Quick answer
A standby generator is a permanently installed engine generator, usually fed by natural gas or propane, that starts on its own and powers a home through an automatic transfer switch when utility power fails.
It sits on a pad outside, tests itself on a schedule, and shuts down after the grid comes back. In code terms, a home unit is an optional standby system under NEC Article 702.
Quick facts
The key facts about Standby Generator, with sources:
- Switching device
- An automatic transfer switch listed to UL 1008 1
- Self-test
- Weekly to monthly exercise, about 5–12 minutes on Generac models 2
- Clearance (NFPA 37, per Generac)
- 5 ft from windows, doors and vents; 5 ft open above; 3 ft front and ends 3
- Reduced wall clearance
- 18 in from the back to a wall, only for units marked with the SwRI logo 3
- 2024 outage average
- 11 hours per U.S. customer; nearly 53 in South Carolina 4
Key takeaways
- A standby generator is wired in and fueled from a gas line or tank, so it runs without anyone at home.
- It works with an automatic transfer switch that senses the outage and moves the load.
- Placement is set by fire-safety clearances, not looks: 5 feet from openings under NFPA 37.
- It tests itself on a schedule and needs routine engine service.
- Pick the size from a load calculation, not from the biggest model on the lot.
What a standby generator is
A standby generator is an engine and alternator in a weatherproof cabinet, set on a pad beside the house. It connects to your gas supply and to your wiring. Unlike a portable generator, you never roll it out, fuel it or plug it in.
In code language, a home unit is an optional standby system. That means it is there for comfort and protection of property, not life safety as in a hospital. NEC Article 702 covers it, and its transfer equipment must keep the utility and generator from ever connecting 1.
The partner device is the automatic transfer switch, or ATS. The ATS watches the grid, tells the generator to start, and moves your circuits over once the engine is stable.
How it works, step by step
The engine burns natural gas or propane and spins an alternator. The alternator makes 120/240-volt AC, the same split-phase power the utility delivers. A controller on the generator talks to the ATS.
When the grid fails, the ATS signals a start. The engine warms up, the ATS transfers, and your circuits come back on. When the grid returns and holds steady, the ATS moves back and the engine cools down.
Between outages the controller runs an exercise. Generac says all its home standby systems have an automatic exercise timer, and that during the test power usually is not transferred to the house 2.
Types of standby generators
| Type | How it differs | Common use |
|---|---|---|
| Air-cooled | Fan-cooled engine, smaller output range | Most single-family homes |
| Liquid-cooled | Radiator-cooled engine, larger output | Large homes, small businesses |
| Natural gas fueled | Runs from the utility gas line | Homes with gas service |
| Propane fueled | Runs from an on-site tank | Rural homes without gas service |
| Diesel | On-site fuel tank; more common in commercial work | Commercial and critical sites |
Example: siting a unit on a typical lot
Placement is often the first hard limit. These distances come from Generac’s summary of NFPA 37; local codes take precedence 3.
| Clearance | Distance | What it rules out |
|---|---|---|
| From windows, doors, vents, window wells | 5 ft | Tucking it under a bedroom window |
| Back of unit to wall (SwRI-marked units) | 18 in | Closer placement for unmarked units |
| Unmarked units to combustible material | 5 ft | Placing against wood siding without the mark |
| Open space above | 5 ft | Under a deck or low eave |
| Front and both ends | 3 ft | Shrubs crowding the service doors |
Water from downspouts, sprinklers and sump discharge should also be directed away from the enclosure 3. The installer’s manual adds model-specific rules, such as distance from the gas meter.
Where you meet the term
On a quote, “standby generator” usually covers the unit, the ATS, the pad, the gas connection, wiring and permits. On a permit, the electrical and gas work may be separate permits.
On utility and insurance forms, you may be asked whether you have a permanently installed generator. The answer is yes for a standby unit and no for a portable one.
Benefits and drawbacks
What it does well
- Starts on its own, even when you are away.
- No refueling with a gas line; long run time with a propane tank.
- Can carry most or all of a home with the right size and load management.
- Tests itself on a schedule 2.
Where it falls short
- Higher upfront cost than a portable unit.
- Needs a pad, gas work, permits and siting space.
- Routine engine service is required.
- Burns fuel and makes exhaust and noise; clearances must be kept 3.
What a standby generator cannot do
- A short gap occurs before the generator takes over; computers need a UPS to ride through it.
- Natural gas supply can be affected in some disasters; propane relies on tank level.
- Lot size and setbacks may rule out a compliant spot.
- HOA or local noise rules may limit exercise times.
- It does nothing for a fault inside your own wiring; a tripped main or damaged service still needs an electrician.
What drives the cost
The HyreElectrical glossary does not quote standby generator prices. These drivers explain why quotes differ; see generator installation for what an electrician includes.
- Size in kW. Larger units cost more and need larger gas supply.
- ATS type. Whole-house, service-rated or load-managing switches differ.
- Gas line run. Distance from the meter or tank and pipe size.
- Site work. Pad, trenching and clearances.
- Panel condition. An old panel may need an upgrade first.
- Service plan and warranty. Terms vary by brand and dealer.
How an installation runs
A licensed electrician and a gas fitter do this work, in roughly this order.
- Load calculation to pick generator size and ATS type.
- Site plan using NFPA 37 clearances and the maker’s manual 3.
- Electrical and gas permits.
- Pad, unit, gas line, ATS and wiring installed.
- Start-up, simulated outage test and inspection.
Upkeep
Warning signs and when to call
Call your dealer if the unit fails its exercise, shows a fault, or runs but the house stays dark. Generac says a faint gas smell during exercise can be normal, but a gas smell at other times means calling the dealer or gas company right away 2.
