Generator Sizing + Selection Reference
Why this matters
Customer wants backup power but doesn't know what size. Wrong size = critical loads don't run when needed OR overspending on capacity never used. The tech who sizes correctly + installs properly delivers peace of mind. This is the field card.
Generator types
Portable generator: 1,000 - 12,000 W; gasoline OR propane; plug appliances directly OR through generator panel.
Inverter generator: small portable; quiet; sine-wave electricity (electronics-safe); 1,000 - 7,000 W.
Standby (stationary) generator: 7,000 - 50,000+ W; installed permanently; auto-starts on power loss; natural gas OR propane OR diesel.
Portable standby (skid-mounted): large portable; commercial use; 20,000 - 100,000+ W.
Commercial / industrial: 50,000 W+ to megawatts.
Standby generator sizes (residential)
| Size (kW) | Watts | Coverage |
|---|---|---|
| 7 - 12 | 7,000 - 12,000 | Essentials (lights, fridge, small AC) |
| 14 - 22 | 14,000 - 22,000 | Half-house OR whole-house essentials |
| 22 - 28 | 22,000 - 28,000 | Whole house typical residential |
| 30 - 50 | 30,000 - 50,000 | Large home, multiple AC zones |
Modern trend: 22 kW Generac OR Kohler is most-common residential install.
Sizing approach
Watts on the appliance label: running watts + starting (surge) watts.
Running watts: continuous operation.
Starting watts: brief surge at startup (motors, compressors); 2 - 5x running watts.
Generator must handle both:
- Sum running watts of expected simultaneous load
- Add largest motor's starting surge
Common appliance watts
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 600 - 800 | 1,200 - 2,000 |
| Window AC 5000 BTU | 500 - 700 | 1,500 |
| Central AC 3 ton | 3,500 - 5,000 | 8,000 - 11,000 |
| Heat pump 3 ton (heat) | 3,500 - 5,000 | 8,000 - 11,000 |
| Sump pump | 800 | 1,500 |
| Well pump 1 HP | 1,500 | 4,000 |
| Furnace blower | 700 - 1,000 | 2,000 |
| Garage door opener | 350 | 700 |
| Microwave 1000W | 1,500 | 1,500 |
| Toaster | 1,200 | 1,200 |
| TV LED 55" | 100 | 100 |
| LED lighting 8 bulbs | 70 | 70 |
| Hairdryer | 1,500 | 1,500 |
| Electric stove (1 burner) | 2,000 | 2,000 |
| Electric water heater | 4,500 | 4,500 |
| Electric dryer | 5,000 | 5,000 |
Whole-house sizing example
Residential 3-bedroom home with central AC + gas water heater:
Essentials only:
- Refrigerator: 800W running + 2,000 surge
- Sump pump: 800W running + 1,500 surge
- Gas furnace blower: 1,000W running + 2,000 surge
- LED lights (10 × 10W): 100W
- Internet / router: 50W
- TV + sat / cable: 200W
- Microwave (intermittent): 1,500W
- Coffee maker (intermittent): 1,200W
Running total: ~ 5,000W
Surge from heaviest motor (refrigerator + sump compounds): + 2,000W
Generator size: 7,000 - 8,000W minimum.
Whole-house (include AC):
- Above + central AC (5,000W run / 11,000 surge): + 5,000W run + 6,000W surge
- Total: 10,000W run + 16,000W surge
- Need: 16 - 22 kW generator
Standby generator install (residential)
Concrete pad: outside; level + drained.
Distance from house: 18" min from the wall; 5 ft (60") from any door, window, or fresh-air intake; 36" from combustibles.
Gas line: natural gas OR propane; sized for load (often 1" gas line for 22 kW).
Transfer switch: required; auto OR manual; matches generator size.
Transfer switch types:
- Whole-house ATS (automatic transfer switch): full panel
- Subpanel ATS: critical loads only
- Manual transfer switch: requires homeowner activation
Permit + inspection: required for permanent install + transfer switch.
Cold-weather considerations
Battery: starts hard in cold; consider battery warmer.
Engine oil: synthetic OR cold-rated.
Coolant: rated for climate.
Air intake: clear of snow.
Exhaust: clear of snow drifts.
