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 window OR door; 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
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