Exhaust from any engine contains carbon monoxide. Keep CO alarms working on every level of the home 6.
Clearances are a safety rule
Placing a generator closer than 5 ft to windows, doors or vents can let exhaust into the home 3. Never enclose the unit or block its vents, and leave all service inside the cabinet to a qualified technician.
Codes and standards
| Authority | Rule | What it covers |
|---|---|---|
| NFPA 70 (NEC) | 2017 ed. as cited by UL, Article 702 and 702.5 | Optional standby systems; transfer equipment must prevent interconnection 1 |
| NFPA 70 (NEC) | 2023 ed., Section 230.85 | Outdoor emergency disconnect for one- and two-family homes 5 |
| NFPA 37 | Engine installation (as summarized by Generac) | Clearances from openings and combustibles 3 |
| UL | UL 1008 | Transfer switch listing 1 |
| Local | Zoning, setbacks, noise | Varies by town |
Standby vs other backup
| Option | Starts | Run time | Typical coverage |
|---|---|---|---|
| Standby generator | Automatically | As long as fuel lasts | Most or all of the home |
| Portable unit | By hand | One tank at a time | Must-run circuits |
| Battery storage | Automatically | Limited by kWh and solar | Often a critical loads panel |
| Plug-in UPS | Instantly | Minutes | One device |
Myths
- Myth A standby generator gives zero interruption.
- Reality There is a gap of seconds while it starts and the ATS transfers.
- Myth It can go anywhere outside.
- Reality NFPA 37 clearances, summarized by Generac, set real limits 3.
- Myth Bigger is always better.
- Reality Oversizing raises cost and fuel use; a load calculation sets the right size.
Where it matters most
Outage exposure varies widely. EIA reported 11 hours without power for the average U.S. customer in 2024, and nearly 53 hours in South Carolina, driven by hurricanes 4. Code details depend on the NEC edition your state has adopted.
Should you get one?
A standby generator makes sense when outages are long or frequent, when someone at home depends on powered equipment, or when nobody can be there to run a portable unit.
Begin with the generator sizing calculator and an electrical load estimate. Then get quotes that list the ATS type, gas work and clearances.
- If Outages last days
- Then Standby generator or large battery
- If No gas service
- Then Propane tank or battery
- If Small lot, tight setbacks
- Then Check clearances first; a battery may fit better
- If Short, rare outages
- Then Portable unit with a transfer switch
Next step
Get the load calculation done first; it decides generator size, ATS type and gas line size together.
Questions about Standby Generator
What is the difference between a standby generator and a portable generator?
A standby generator is installed permanently, runs on natural gas or propane, and starts on its own through an automatic transfer switch. A portable generator is rolled out by hand, usually burns gasoline, and connects through cords or a manual transfer switch. Standby units cost more but work when no one is home.
How close to the house can a standby generator be?
Generac’s summary of NFPA 37 calls for 5 feet from windows, doors, vents and window wells, 5 feet of open space above, and 3 feet at the front and ends.
Units marked with the SwRI logo may sit 18 inches from a wall at the back. Local codes take precedence, so check with your installer and town.
How often does a standby generator run its self-test?
Generac says its home standby units have an automatic exercise timer. Depending on the model, the test runs weekly, every two weeks or monthly, for about 5 to 12 minutes. The test keeps the engine lubricated and checks function, and power usually is not transferred to the house while it runs.
Does a standby generator need a transfer switch?
Yes. It connects through an automatic transfer switch that senses the outage, starts the generator and moves the load. NEC 702.5 requires transfer equipment that prevents the utility and generator from connecting by accident. The switch is usually listed to UL 1008 and installed by a licensed electrician.
What size standby generator do I need?
The right size comes from a load calculation of what you want to run at the same time, including starting surges from motors like air conditioners and well pumps. Some systems use load management to shed big loads so a smaller unit covers the house. Our generator sizing calculator is a good first pass.
Can a standby generator run on natural gas and propane?
Many home models are built to run on either fuel, set up at installation for the one you have. Natural gas comes from the utility line, so you never refuel.
Propane comes from a tank on site, which works where there is no gas service but needs refills. Check the model’s fuel options and output on each fuel.
Is a standby generator required to have a permit?
In most places, yes. The electrical connection and the transfer switch need an electrical permit, and the gas line often needs its own. Inspectors check the switch, wiring, clearances and fuel connection. Rules depend on your local code and the NEC edition your state adopted.
How long can a standby generator run?
On natural gas, it can run as long as the gas supply holds and the engine is serviced on schedule. On propane, run time depends on tank size and load. Engines need oil checks during long runs, so follow the maker’s guidance for extended outages.
Is a battery a good alternative to a standby generator?
It can be, especially with solar. A battery switches over automatically, makes no exhaust and runs quietly, but its run time is limited by its stored kWh unless solar recharges it. Many battery systems back up a critical loads panel rather than the whole home. Compare both against your outage history and loads.
Sources
- IAEI Magazine, UL Question Corner: NEC 702.5 transfer equipment, UL 1008, 2017 NEC, May/June 2017, retrieved .
- Generac Support, What Does the Exercise Timer Do on a Home Standby Generator, retrieved .
- Generac Support, Clearance Requirements When Installing My Generator (NFPA 37), retrieved .
- U.S. EIA, Today in Energy: hurricanes in 2024 led to the most hours without power in 10 years, retrieved .
- Larimer County (CO), 2023 NEC Significant Code Changes, retrieved .
- Ready.gov, Power Outages, retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Facts checked against Generac support articles, IAEI’s UL column on NEC 702.5, Larimer County’s 2023 NEC summary, Ready.gov and EIA outage data on 2026-10-10.
Suggest a correction. We fix errors and say what changed.