Generator install pricing
Quote the project, not the generator. Customers shop the equipment price online and then get surprised, because on a typical residential standby the installed job runs somewhere around double the equipment cost, and can run higher when the gas or electrical work is heavy.
What the quote has to include:
- Generator and transfer switch (the switch is a real line item, not an accessory)
- Site work: pad or gravel base, leveling, and any trenching
- Gas: line sizing, tapping the meter, new run to the unit, and possibly a meter upsize from the utility. On propane, tank sizing or a second tank, and the regulator
- Electrical: feeder from the transfer switch to the panel, a subpanel if it is a critical-loads install, grounding electrode, and any panel work
- Permits and inspections, usually both electrical and gas or mechanical
- Utility coordination for the gas meter, and notification to the electric utility
- Startup, load test, and customer orientation
- First-year warranty registration and the maintenance conversation
What drives the number up:
- Distance from the panel and from the gas meter; every extra foot is conduit, wire, and pipe
- Undersized existing gas service, which forces a utility meter change and a lead time
- Rock, hardscape, or a finished basement between the unit and the panel
- Whole-house automatic transfer versus a critical-loads subpanel; whole-house costs meaningfully more in both switch and feeder
- Panel that is full or obsolete and has to be replaced first
- Jurisdictions with plan review or engineered pad requirements
How to present it: give the customer a single installed price with the scope written out, and show the two decisions that actually move the number, which are generator size and whole-house versus critical loads. Offering a smaller unit on a critical-loads switch is usually the honest lower-cost option, and it beats losing the job to a competitor who quoted equipment only and will change-order the rest.
Recurring revenue: price the annual maintenance agreement at the same time. A standby generator is a machine that sits idle and must start on the worst day of the year, and the service plan is a small fraction of the install that protects the whole investment.
Never quote from the phone. Gas line size, panel condition, setback distances, and the run length are site facts. A phone number becomes a change order, and change orders on a project this size are what generate complaints.
Portable generator considerations
Run on gas / propane / diesel: typically 8 - 20 hr per tank.
Inverter portable (Honda EU2200, EcoFlow): quiet, electronics-safe.
Plug into transfer switch (interlock kit OR manual transfer switch): safer than running cords inside.
NEVER use indoors / garage / open window: CO poisoning kills.
Run > 25 ft from any open window OR door.
Common generator install mistakes
Undersized for AC surge (AC won't start). Wrong gas line size (engine starves). No transfer switch (illegal to back-feed grid via outlet). Generator too close to house (exhaust + fire risk). Not test-running periodically (battery + fuel issues). Skipping permit (insurance + code issue). Cold-weather not addressed.
Maintenance schedule
Monthly: test run for 15 - 20 min (Generac auto-test).
Annually: oil change, air filter, spark plug, battery, transfer switch test.
Every 5 years: load test + inspection.
Fuel storage (portable): rotate fuel; use stabilizer for long storage.
Power transfer modes
Whole-house ATS: switches entire panel from utility to generator + back.
Subpanel ATS: critical-load panel switched; main panel stays utility.
Manual transfer switch: homeowner flips switches manually.
Interlock kit: physical block on panel to prevent backfeed when generator running.
Customer education
Run time per gallon of fuel (for portable): set realistic expectations.
Maintenance commitment: weekly visual + annual service.
Test schedule: turn off main + verify generator picks up.
Cold-weather prep: battery, oil, intake clear.
Don't store gas long-term: stabilizer + rotate annually.
CO detector inside: protect from any exhaust intrusion.
The single most-useful generator sizing habit is the AC SURGE CHECK first. Customer says "I want backup for everything" - but if their 3-ton central AC compressor requires 9,000 W to start, an 8 kW generator won't get the AC running (the compressor stalls + fault clears, repeatedly). For whole-house with AC running, size for AC starting surge plus all other simultaneous loads. For essentials-only, leave AC off and size for the small loads only. Educate customer that whole-house running EVERYTHING simultaneously requires a much larger (more expensive) generator + larger gas line - most stick with critical-loads ATS + selective use.
References
- NEC Article 702 (Standby Power Systems)
- NFPA 110 (Emergency + Standby Power)
- Manufacturer technical data (Generac, Kohler, Cummins, Briggs + Stratton)
- ICC + IRC generator install requirements
- Manuall internal: Electrical Conversion Math, Common Codes Overview