How Does a Home Generator Work? Types and Sizing
Chameera Sampath · 10/18/2026
How a home generator works: portable, inverter and standby types, transfer switches and backfeed dangers, a worked sizing example and carbon monoxide safety.
A home generator converts the chemical energy of fuel into electricity so that a house can keep essential circuits running during a utility outage. It works by electromagnetic induction: an engine turns a magnetic rotor inside coils of wire, and the changing magnetic field induces an alternating voltage in the coils. Choosing one comes down to type, size and how it connects to the house.
Quick answer. An engine spins an alternator, which produces 120/240 V AC. Portable generators power selected loads through cords or a transfer switch; standby generators start automatically through an automatic transfer switch. Size the generator for the running watts of everything on at once plus the largest motor's starting surge. Never run a generator indoors or near openings.
How a generator makes electricity
- Engine. Gasoline, propane, natural gas or diesel drives a crankshaft.
- Alternator. The crankshaft turns a rotor carrying a magnetic field inside stationary windings, the stator.
- Induction. The rotating field induces alternating voltage in the stator windings.
- Regulation. A voltage regulator holds the output near 120/240 V, and the engine governor holds speed so that frequency stays near 60 Hz.
Inverter generators add a stage: the alternator's raw output is rectified to DC and converted back to clean AC electronically, giving steadier voltage and frequency and allowing the engine to slow down at light load.
Types of home generator
| Type | Typical size | Fuel | Starts | Connects |
|---|---|---|---|---|
| Portable | 3–10 kW | Gasoline, propane, dual fuel | Manually | Cords, or an inlet with a transfer switch or interlock |
| Inverter (portable) | 1–7 kW | Gasoline, propane | Manually | Cords; sensitive electronics |
| Standby | 10–26 kW and up | Natural gas or propane | Automatically | Automatic transfer switch |
Connecting to the house safely
A generator must never be connected to house wiring through a receptacle with a "double male" cord. Doing so backfeeds the utility lines: the generator's output travels back through the service and transformer, where it can electrocute line workers and others, and it energizes the house wiring without any protection.
Safe connections are:
- Extension cords rated for the load, run directly from the generator to individual appliances.
- A manual transfer switch or a listed interlock kit on the main panel, fed from a generator inlet box. Both make it physically impossible to have the utility and the generator connected at the same time.
- An automatic transfer switch with a standby generator, which disconnects the utility, starts the generator and transfers the load when power fails.
Transfer equipment is installed under permit, normally by a licensed electrician.
Sizing a generator
Size depends on what must run at the same time, not on everything in the house.
- List the loads to be powered during an outage.
- Note each load's running watts, and for motor loads its starting watts, from the nameplate or manual.
- Add the running watts of everything on at once.
- Add the single largest starting surge (starting watts minus running watts), since motors rarely start together.
- Add a margin, commonly 20%.
The generator sizing calculator performs these steps and suggests a common rating.
Worked example
A refrigerator (700 W running, 2,200 W starting), sump pump (800 / 2,100), furnace blower (750 / 2,350), lighting (400 / 0) and well pump (1,000 / 2,000) total 3,650 W running. The largest surge above running is the furnace blower's 1,600 W, giving a peak of 5,250 W. With a 20% margin the requirement is 6,300 W, so a 6.5 kW generator covers it.
Central air conditioning, electric ranges, water heaters and electric heat each add several kilowatts; covering them usually means a standby generator in the 15–26 kW range.
Carbon monoxide and placement
Generator exhaust contains carbon monoxide, which is colorless, odorless and lethal in enclosed spaces within minutes. Generators must never run indoors, in garages (even with the door open), in basements, crawl spaces or sheds. US safety agencies recommend placing a portable generator at least 20 feet from the house with the exhaust directed away from doors, windows and vents. Battery-backed carbon monoxide alarms should be installed in the home.
Maintenance
- Run the generator under load periodically; standby units exercise themselves on a schedule.
- Change oil, air filter and spark plugs at the manufacturer's intervals.
- For gasoline units, use fuel stabilizer or run the carburetor dry before storage.
- Test the transfer switch or interlock before storm season.
Frequently asked questions
How does a generator work? An engine turns a magnet inside coils of wire; the changing magnetic field induces an alternating voltage in the coils, which is regulated to household voltage and frequency.
What size generator does a house need? For essential circuits, typically 5 to 8 kW. For a whole house with central air and electric appliances, typically 15 to 26 kW. A sizing calculation based on the actual loads is more reliable than either range.
Can a generator be plugged into a wall outlet? No. Backfeeding through a receptacle is dangerous and illegal. A transfer switch or interlock is required.
How long can a generator run? Portable generators run for the duration of a tank of fuel, often 8 to 12 hours at half load. Standby generators on natural gas can run for days, subject to the manufacturer's maintenance intervals.
Scope
This article describes residential generators in the United States. Transfer equipment and permanently connected generators must be installed to the National Electrical Code and local requirements, normally under permit by a licensed electrician.
Written by Chameera Sampath. Not yet reviewed by a licensed electrician — see the methodology page. To report an error, use the contact page.