Standby Generator Sizing Calculator: How to Choose the Right kW

Reading time: ~14 min read

Selecting the right standby generator capacity is the decision that makes or breaks a backup power installation. Undersize the unit and you get nuisance tripping, voltage sag, and compressors that never quite start. Oversize it and you burn capital on capacity that idles along at 20% load, wet-stacking and drinking fuel. I've sized these systems at the Portlandia Electric Supply counter for years, and the same five-step worksheet handles 95% of the homes that call. This guide walks that worksheet — load inventory, running watts, starting surge, the 80% rule, surge verification — then layers on NEC Article 220, fuel derating, altitude correction, and a straight comparison of Generac, Cummins, and Kohler.

For a full selection of standby and portable units, browse our generators collection and standby generators. If you want the homeowner-friendly version first, our whole-home generator sizing guide covers the same math without the code references.

Understanding Generator Power Ratings

Standby generators carry two ratings every installer must separate before touching a calculator:

• Continuous (running) rating, kW: the sustained power the unit delivers indefinitely under rated conditions. This is the number you size loads against.
• Surge (starting) rating, kW: the momentary overload capacity — typically 10–30% above continuous for conventional sets, available for 2–5 seconds to start motors. Inverter generators can deliver up to 200% of continuous rating for 1–3 seconds.

The 80% rule is the industry standard for continuous loading: sustained load stays at or below 80% of the continuous rating. That margin covers voltage regulation, thermal headroom, altitude losses, and the load somebody adds two years from now. A generator loafing at 60–80% outlasts one screaming at 95% every time.

Starting Watts vs. Running Watts: The Motor Surge Problem

The most common sizing mistake is ignoring motor inrush. Every load falls into one of two categories:

Resistive loads

Resistive loads draw the same wattage at startup as in steady state — no surge at all. Incandescent lighting, electric water heater elements, space heaters, and toaster elements all behave this way.

Inductive (motor) loads

Inductive loads pull a starting surge of 1.5 to 7 times running wattage for 0.1 to 5 seconds. Two nameplate terms run the calculation:

• FLA (Full Load Amps): the steady-state running current.
• LRA (Locked Rotor Amps): the current drawn the instant the motor energizes. The LRA/FLA ratio is your surge multiplier.

I watched a 14 kW unit lug down and trip every time a 5-ton compressor kicked — undersized by exactly one surge event. The fix wasn't a bigger generator; it was a soft-start kit that cut the compressor's inrush by more than half. Check LRA on the condenser nameplate before you blame the generator.

The table below shows typical running and starting watts for common residential and light-commercial loads:

Load	Running Watts	Surge Multiplier	Starting Watts
Refrigerator (18 cu ft)	600–800	3×	1,800–2,400
Deep freezer (15 cu ft)	500–700	3×	1,500–2,100
Sump pump (½ HP)	800–1,050	3–5×	2,400–5,250
Well pump (½ HP, 240V)	1,000–1,200	5–7×	5,000–8,400
Furnace blower (½ HP)	500–800	2–3×	1,000–2,400
Central AC (3-ton)	3,500–4,000	3–5×	10,500–20,000
Window AC (12,000 BTU)	1,000–1,200	2–3×	2,000–3,600
Electric water heater	4,500	1× (resistive)	4,500
Microwave (1,000W cooking)	1,500–1,700	1×	1,500–1,700
LED lights (10 bulbs)	100	1×	100

For loads rated in amps rather than watts, use the fundamental power formula: Watts = Volts × Amps. Use 120V for standard receptacles and 240V for well pumps, water heaters, dryers, and central air equipment.

Step-by-Step Generator Sizing Calculation

Step 1: Build a load inventory

Walk the panel and list every load the generator must support — voltage, running watts, and starting surge for each. Include essential and optional loads, then prioritize. A load management system that starts big motors sequentially rather than simultaneously lets a smaller generator carry the same house by avoiding stacked surges.

Step 2: Calculate total running watts

Sum the running wattage of all loads that operate simultaneously: Total Running Watts = Σ (running watts of all simultaneous loads).

Step 3: Determine the starting surge

The surge calculation depends on the transfer equipment:

• Manual transfer switch (MTS): the operator starts loads sequentially. Required surge = total running watts + the single largest starting surge.
• Automatic transfer switch (ATS): multiple motors may start the instant power transfers. Required surge = total running watts + the sum of starting surges for every motor that starts at once — unless load management staggers them.

Step 4: Apply the 80% continuous load rule

Required Continuous Rating = Total Running Watts ÷ 0.80. This keeps the unit inside thermal limits and holds voltage regulation under sustained load.

