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 and interlock install is the smaller half of the job; the generator itself is the larger

Stationary / standby:

• 7-50 kW residential typical
• Natural gas or propane (preferred); some diesel
• Automatic transfer switch
• Auto-starts during outage
• Equipment and installation are comparable line items on a typical job; the install side carries the pad, the gas run, the transfer switch and the permits, so it is not the small half
• 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 adds the sub-panel, the transfer switch and the circuit moves
• Installed cost lands between a portable setup and a whole-home system

Automatic transfer switch (stationary):

• Senses grid outage
• Switches to generator (after generator starts)
• Returns to grid when restored
• transfer switch
• Installed cost is the highest of the three options, and the automatic transfer switch and gas run are a meaningful share of it

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 and inspections are a customer-paid pass-through. Quote them separately from your labor so the customer can see what is yours and what is the jurisdiction's.

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 is the cheapest way in, and the trade is that you handle it: you roll it out, start it, run the cord, and refuel it in the weather that caused the outage."
3. "Critical-load costs more than a portable and less than whole-home. We pick the circuits that matter, usually the furnace, the fridge, the well pump and a few outlets, and those come back automatically."
4. "Whole-home is the most expensive and the least thought about. Everything runs, it starts itself, and you find out there was an outage from the neighbours."
5. "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

An annual service contract is the recurring line that makes generator work worth carrying. Cherry-pick high-density neighborhoods so the route economics work; scattered rural units eat the margin in drive time.

Common pitfalls

Any connection that can backfeed the utility. No listed transfer means, or a customer using a double-ended cord into a dryer receptacle, energizes the service drop and can kill a lineman working the outage. If a customer already owns a portable and a homemade cord, that conversation happens before anything else on the visit, and it ends with an inlet box and a transfer switch or it ends with you walking.

Exhaust and clearances treated as a formality. Keep the unit away from windows, doors, and intakes, out of alcoves and from under decks, and pointed so exhaust cannot wash back at the house or at a neighbor's intake. Manufacturer clearances for airflow and for service access are frequently larger than the code minimum, and the inspector enforces both. Also site it above local snow depth and out of any area that floods.

Undersized gas supply. The most common commissioning failure by a wide margin. The unit starts fine and then stumbles or shuts down the moment load hits, because the meter and the piping cannot deliver the volume at the required pressure. Add up the whole connected load, not just the generator, size the piping for the run, and get the utility to upsize the meter or regulator when the numbers say so. Verify pressure at the unit while it is running at full load, never at idle.

Getting the neutral wrong. Whether the installation is a separately derived system determines where the neutral-to-ground bond belongs and which transfer switch (switched or solid neutral) is correct. Guessing here produces nuisance tripping, objectionable current on grounding conductors, and ground-fault protection that does not do what anyone thinks it does. Confirm the generator's internal bonding configuration before you order the switch.

Sizing off square footage instead of a calculation. Run the load calc, and account for motor starting, not just running load. An air conditioner and a well pump trying to start together is what actually stalls an undersized unit. Where the numbers are tight, a soft starter on the compressor or a load management module is cheaper than the next generator size up. Ask about the EV charger, the heat pump, and the pool that are coming in two years.

Voltage drop on the run to the generator. Back-of-lot placements are common and the conductor size has to reflect the distance, not just the ampacity.

Control wiring bundled with power conductors where the manufacturer prohibits it. The symptom is intermittent false starts and comms faults months later, and it is miserable to chase.

Commissioning with no load. A unit that runs beautifully with nothing connected has proven nothing. Transfer it and load it, with the air conditioning running, and watch voltage and frequency hold.

Forgetting the battery. The starter battery is the leading cause of a standby unit failing to start when it is finally needed. Confirm the charger is fed from the utility side, and tell the customer plainly that the battery is a wear item on a schedule.

Exercise settings and the neighbors. Set the weekly exercise for a reasonable hour and check the local noise rules. A unit that test-runs before dawn generates complaints that come back to you.

Leaving without the handoff. The customer needs to know where the gas shutoff is, how to shut the unit down, what the exercise cycle sounds like, and exactly which circuits are backed up. Label the panel directory with the backed-up circuits while you are standing there.

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