How to choose a standby generator size (checklist)

Also see: How to maintain a standby generator.

Homeowners and small-business managers facing a power outage often assume bigger is better. In reality, an oversized standby generator wastes fuel, costs more up front, and may strain your home’s electrical system if not properly matched. This checklist walks you through the practical steps of sizing a residential standby generator, from identifying essential loads to understanding fuel constraints and permit requirements.

Important: This guide is educational only. Always engage a licensed electrician for a NEC Article 220 load calculation. Never backfeed a home panel. A transfer switch (ATS) is required for safe isolation from the grid. Fuel type, gas-meter capacity, and local codes all affect what you can install. Cost bands below are illustrative ranges from secondary contractor sources; they are not quotes.

1. Start with a load inventory

Before requesting quotes, list every circuit or appliance you want to keep running during an outage. Prioritize into tiers:

Must-run (essential for safety and basic living)

• Refrigerator / freezer
• Sump pump (if applicable)
• Well pump (if municipal water is unavailable)
• Medical equipment (CPAP, oxygen, etc.) — verify with provider
• Lighting for key areas
• Security system / alarm panel

Important (comfort and property protection)

• HVAC system (see seasonal considerations below)
• Water heater (electric or gas, depending on setup)
• Garage door opener (if used for egress)
• Home office equipment (router, computer)

Nice-to-run (luxury loads)

• Pool pump
• EV charger (Level 2)
• Hot tub
• Range / oven (usually not recommended on standby due to high draw)
• Clothes dryer

2. Seasonal HVAC considerations

Heating and cooling are typically the largest electrical loads in a home. When sizing:

• Winter: If you have an electric heat pump or electric furnace, the compressor or blower motor may draw 3–5 kW or more. A gas furnace blower motor is usually smaller (1–2 kW), but the ignition system may add load.
• Summer: Central AC compressors can range from 2 kW to 5 kW depending on tonnage. A 3-ton unit may draw ~3.5 kW running, with a higher surge at startup.
• Hybrid systems: Some homes have dual-fuel or zone systems. Confirm the exact nameplate kW of the outdoor unit and blower motor.

Rule of thumb: Do not size a generator to run the entire home’s HVAC at once unless the electrician confirms the service panel can handle the combined start-up surges. Many homeowners opt to power only the thermostat/blower circuit and use window fans or portable heaters during outages.

3. Fuel type and nameplate rating

A common sizing trap is comparing the propane (LP) headline kW to what you will actually get on utility natural gas (NG). On many air-cooled residential units, the advertised kW is the LP figure; NG output is often roughly 8–12% lower (dealer secondary guides cite examples such as a “22 kW” class unit delivering closer to ~19.5 kW on NG). Always compare the NG column if that is your fuel.

• Check the manufacturer spec sheet for both NG and LP continuous ratings — do not size off the LP headline alone.
• Verify your fuel source: Most suburban installs use utility natural gas. If you need propane tanks, confirm the model’s LP configuration and that tank size supports the outage duration you care about.
• Gas-meter capacity: Full-load residential standby units can pull hundreds of cubic feet per hour. Have your gas utility or a licensed plumber verify meter and service-line capacity, especially with other gas appliances running at the same time.

4. Transfer switch type and ATS sizing

A standby generator is useless without a transfer switch (ATS — automatic transfer switch). The ATS isolates the circuits you want to power and switches them from the grid to the generator when an outage occurs.

• Circuits vs. sub-panel: You can wire the ATS to specific circuits in your existing panel, or install a dedicated sub-panel for standby loads. A sub-panel makes it clearer which circuits are “essential” and simplifies future expansions.
• ATS amperage: The ATS rating must match or exceed the total expected load. An electrician will sum the kW of your selected circuits, convert to amperage (kW ÷ voltage × power factor), and select an ATS with appropriate capacity.
• Manual vs. automatic: For residential standby, an automatic transfer switch is standard. Manual transfer switches exist but require someone to be present to flip the switch, defeating the convenience purpose.

5. Permits, HOA, and local codes

Standby generator installation is subject to local electrical codes, zoning, and permitting. Requirements vary by municipality and county.

