Standby Generator Sizing Chart: kW Lookup for Home & Battery Backup

For a standard 200-amp residential panel with central air conditioning, you need a 22kW to 24kW air-cooled standby generator (such as the Generac 7043). If you are also charging a 48V LiFePO4 battery bank via a hybrid inverter like a Sol-Ark or Schneider XW-Pro, you must bump that capacity to 26kW–30kW to handle the continuous DC charge current without tripping the generator's output breaker. Sizing a generator is not just about adding up appliance wattages; it is about matching the alternator's continuous thermal limit to your worst-case simultaneous loads while respecting NEC Article 702 guidelines for optional standby systems.

The Master Standby Generator Sizing Chart (NEC 702 & NFPA 110 Baseline)

This chart provides baseline continuous kilowatt (kW) requirements for whole-home backup. How to read this table: The 'Base Continuous kW' column represents the running load for standard appliances and lighting. The 'Peak/Surge kW' column accounts for motor starting currents (Locked Rotor Amps). The 'Battery/Hybrid Adder' column is the extra continuous kW required if a hybrid inverter is simultaneously passing through AC loads while bulk-charging a 48V battery bank. All base kW values assume a standard 40°C (104°F) ambient temperature alternator rating and natural gas fuel delivery at 7–9 inches of water column pressure, aligning with NFPA 110 Type 10, Class X standby performance standards.

Quick-Jump Rows: High-Demand Load Adders

Bookmark this section for the specific heavy loads that routinely cause undersized generators to stall or throw overcurrent faults. Add these values to your base continuous calculation if the load is not managed by an automatic load shedding module.

• Central AC (3 to 5 Ton): Add 4 to 6 kW. Older compressors without soft-starts can pull 5x their Rated Load Amps (RLA) for the first 500 milliseconds. If your generator lacks a 24V AC load lockout from the thermostat, size for the surge.
• EV Charger (Level 2, 48A continuous): Add 11.5 kW. Never run a 48A EVSE on a 20kW generator without a smart load shedder (like the Generac PWRview or Emporia VUE). The continuous draw will instantly bottleneck a 20kW alternator.
• Well Pump (1.5 HP Submersible): Add 3.5 kW surge. Submersible pumps operate deep underground with long wire runs, increasing voltage drop and extending the time it takes the motor to reach synchronous speed.
• 48V LiFePO4 Battery Charging: Add 6 to 8 kW continuous. When a hybrid inverter (e.g., Sol-Ark 15K) acts as a UPS, it passes through AC loads and pulls DC charge current from the generator. A 100A battery charge rate at 52V requires roughly 5.2kW of pure DC power, which translates to ~6.5kW of AC generator draw accounting for inverter efficiency losses.

Derating Factors: How Environment and Inverters Modify Base Values

The chart above assumes ideal sea-level conditions. In the real world, physics and local infrastructure force you to derate the generator's nameplate capacity. Here is how derating rows modify the base value:

Altitude Derating (Air-Cooled Engines)

Air density drops as elevation increases, starving the engine of oxygen. For air-cooled standby generators (like the Generac Guardian series), you must derate the engine output by 3.5% for every 1,000 feet of elevation above 3,500 feet. A 24kW generator installed at 6,500 feet in Colorado loses roughly 10.5% of its capacity, effectively becoming a 21.5kW unit. Liquid-cooled engines (Kohler RCL series, Cummins QSNT) handle altitude better but still require ECU tuning and roughly a 2% per 1,000 ft derate.

Temperature Derating (Alternator Windings)

Generators are typically rated at a 40°C (104°F) ambient temperature. If installed in a location where summer ambient temperatures routinely exceed 100°F (like an unshaded side yard in Arizona), the copper windings in the alternator experience increased electrical resistance. Derate the alternator output by 2% for every 10°F above 100°F to prevent thermal degradation of the insulation.

Fuel Pressure and BTU Content

Natural gas (NG) delivers roughly 1,000 BTU/cu ft, while liquid propane (LP) delivers 2,500 BTU/cu ft. Most dual-fuel generators lose 10% to 15% of their nameplate kW capacity when switched to NG due to the lower energy density and typical residential meter pressure limits (7 inches water column). Always size your generator based on its NG rating if you are connected to the municipal gas grid.

Decision Tree: Pick Your Exact Generator Model

Use this decision path to terminate your research and select a specific, proven unit. Do not rely on generic 'whole home' marketing claims; match your exact infrastructure to the hardware.

What This Chart Cannot Tell You (And When to Call an Engineer)

While this chart covers 95% of residential and light-commercial battery-backup scenarios, it has hard limits. First, it cannot predict the exact Locked Rotor Amps (LRA) of aging, off-brand HVAC compressors. If your AC unit is over 15 years old, the startup spike may exceed standard NEMA design B motor assumptions, requiring a hard-start capacitor kit or a dedicated soft-start device (like a Micro-Air EasyStart) to prevent the generator from bogging down.

Second, this chart assumes standard utility drop tests. If your local utility experiences 'brownouts' (voltage sags to 105V) rather than clean blackouts, your hybrid inverter's transfer switch might chatter or fail to disconnect the grid, backfeeding the grid and tripping the generator. In these cases, you need a generator with an integrated utility isolation contactor or a smart transfer switch with adjustable voltage dropout thresholds.

Finally, local Authorities Having Jurisdiction (AHJ) may enforce strict load-shedding rules under NEC 702. If your calculated loads exceed the generator's capacity, the inspector will require an automatic load management module (like a Generac Smart Management Module) to physically disconnect heavy loads like EV chargers or secondary AC units during peak generator demand. Always submit your final load calculation sheet to your local AHJ before pouring the concrete pad.