Backup Generator Sizing and Installation Reference

Why this reference exists

Whole-home + critical-load backup generators are a installation that combines electrical + gas/propane work. Customer demand surged after major weather events + grid uncertainty. A typical install: 6-10 hours labor + permit. Recurring service revenue: annual maintenance per generator. This reference covers sizing + install fundamentals.

Generator types

Portable:

• 3-10 kW typical
• Gasoline fuel, sometimes propane
• Plug-connected to inlet box OR manual transfer switch
• Customer-operated
• • inlet box install

Stationary / standby:

• 7-50 kW residential typical
• Natural gas or propane (preferred); some diesel
• Automatic transfer switch
• Auto-starts during outage -+ for the generator; + install
• Most common for "whole-home backup"

Solar + battery (covered in Battery Storage Reference):

• Different category; not a fuel-burning generator
• Sometimes complementary

For most modern customers: stationary natural gas + automatic transfer switch.

Sizing

Whole-home backup: generator must serve ALL loads when grid is down.

Calculate per NEC 220 (standard load calc):

• Lighting + receptacles: 3 VA/sq ft
• Kitchen + bath fixed: nameplate
• HVAC: usually largest load - calculate startup + run
• Water heater (if electric)
• Well pump (rural)

Typical sizing:

• 2,000 sq ft no AC: 14 kW
• 2,500 sq ft with central AC: 18-22 kW
• 3,500 sq ft with central AC: 22-26 kW
• 4,500 sq ft with multiple AC: 30+ kW
• Add for: hot tub, pool, EV charger, electric range, electric dryer

Critical-load (essential circuits only): generator serves selected loads via sub-panel.

• 8-14 kW typical
• Refrigerator, freezer, lights, well pump, furnace fan, internet, garage door, a few outlets
• Lower cost, smaller generator
• Customer accepts not having "everything" during outage

Load shedding (modern generators):

• Manage which loads run when generator can't carry all
• Smart panel + generator integration (Generac PWRcell, Tesla Backup, FranklinWH)
• Allows smaller generator with smart prioritization

Fuel sources

Natural gas (preferred):

• Unlimited supply during outage (utility gas usually independent of electrical grid)
• Standard residential gas service supports most generators
• Higher operating cost than propane (slightly)

Propane:

• 250-1,000 gallon tank typical (residential)
• Runs out (sizing tank for duration: ~1 gallon per hour per 10 kW)
• Higher BTU per gallon than gas
• Common in rural without natural gas

Diesel:

• Most expensive fuel
• Larger commercial generators
• Refueling concerns for long outages

Gasoline (portable only):

• Refueling concerns
• Stale fuel issues
• Not for standby

For most residential install: natural gas if available, propane otherwise.

Transfer switch options

Manual transfer switch (portable generators):

• Customer plugs portable generator into inlet box
• Manually flips switch to "generator" position
• Powers selected critical-load sub-panel
• • install
• total system installed

Automatic transfer switch (stationary):

• Senses grid outage
• Switches to generator (after generator starts)
• Returns to grid when restored
• transfer switch -+ total system installed

ATS sizing:

• Whole-home: ATS rated equal to or greater than service amperage (200A typical)
• Critical-load: smaller ATS feeding sub-panel

Permitting + inspection

Required for nearly all install:

• Electrical permit (panel + ATS work)
• Gas permit (if natural gas)
• Building permit (foundation pad, generator placement)
• HOA approval (some require)
• AHJ inspection at completion

Permits + inspections:. Customer-paid pass-through.

Installation procedure

Step 1: Site assessment

1. Generator location: 5 ft from windows, doors, intake vents (per NFPA 37)
2. Concrete pad or foundation
3. Distance from main panel (longer run = larger conductors)
4. Gas line route (if natural gas)
5. Permits filed
6. Customer-approved location + permission

Step 2: Site prep

1. Pour concrete pad 4-6" thick, sized to generator
2. Trench for gas line + electrical conduit
3. Bonding (per NEC 250)
4. Drainage around pad

Step 3: Gas connection

1. Tap natural gas at supply main (1.5-2 psi typical; verify)
2. Sized to generator BTU demand (often 3/4" or 1" line)
3. Shutoff valve at generator
4. Pressure test per code
5. Soap-test all joints

Step 4: Electrical conduit + wire

1. Conduit from generator to ATS
2. 4-conductor: 2 hot + neutral + ground
3. Sized per NEC for distance + amperage
4. Both ends ready for termination

Step 5: ATS install

1. Mount in proper location (often near main panel or on wall)
2. Wire from main service to ATS (from utility side)
3. Wire from ATS to main panel (to load side)
4. Wire from generator to ATS
5. Control wiring from ATS to generator (sensing + start signal)

Step 6: Generator install

1. Place generator on pad
2. Bolt to pad with vibration isolators
3. Connect gas + electrical
4. Initial fuel-up + leak check

Step 7: Commission

1. Start generator (manual start)
2. Run no-load 10-15 minutes; warm engine
3. Connect to ATS; verify voltage + frequency
4. Test ATS transfer: simulate outage; ATS should switch to generator
5. Test restoration: restore grid; ATS should return to utility
6. Run under load 20-30 minutes; verify stable operation
7. Customer briefing + paperwork

Step 8: Customer briefing

1. Manual start procedure (if needed)
2. Annual maintenance recommendation
3. App pairing (Generac WiFi, Kohler OnCue, Champion)
4. Warranty registration
5. Service contract offer

Customer talking points

For outage-resilience customer:

1. "Three options: portable, critical-load, whole-home."
2. "Portable: cheapest; you manage during outage.K total."
3. "Natural gas: unlimited fuel during outage. Propane: tank-limited. Battery: silent, but limited capacity."

For specific concerns:

• Medical equipment: whole-home or critical-load with redundancy
• Well pump: critical-load including well
• Home office: critical-load including office + internet
• Family with kids: whole-home (food preservation + comfort)

Service + maintenance

Generators need maintenance:

• Oil change every 100-200 hours OR annually
• Air filter
• Spark plugs (smaller engines)
• Battery check (battery starts engine)
• Fuel filter
• General inspection + run test

Annual contract:. Cherry-pick high-density neighborhoods; service-route economics.

Common pitfalls

References

• NEC Article 445 + 700 (Generators + Emergency Systems)
• NEC Article 702 (Optional Standby Systems)
• NFPA 37 (Stationary Combustion Engines)
• NFPA 54 (Fuel Gas)
• Manufacturer install manuals (Generac, Kohler, Briggs, Champion, Westinghouse)
• Manuall internal: Generator Transfer Switch Installation, Electrical Panel Upgrades for Electrification Reference