• Permits: Most jurisdictions require an electrical permit and a separate mechanical/gas permit if the fuel line is modified.
• HOA restrictions: Some communities regulate generator placement, noise levels, and even fuel type. Review your covenants before purchasing.
• Setbacks and enclosures: Local codes may dictate how close a generator can be to property lines, windows, or doors. Outdoor enclosures must provide adequate ventilation while protecting the unit from weather.
• Flood zones: If you live in a flood-prone area (including SE Florida examples), codes may require the generator to be elevated on a pad or platform above base flood elevation. This is an illustrative example; always check your local flood maps and building codes.
• Florida SE examples (illustrative only): Contractors in South Florida often cite permit timelines and HOA submission packets as part of the installation process. These are not national standards but reflect local AHJ (Authority Having Jurisdiction) practices.

6. Cost bands (illustrative only)

Cost for a turnkey standby generator installation varies widely based on region, generator size, electrical complexity, and fuel infrastructure. The following bands are from secondary contractor sources labeled as illustrative Southeast Florida ranges and should not be taken as quotes:

• 22 kW class: Approximately $12,000 – $16,000 turnkey (SE Florida illustrative)
• 26 – 36 kW class: Approximately $16,000 – $22,000 turnkey (SE Florida illustrative)
• National Generac guidance: Often cited as ~$8,000 – $16,000, but regional uplift for labor, permits, and gas-meter upgrades should be expected.

Never rely on online pricing calculators as final quotes. Get at least one licensed electrician site visit and load calculation before committing.

7. Practical checklist: what to verify before you buy

1. List must-run loads (refrigerator, well, medical, lighting, sump)
2. Note seasonal HVAC loads (heating vs. cooling, tonnage, blower motor)
3. Confirm fuel type (natural-gas line vs. propane tank size and location)
4. Have gas utility or plumber verify meter capacity for generator load + existing appliances
5. Obtain NEC Article 220 load calculation from a licensed electrician
6. Determine ATS type (specific circuits vs. sub-panel)
7. Check local permit requirements and HOA covenants
8. Review setback and flood-zone regulations for your property
9. Get at least two written quotes from licensed contractors after the load calc
10. Ask about warranty and maintenance plans (refer to manufacturer literature, do not infer claims)

8. Questions to ask your installer

When you meet with a licensed electrician or generator contractor, consider asking:

1. Can you perform a NEC Article 220 load calculation for my panel, and will you provide a written report?
2. Which circuits or sub-panel do you recommend for my essential loads, and why?
3. Will the natural-gas meter and service line support the generator’s fuel demand at full load?
4. What transfer switch model do you specify, and how is it sized relative to the expected load?
5. What permits are required in this municipality, and do you handle the application process?
6. Are there HOA or setback restrictions I should know about before finalizing the unit location?
7. What is the expected run time at 50% load given my gas supply and tank size (if applicable)?
8. Do you provide a maintenance schedule or service agreement for the first year?

Sources

1. Generac sizing guide PDF 2025 (NEC 220.82 / 220.87 framing): https://www.denneyelectricsupply.com/content/Generac_Generator_Sizing_Guide%202025.pdf
2. Transfer switch quote checklist / NEC 445/700/701/702 framing: https://www.documentorium.com/blog/generator-transfer-switch-quotes-for-electrical-contractors
3. Generac vs Kohler NG vs LP output trap (secondary dealer): https://www.portlandiaelectric.supply/blogs/energy-solutions/generac-vs-kohler-standby
4. ATS sizing 2026 guide (secondary): https://www.portlandiaelectric.supply/blogs/brand-guides/transfer-switch-sizing-guide-amps-circuits-ats-vs-manual-2026-pes
5. South Florida sizing/permit/cost ranges (secondary contractor; label as FL SE example): https://www.trophyelectricllc.com/standby-generator-installation-in-south-florida-sizing-transfer-switches-and-the-2026-permit-process
6. NFPA 110-2025 load-bank notes (maintenance adjacency, not sizing core)

Affiliate relationships will be disclosed when live. This guide is not tax, legal, or engineering advice. Always consult a licensed professional for your specific situation.