Step 5: Verify surge capacity

Confirm the selected generator's surge rating exceeds the Step 3 total. If it doesn't, either step up a size or add soft-start/load management to shrink the surge.

Worked Example: Whole-Home Sizing

A residential installation on an ATS with these connected loads:

• Furnace blower: 600W running, 1,800W surge
• Refrigerator: 700W running, 2,100W surge
• Deep freezer: 600W running, 1,800W surge
• LED lights (10 bulbs): 100W, no surge
• Modem/router: 20W, no surge

Total running watts: 600 + 700 + 600 + 100 + 20 = 2,020W
Derated continuous rating: 2,020 ÷ 0.80 = 2,525W (≈ 2.5 kW)
ATS starting surge (simultaneous motor start): 2,020 + 1,800 + 2,100 + 1,800 = 7,720W
Selected generator: at least 2.5 kW continuous with 8 kW surge. In practice a 7–8 kW standby unit with ~10 kW surge covers this profile with headroom to spare.

Add a well pump, a 3-ton central AC, and an electric water heater and the picture changes fast: that 3-ton AC alone can demand 10,500–20,000 starting watts, which pushes the system into the 18–22 kW class. For a middle path, select-circuit systems like the Champion 12.5kW (select-circuit ATS compatible) or the Kohler 10kW with 100A 12-circuit ATS cover essentials without paying for whole-home capacity.

kW Quick Reference: Sizing Bands by Home

Prefer a shortcut before running the full worksheet? These bands cover most of what we quote:

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 / 3-phase service	Full facility backup	50–150 kW liquid-cooled

Load management is why an 18–22 kW air-cooled unit can genuinely run most of a 2,500 sq ft home: smart modules briefly shed the AC or water heater if capacity runs short, so the big 240V loads never start in the same second. Size the generator to the managed load, not the theoretical everything-at-once load.

NEC Article 220 Load Calculation Method

The wattage inventory is the practical method; NEC Article 220 is the formal one many jurisdictions require for permit submittal. The standard method for dwelling units:

• General lighting and receptacles: 3 VA per sq ft of living space (NEC 220.41)
• Small-appliance branch circuits: two 1,500 VA kitchen/pantry circuits (NEC 220.52(A))
• Laundry circuit: one 1,500 VA circuit (NEC 220.52(B))
• Fixed appliances: nameplate ratings for water heater, dishwasher, disposal
• Dryer and range: demand factors per NEC 220.54 and 220.55
• HVAC and motors: nameplate FLA/LRA per NEC 430, with 25% additional on the largest motor

Apply Table 220.45 demand factors to general lighting: first 3,000 VA at 100%, the remainder at 35%. For optional standby systems under NEC Article 702, the generator must carry every load intended to run simultaneously — or the transfer equipment must prevent simultaneous operation beyond capacity. The optional method in NEC 220.82, available for dwellings with 100A+ service, usually yields a lower calculated load than the standard method and is worth running before you commit to a size. Conductor and breaker sizing for the install side lives in our NEC wire sizing and ampacity guide and standard breaker sizes chart.

Fuel Type Impact on Sizing and Performance

Fuel choice moves the power number, not just the operating cost:

Fuel Type	Typical Sizes	Power Derating vs Natural Gas	Key Considerations
Natural Gas	7–60+ kW	Baseline (0%)	Continuous supply, no refueling, lower BTU content per cubic foot
Propane (LP)	7–60+ kW	+0 to +5% more power	Higher energy density, requires tank, vaporization limits in cold weather
Diesel	10–500+ kW	+10 to +15% more power	Highest power density, fuel stability concerns, ideal for large/commercial

Here's the trap: many air-cooled units rated for propane produce 5–10% less on natural gas. A "20 kW" generator on NG may only deliver 18 kW. Always verify the rated kW for the specific fuel on the manufacturer's spec sheet — not the marketing headline — and size to the lower (NG) number on dual-fuel units.

Fuel consumption scales with size, which is the running-cost argument against oversizing. Typical published spec-sheet ranges:

Unit class	LP @ half load (gal/hr)	LP @ full load (gal/hr)	NG @ half load (ft³/hr)	NG @ full load (ft³/hr)
10 kW	~1.0–1.4	~1.6–2.0	~110–140	~160–200
14 kW	~1.4–1.8	~2.3–2.9	~150–190	~220–280
22 kW	~1.8–2.3	~3.0–3.8	~200–280	~300–380
26 kW	~2.0–2.5	~3.5–3.9	~230–300	~330–420

A 26 kW unit at half load drinks 2–2.5 gallons of LP every hour; a 14 kW doing the same real work sips closer to 1.4–1.8. Over a week-long outage that's a 500-gallon tank versus two-thirds of one. The full reference table is in our generator fuel consumption chart.

