Whole-Home Generator Sizing Guide: What Size Do You Need?

Buying a whole-home standby generator is a $5,000–$15,000 decision, and the single most expensive mistake is buying the wrong size. Undersize it and the unit overloads, trips, or damages the compressor on your air conditioner. Oversize it and you spend thousands extra on capacity you never use — plus more fuel every hour it runs. This guide walks you through the same sizing process our team uses when a customer calls Portlandia Electric Supply: a load worksheet, a kW quick-reference table, a fuel-type comparison, and the transfer-switch requirements that have to be in place before any standby unit goes live.

If you're still comparing brands, start with our roundup of the best whole-house generators, then come back here to dial in the size. Shopping for a specific output class? Browse our standby home generators and full standby generator collection while you read.

Step 1 — Understand running watts vs. starting watts

Every electrical load in your home has two numbers:

  • Running (rated) watts — what the appliance draws continuously.
  • Starting (surge) watts — the 2–3x spike a motor-driven load pulls for a few seconds when it kicks on. Air conditioners, well pumps, refrigerators, and furnace blowers all have compressors or motors with significant surge.

A standby generator is sized in kilowatts (kW) of continuous output, with a smaller surge reserve. The sizing question isn't "what's my total?" — it's "what runs at the same time, and what's the biggest motor that has to start while everything else is already running?"

Step 2 — The whole-home generator sizing worksheet

Walk your panel and list every circuit you want backed up. Use these typical values (check your equipment nameplates for exact figures):

Load Running watts Starting watts
Central AC, 3-ton (36,000 BTU) ~3,500 ~7,000–10,500
Central AC, 4-ton ~4,700 ~9,400–14,000
Central AC, 5-ton ~5,900 ~11,800–17,700
Furnace blower (1/2 HP) ~800 ~2,000
Well pump (1 HP) ~1,000 ~2,500–3,000
Refrigerator/freezer ~700 ~2,100
Sump pump (1/2 HP) ~1,050 ~2,600
Electric water heater ~4,500 ~4,500
Electric range (one oven element) ~2,000–3,000 —
Electric dryer ~5,000 ~6,000
Microwave ~1,000 —
Lights (whole home, LED) ~400–800 —
Home office + networking ~300–500 —
Garage door opener ~550 ~1,100

Worksheet math:

  1. Add the running watts of everything you expect to run simultaneously.
  2. Identify the single largest motor load and add its starting watts instead of its running watts.
  3. Add a 20–25% headroom margin — generators run happiest at 60–80% of rated output, and headroom covers future loads.
  4. Divide by 1,000 to get your minimum kW.

Worked example: 4-ton AC (4,700W), refrigerator (700W), furnace blower (800W), lights and outlets (1,200W), sump pump (1,050W) = 8,450W running. Biggest motor is the AC: swap in its ~9,400W starting figure → 4,700 + 9,400 − 4,700 + 3,750 = roughly 13,150W. Add 25% headroom → ~16.4 kW minimum → shop the 18–22 kW class.

Step 3 — kW quick-reference table

Prefer a shortcut? These bands cover most homes we see:

Home / scenario Typical coverage Recommended size
Small home / essentials only (fridge, furnace fan, lights, sump, outlets) No central AC, gas heat 10–14 kW air-cooled
Average home, 1,500–2,500 sq ft, one 3–4 ton AC Managed whole-home with load shedding 18–22 kW air-cooled
Large home, 2,500–4,000 sq ft, one 5-ton or two smaller ACs True whole-home 24–26 kW air-cooled
Estate / 2+ large ACs / heavy electric appliances Whole-home, no compromises 36–48 kW liquid-cooled
Light commercial / very large residential, 3-phase service Full facility backup 50–150 kW liquid-cooled

A note on load management: modern 18–26 kW air-cooled units pair with smart load-management modules that briefly shed the AC or water heater if capacity runs short. That's why an 22 kW unit can genuinely power most of a 2,500 sq ft home — a question we see constantly ("what can a 22kW generator power?"). The honest answer: nearly everything, as long as the big 240V loads don't all start in the same second.

Step 4 — Fuel type: natural gas vs. propane vs. diesel

Air-cooled residential standby units run on natural gas (NG) or liquid propane (LP). Liquid-cooled units 30 kW and up add diesel to the mix.

