing scenarios
Conservative example
Suppose the refrigerator is 300W running, furnace blower 600W, sump pump 800W, and small electronics/lights 300W. Continuous load is about 2,000W. If the sump pump needs roughly 2,000W to start, the generator needs enough headroom for that surge while the other loads remain on. A 4,000W-class inverter may work; a 5,000–6,000W class gives more breathing room.
Why not buy exactly 2,000 running watts?
Because motors do not start politely one at a time just because the spreadsheet says so. A compressor can cycle while the pump starts. Headroom reduces nuisance overloads and lets the engine operate below its ceiling.
What if the furnace is electric?
This guide assumes a gas/oil furnace where electricity mainly powers controls and the blower. Electric resistance heat is a completely different load and can require many thousands of watts by itself.
120V vs 240V
Many furnaces and smaller sump pumps are 120V, but home-backup plans often include 240V circuits. Check the actual equipment and connection plan before buying.
What to shop for
4,000–4,500W inverter class
A strong fit when the measured loads are modest and you value lower noise/clean inverter output.
5,000–6,000W dual-fuel class
Adds margin for a larger sump pump, additional circuits, and propane backup.
Generator safety is not optional
Portable generators are combustion engines. The U.S. Consumer Product Safety Commission says to operate them outside only, at least 20 feet from the home, with exhaust pointed away from the home and other occupied buildings. A garage, shed, carport, porch, basement, crawlspace, or partly enclosed area is not made safe by opening doors or windows. Install and maintain carbon-monoxide alarms on every level of the home and outside sleeping areas.
For home circuits, do not improvise a connection and never backfeed a house through a receptacle. A transfer switch or properly approved interlock/inlet arrangement should be selected and installed in accordance with the generator manual, electrical code, utility requirements, and qualified professional guidance. For rain or snow, follow the manufacturer's weather instructions rather than moving a running unit into an enclosure.
Output ratings also change by fuel. Propane and natural gas commonly produce less power than gasoline, so size the system around the fuel you actually expect to use. Verify the appliance nameplate, motor starting behavior, voltage, receptacle type, and generator manual before purchase.
FAQ
Will a 3500-watt generator run these three loads?
It may in a favorable setup, but pump and blower starting surge can make 3,500W tight. Verify actual equipment data.
Should I start the furnace and pump separately?
Sequencing large motor loads can reduce simultaneous starting surge, but an automatic sump pump may start whenever water rises. Size for realistic overlap.
Can I use extension cords for the furnace?
Permanent furnace connection and home circuits should follow electrical code and approved transfer/inlet methods. Do not improvise wiring.
Primary spec & safety sources
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