Generac vs. Cummins vs. Kohler: Sizing and Selection

The three dominant manufacturers each bring a different strength to the sizing conversation.

Generac

• Residential range: 7.5–26 kW (air-cooled and liquid-cooled)
• Commercial range: 22–500 kW (liquid-cooled, diesel and bi-fuel)
• Key advantages: widest dealer network, Evolution controller with Wi-Fi monitoring, Power Management System (PMS) that starts two central AC units sequentially instead of sizing for simultaneous start

Cummins (Quiet Connect)

• Residential range: 10–60 kW, all liquid-cooled
• Commercial range: 10–2,500 kW
• Key advantages: liquid-cooled across the line (longer service life), sound levels as low as 56 dB at 23 ft, PowerCommand digital controller. Entry price runs higher, but 100% load acceptance on some models simplifies surge verification. The Cummins RS50 50kW anchors our liquid-cooled residential/commercial quotes.

Kohler

• Residential range: 8.5–150 kW (air-cooled and liquid-cooled)
• Commercial range: 10–4,000 kW
• Key advantages: fastest response in the industry (power restored in ~10 seconds), ±1% voltage regulation — the pick for sensitive electronics and variable-speed motor loads. The Kohler 10RESV is the compact entry we ship most for essentials-only systems.

Feature	Generac	Cummins	Kohler
Air-cooled options	Yes (7.5–26 kW)	No	Yes (8.5–30 kW)
Liquid-cooled residential	22+ kW	10+ kW	14+ kW
Transfer time	~30 sec	~30 sec	~10 sec
Voltage regulation	±2%	±2%	±1%
Remote monitoring	Wi-Fi standard	Wi-Fi optional	OnCue Plus
Load management	PMS (2 AC units)	PowerCommand	OnCue Plus
Best for	Value + features	Quiet operation	Tight regulation

Whichever brand lands, the transfer switch has to match. See our guides to manual vs. automatic transfer switches and the transfer switch sizing chart, and browse automatic transfer switches from 100A up. For brand-level shopping, start with the best whole-house generators roundup.

Altitude and Temperature Derating

Engines breathe air, and thin or hot air costs power. Most manufacturers derate roughly 3–4% per 1,000 feet above sea level and about 1% per 10°F above the rated ambient (typically 104°F / 40°C). A 22 kW unit installed at 6,000 feet is effectively an 18–19 kW unit before you burn a drop of fuel. Above 5,000 feet or in extreme-heat climates, verify the derated capacity still clears the calculated load — and lean toward propane or diesel, which tolerate thin-air combustion better than natural gas.

Common Sizing Mistakes to Avoid

• Ignoring motor starting surge: the single most common error. A 5 kW continuous load with a well pump can demand 8+ kW of surge.
• Sizing for connected load instead of demand: nameplate totals without NEC demand factors or load shedding lead to needless oversizing.
• Forgetting fuel derating: a 20 kW unit on natural gas may only deliver 18 kW.
• Neglecting future expansion: the hot tub or workshop circuit added later can strand an exactly-sized unit.
• Assuming all loads are simultaneous: load sequencing and smart transfer switches cut required capacity significantly.

Sizing Checklist for Contractors

• Complete a load inventory with running watts and starting surge for every load
• Determine whether loads start sequentially (MTS) or simultaneously (ATS)
• Apply the 80% continuous loading rule for minimum continuous kW
• Verify generator surge rating exceeds total starting wattage
• Run the NEC Article 220 calculation for permit submittal where required
• Confirm fuel type and apply natural-gas derating
• Apply altitude and ambient-temperature derating per spec sheet
• Consider load management to reduce required size
• Plan 25–30% future load growth in commercial applications
• Select a transfer switch rated for the generator's continuous output

Run the worksheet, check the compressor's locked-rotor amps, pick the fuel, and derate for the site — the kW number picks itself after that. Bring us the panel schedule and we'll confirm the size and quote the generator, ATS, and freight together. Start in the standby generators collection or the full generators inventory.

Shop This Article

• Kohler 10kW Home Standby Generator with 100A 12-Circuit ATS
• Kohler 10RESV Home Standby Generator
• Champion 12.5kW Generator (Select-Circuit ATS Compatible)
• Cummins RS50 50kW Quiet Connect Standby Generator
• Cummins 100A RA Series Automatic Transfer Switch, NEMA 3R

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