Factor Natural gas Propane (LP) Diesel
Availability Utility piped to the home — no tank On-site tank (typically 250–500 gal) On-site tank, integrated base tanks common
Output derating ~8–10% less than LP rating Full rated output Full rated output
Runtime during outage Unlimited while gas utility is up Limited by tank size Limited by tank size
Fuel cost per kWh Usually lowest Moderate Moderate; best efficiency at heavy load
Cold-weather behavior Unaffected Pressure drops in extreme cold with small tanks Needs winter-blend/additives in severe cold
Maintenance Lowest Low Higher (fuel polishing, wet-stacking risk at light load)

How much propane does a 26 kW standby generator use per hour? A popular search, and a fair one: a 26 kW air-cooled unit burns roughly 2–2.5 gallons of propane per hour at half load and 3.5–3.9 gallons per hour at full load (check your model's spec sheet for exact figures). On natural gas, the same class of unit consumes roughly 200–300 cubic feet per hour. Practical takeaway: a 500-gallon propane tank (filled to ~400 gallons) gives you roughly 4–7 days of continuous runtime for a large home — one more reason correct sizing matters, since every extra kW of capacity burns fuel whether you need it or not. At the small end, a 10 kW generator running at half load sips closer to 1–1.4 gallons of LP per hour.

Step 5 — The transfer switch is not optional

A standby generator is a permanently installed appliance, and code (NEC Article 702) requires a listed transfer switch so your generator can never backfeed the utility grid — a lethal hazard for line workers and a warranty-voider for you. Key decisions:

  • Automatic transfer switch (ATS): standard on every true standby system. Detects the outage, starts the generator, and transfers the load in about 10–30 seconds, then reverses the process when utility power returns.
  • Service-entrance-rated ATS: for whole-home coverage, the ATS sits between your meter and main panel and must match your service size — typically 200 amps for modern homes. This is the most common pairing for 18–26 kW air-cooled units.
  • Load-center ATS: combines the switch with a sub-panel for essentials-only coverage on smaller (10–14 kW) systems.
  • Sizing rule: the switch ampacity must meet or exceed the breaker feeding it; a 26 kW unit on 240V single-phase delivers about 108 amps, which is why 200A service-entrance switches dominate.

We stock a deep bench of automatic transfer switches sized from 100A to 800A — match the switch to your service panel, not just the generator.

Step 6 — When to step up to liquid-cooled (50 kW and beyond)

Air-cooled units top out around 26 kW. Above that — estates with multiple large AC systems, homes with 400A service, small commercial buildings, agricultural shops — you move into liquid-cooled territory: automotive-style engines, quieter enclosures, longer service intervals, and three-phase options.

This is where our current inventory is strongest. As of this writing, these Cummins Quiet Connect liquid-cooled standby generators are in stock and ready to ship:

Three-phase configurations (120/208V and 277/480V) of the same RS50–RS150 line are also available — browse the full standby generators collection or the master generators collection for Generac Guardian air-cooled units from 10 kW to 26 kW, Briggs & Stratton Fortress liquid-cooled models, and Kohler options.

Common sizing mistakes to avoid

  1. Sizing to the panel, not the load. A 200A panel does not mean you need a 48 kW generator. Your diversified load is what matters — most 200A homes peak under 15 kW.
  2. Forgetting the AC's starting surge. Locked-rotor amps on an older compressor can demand 3x running watts. A soft-start kit on the AC can drop that dramatically and let you buy a smaller generator.
  3. Ignoring natural-gas derating. A "26 kW" unit on NG typically delivers ~24 kW. Size to the NG number if that's your fuel.
  4. No headroom. Running a generator at 95%+ continuously shortens its life. Target 60–80% of rated output for your expected load.
  5. Skipping the load calc for 3-phase. Commercial buyers: single-phase kW math doesn't transfer. Bring us your panel schedule and we'll size it properly.

The bottom line

For most homes the answer lands in a narrow band: 14 kW for essentials, 18–22 kW for a managed whole home, 24–26 kW for large homes, 50 kW+ liquid-cooled for estates and commercial. Run the worksheet, check your AC's starting draw, pick your fuel, and pair the unit with a properly sized automatic transfer switch.

Have your panel schedule or a list of circuits? Send it to the Portlandia Electric Supply team — we size standby systems every day and can quote the generator, the ATS, and freight in one shot.

Shop the collections: Standby Home Generators · Standby Generators · Automatic Transfer Switches · All Generators

Related charts & calculators

Keep these quick-reference charts handy while you plan:

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