A permanent standby generator plus an automatic transfer switch (ATS) is the safest, code‑compliant way to power a home during utility outages. Two main sizing approaches exist: a whole‑house unit sized to carry essentially all household loads, or an essential‑loads design where the generator is smaller and chosen circuits are prioritized/managed. Manufacturers publish sizing tools and guidance for both approaches to help match generator capacity to expected running and motor‑starting loads[4][5].
Sizing: running loads, motor starting, and practical choices
When sizing a genset, allow for both continuous running loads and the motor starting (inrush) currents of appliances and HVAC compressors. Motor starting surge can be a dominant factor for refrigerators, heat pumps, air conditioners, pumps, and other motor loads; manufacturer calculators and sizing tools explicitly include allowances for those starting surges[4].
Two practical approaches homeowners use:
Whole‑house sizing — pick a generator large enough to carry essentially all major loads. This simplifies use (no manual circuit switching) but increases equipment and installation cost and site requirements[4].
Essential‑loads / managed sizing — install a smaller generator and feed only prioritized circuits or an essential loads subpanel. Load‑management hardware and staged shedding can let a smaller genset serve critical circuits by cycling or shedding lower‑priority loads during high demand[5][6].
Automatic transfer switch selection and required markings
Select an ATS that is rated at least to the generator’s continuous ampere/kW output and that is marked with its short‑circuit/fault‑current rating. UL 1008 is the recognized product safety standard that defines required ratings and tests for transfer switches, and manufacturers’ instructions call out matching the ATS to the genset rating and marking the short‑circuit current rating for coordination with upstream equipment[9][5][7].
In addition to ampere/kW rating, confirm the ATS’s short‑circuit current rating (SCCR) and that protective devices upstream (mains breaker, fuses) are coordinated with that rating per NEC commentary and manufacturer guidance[8][7].
Load prioritizing and load‑management options
Manufacturers offer hardware to prioritize and manage loads. Examples include switched ATS configurations that feed a dedicated subpanel of selected circuits or external load‑management modules that shed nonessential loads during peak demand. Generac’s Smart Management Module (SMM) is one documented example: it provides staged/prioritized load control, features a priority dial, supports multi‑module configurations, and works with compatible ATS equipment when installed per the manual[5][6].
If you plan to use load‑management modules, follow the manufacturer installation manual for allowed configurations and limits (number of modules, supported voltages) and verify compatibility with the specific ATS model being installed[6][5].
Fuel choices and regulatory considerations
Common residential fuel options are natural gas and propane (LP); diesel is more common for larger commercial units and can have stricter emissions oversight. Manufacturers list available model fuel options and typical residential choices when selecting equipment[5][4].
Before planning extended non‑emergency operation (long test runs or maintenance running) or installing a large stationary engine, check EPA stationary‑engine rules and state/local air‑district regulations because those rules can limit non‑emergency operation and may impose emissions controls on certain stationary engines[11].
Codes, testing, commissioning, and maintenance
For residential whole‑house standby generators, NEC Article 702 (Optional Standby Systems) is typically the applicable NEC article; it addresses wiring, marking, and some installation/testing rules. NFPA 110 governs emergency/legally‑required systems (life‑safety) and contains prescriptive testing protocols; manufacturers often recommend testing and maintenance routines similar to NFPA practices even when the installation is an optional standby system under NEC 702[8][7].
Manufacturers and industry guidance recommend commissioning and periodic exercising of the ATS and generator controls: test the ATS automatic control system after installation, perform loaded exercise tests per controller instructions, and follow the manual’s suggested periodic inspection/testing interval (manufacturers commonly advise monthly control/function checks and periodic loaded exercises as directed in the owner/installation literature)[7][3].
Because AHJs and installers may interpret local code/testing requirements differently, confirm local permitting and AHJ expectations for testing and maintenance in advance[7][8].
Safety: CO, backfeed, and who should install
Never operate fuel‑powered generators indoors, in garages, basements, or other enclosed or partially enclosed spaces. Keep exhaust directed away from doors, windows, and vents and install working carbon monoxide (CO) alarms inside the home per federal safety guidance[1][2][3].
Backfeeding the utility is a grave hazard. Properly installed, code‑compliant ATS equipment prevents backfeeding and isolates the home wiring from the utility; using a portable generator without the correct transfer equipment risks energizing utility lines and endangering utility workers. Manufacturer, FEMA, and CPSC guidance all warn against portable generator backfeeding and emphasize using a listed, installed ATS or an approved interlock/transfer method where permitted[1][3][5].
Have a licensed electrician or factory‑authorized installer perform the ATS and generator installation, verify that the ATS prevents utility backfeed, confirm protective device coordination, and obtain required permits and inspections from the Authority Having Jurisdiction (AHJ)[5][7][8].
Costs and next steps
Installed costs vary substantially with model, fuel choice, transfer equipment, site work, and local labor rates. Consumer guidance provides market overviews and typical installed cost ranges—consult current cost pages and get local installer quotes for exact pricing for your situation[10].
Practical next steps for homeowners considering a standby generator:
Inventory major loads and note motor‑starting loads (HVAC, well pump, major appliances) to inform sizing; use manufacturer sizing tools where available[4].
Decide whole‑house vs essential‑loads approach based on budget, site fuel availability, and tolerance for load management[4][5].
Select an ATS rated to the generator and marked with its short‑circuit current rating; confirm coordination with upstream protective devices per the ATS and generator manuals[9][5][7].
If using load‑management (modules or switched ATS circuits), follow the manufacturer manuals for allowed configurations and verify ATS compatibility[6][5].
Check EPA and local air‑district rules before planning extended non‑emergency operation or choosing a large stationary engine fuel option[11].
Engage a licensed electrician/authorized installer, obtain permits, commission and test the system per the manual, and set up a periodic exercise and inspection routine[7][3].
What varies by installer or AHJ
Naturally, local AHJs and installers may interpret testing, maintenance, and certain installation details differently. NFPA 110 is prescriptive for emergency/legally required systems, while NEC 702 covers optional standby systems; manufacturers frequently recommend NFPA‑style maintenance even where full NFPA compliance is not legally required for residential optional standby systems, but confirm expectations with your installer and AHJ[7][8].
Where to find more detail
Manufacturer support pages and product manuals are the right place for model‑specific sizing, ATS compatibility, and load‑management configuration details. Consult generator and ATS manuals for the exact installation limits, wiring diagrams, and commissioning steps that apply to the equipment you plan to buy[4][5][6].
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Which generator sizing approach fits your home?
Quick check
Two main sizing approaches exist: a whole-house unit sized to carry essentially all household loads, or an essential-loads design.
The article does not specify a single alternate recommendation; consider using manufacturer sizing tools and consult a licensed installer to determine the right approach.
Confirm sizing, ATS compatibility, and installation details with the generator/ATS manufacturer and a licensed electrician or factory-authorized installer.