Most 200-amp residential services require a 22kW to 24kW air-cooled standby generator to handle whole-home loads without shedding, while 400-amp services demand 36kW to 48kW liquid-cooled units. The exact kW requirement depends on the sum of your continuous running watts plus the single largest motor starting surge (Locked Rotor Amps, or LRA). Sizing a standby unit is not about matching the main breaker's theoretical maximum; it is about calculating the actual simultaneous load per NEC Article 220 and applying environmental derating.
The Master Backup Generator Sizing Chart
This reference table maps standard generator kW ratings to maximum running watts, peak motor surge capacity, and typical service panel matches. The data aligns with load calculation methodologies outlined in NFPA 110 (Standard for Emergency and Standby Power Systems) and practical field sizing data from major manufacturers like Generac.
How to read this table: The Generator Rating is the nominal continuous output. Max Running Watts assumes a 1.0 Power Factor (standard for single-phase residential). Peak Starting Watts represents the alternator's 10-second surge capacity to start the largest single motor (like an HVAC compressor) while all other baseline loads are running. If you have two AC units, you must ensure the peak starting watts can handle the second unit starting while the first is already running.
| Generator Rating | Max Running Watts | Peak Starting Watts | Typical Service Match | Primary Application |
|---|---|---|---|---|
| 10 kW | 10,000W | 15,000W | 100A Subpanel | Essential circuits only (fridge, lights, well pump) |
| 14 kW | 14,000W | 21,000W | 100A - 150A Main | Essentials + one 3-ton HVAC unit |
| 18 kW | 18,000W | 27,000W | 150A - 200A Main | Managed whole home (with load shedding) |
| 22 kW | 22,000W | 33,000W | 200A Main | Whole home (1 central AC, electric range) |
| 24 kW | 24,000W | 36,000W | 200A Main | Whole home (2 central ACs, no shedding) |
| 32 kW | 32,000W | 48,000W | 200A - 320A Main | Large home + Level 2 EV charging |
| 36 kW | 36,000W | 54,000W | 320A - 400A Main | Estate / Light 3-phase commercial |
| 48 kW | 48,000W | 72,000W | 400A Main | Full estate / Zero load shedding |
| 60 kW | 60,000W | 90,000W | 400A+ / 3-Phase | Small commercial / Multi-family |
• Standard 200A Home: Look at the 22 kW row. This is the industry baseline for modern 2,500 sq ft homes with natural gas heat and one central AC.
• 3-Ton AC Surge: A standard 3-ton compressor draws ~18,000W LRA. A 14kW generator (21,000W peak) handles this if baseline loads are under 3,000W.
• Well Pump: A 1HP submersible well pump requires ~3,000W starting surge. Ensure your baseline + 3,000W does not exceed the 'Peak Starting Watts' column.
Derating Factors: Modifying the Base Values
The chart above assumes standard atmospheric conditions: sea level elevation and an ambient temperature below 100°F (38°C). In real-world installations, environmental factors physically restrict the engine's ability to ingest oxygen and the alternator's ability to shed heat. You must apply derating multipliers to the Max Running Watts column based on your site conditions.
- Altitude Derating (Naturally Aspirated): Air-cooled residential generators (like the 22kW and 24kW models) lose approximately 3% of their capacity for every 1,000 feet of elevation above 500 feet. A 22kW generator installed in Denver, CO (5,280 ft) loses roughly 14% of its capacity, dropping its continuous rating to ~18.9kW.
- Altitude Derating (Turbocharged): Liquid-cooled commercial units (32kW and above) use turbochargers that compensate for thin air, losing only about 1% per 1,000 feet above 500 feet.
- Temperature Derating: When ambient air exceeds 100°F (38°C), the alternator's copper windings and insulation limits restrict output. Expect a 1% to 2% drop per 10°F above 100°F. If you live in Phoenix and enclose the generator in a tight, poorly ventilated fence, localized radiant heat can push ambient intake temps to 120°F, triggering thermal shutdowns or permanent alternator damage.
- Fuel Type Derating: Generators produce their rated kW on Liquid Propane (LP) or Natural Gas (NG). However, NG delivers roughly 10-15% less energy density by volume through standard residential piping compared to LP. Always size your NG pipe to deliver the required volume at 7 to 11 inches of water column (WC) under full load.
What This Chart Cannot Tell You
A sizing chart provides the baseline electrical math, but it cannot account for the physical and harmonic realities of your specific installation. Keep these critical blind spots in mind before purchasing:
1. Transfer Switch Ampacity Limits
If you install a 48kW generator on a home with a 200-amp Automatic Transfer Switch (ATS), your maximum continuous draw is physically capped at 200A (48,000W at 240V). The extra 28kW of generator capacity is entirely wasted unless you upgrade to a 400A ATS and run 600 kcmil or parallel 3/0 AWG copper feeders. Match the ATS ampacity to the service panel, not just the generator kW.
2. Inrush Current Duration
The 'Peak Starting Watts' column assumes a standard 10-second surge window. However, high-inertia loads like large workshop air compressors or 5HP well pumps may require 15 to 20 seconds to reach full RPM. If the surge exceeds the alternator's 10-second thermal limit, the generator's voltage regulator will fold back the voltage, stalling the motor and tripping the ATS.
3. Harmonics from VFDs and UPS Systems
If your home has variable frequency drives (VFDs) on HVAC blowers, or large double-conversion UPS systems for server racks, these devices introduce Total Harmonic Distortion (THDi) back into the generator. Standard sizing charts assume clean resistive/inductive loads. High THDi can cause the generator's voltage regulator to hunt or fail. For heavy VFD loads, oversize the alternator by 20% to absorb the harmonic heating.
Backup Generator Sizing FAQ
How do I size a backup generator for a 200-amp residential service?
Do not simply multiply 200A x 240V to get 48kW. A 200-amp panel rarely sees more than 80A to 120A of continuous simultaneous load in a standard home. Perform an NEC Article 220 load calculation: sum your continuous lighting and appliance loads, apply the appropriate demand factors for HVAC and heating, and add the largest motor surge. For 90% of modern 200A homes, a 22kW generator paired with a 200A whole-house ATS provides seamless coverage without the massive expense of a 48kW liquid-cooled unit.
What size backup generator do I need to run a 3-ton central air conditioner?
A standard 3-ton central AC compressor has a Locked Rotor Amp (LRA) rating that translates to roughly 15,000W to 18,000W of starting surge. A 14kW generator (which typically offers 21,000W of peak surge) can start this unit, provided your baseline house loads (fridge, lights, router) are under 3,000W when the AC kicks on. If you want to run a 5-ton unit, or if you have two 3-ton units, you must step up to a 24kW or 26kW model. Alternatively, install a soft-start device like the MicroAir EasyStart on your compressor, which reduces the LRA surge by up to 70%, allowing a smaller 14kW or 18kW generator to handle larger AC loads.
Should I size my backup generator for 100% of my connected load or use load shedding?
Sizing for 100% of your connected load guarantees you will never notice a power outage, but it drastically increases equipment and fuel costs. A 48kW liquid-cooled generator and 400A ATS can cost over $25,000 installed. A far more common and cost-effective approach is to install a 22kW air-cooled generator (approx. $8,000 - $12,000 installed) and utilize the ATS's built-in load shedding modules. These smart relays monitor the generator's frequency and voltage; if the starting surge of a second AC unit or an EV charger threatens to overload the generator, the module temporarily cuts power to that specific non-essential circuit until the surge passes.
