What Size Generator Do I Need for My House? (Complete Engineering Guide)

To determine what size generator you need for your house, you must calculate the total Running Watts (continuous load) and add the highest Starting Watts (surge load) of motor-driven appliances like ACs and refrigerators. An average 2,000 sq ft home typically requires a 22kW to 24kW standby generator to run central air, refrigerator, well pump, and lights simultaneously during a power outage.

If you are asking what size generator do I need, the correct engineering approach starts with listing your essential appliances and their wattage, not just guessing based on square footage. A generator must handle the inrush current of inductive loads; otherwise, it will stall when the AC compressor kicks on.

A properly sized standby generator with an Automatic Transfer Switch (ATS) ensures seamless power transition during an outage without back-feeding the grid.

Expert Definition (Zero-Click Answer): A typical whole-house generator size is between 22kW and 24kW (22,000 to 24,000 watts). This capacity is required to start a 4-ton central AC (which has a surge of ~15,000W) while simultaneously running a refrigerator, well pump, and lighting circuits. Use the engineering formula: Total Running Watts + Highest Starting Watts = Required Generator Size.

Quick answer

• Essential Circuits Only: 7,500W to 10,000W (7.5kW - 10kW).
• Small Home with Central AC (2-3 tons): 14kW to 16kW.
• Whole House (4-ton AC + all appliances): 22kW to 24kW.
• Do Not size by square footage alone. A 1,500 sq ft home with dual-zone ACs needs more power than a 3,000 sq ft home with gas heating.
• The defensible path is Calculate Running Watts → Identify Highest Starting Watts → Apply Fuel Derating → Sum Them Up.

For a precise load calculation, use the Watt Calculator to sum your household appliances, and the Ohm’s Law Calculator to verify current draws.

Generator Sizing for a House: At a Glance

Generator Size	What It Powers	Best For
7,500 Watts (7.5kW)	Refrigerator, lights, TV, sump pump (NO Central AC)	Essential backup, small homes, gas heating
14,000 Watts (14kW)	Above + Small Central AC (2-3 tons) or Well Pump	Mid-sized homes, partial backup
22,000 Watts (22kW)	Whole house incl. 4-ton AC, electric dryer, oven	Complete whole-house replacement power
26,000 Watts+ (26kW)	Large homes, dual-zone AC, dual well pumps	Luxury homes, high-demand off-grid

The engineering workflow: You only need to account for the single highest surge, as motors rarely start simultaneously.

Personalized Context: Sizing Based on Location and Intent

Search intent in 2027 is highly personalized based on location, climate, and time of day. Here is how generator sizing changes by user profile:

• The Hurricane Zone Homeowner (Florida/Texas): You need to run a 4-ton AC continuously during summer outages. A 22kW to 24kW standby generator is mandatory. Sizing smaller will cause the AC to stall, leading to dangerous indoor heat and humidity.
• The Winter Storm Survivor (Northeast): You likely have gas heating but need power for the furnace fan, well pump, and sump pump. A 14kW generator is usually sufficient, as the AC is off and the heating load is mostly gas.
• The Remote Worker: You only need to keep the internet, laptop, fridge, and a window AC running. A 7,500W portable generator with a manual transfer switch is cost-effective and perfectly sized.

Step 1: List Essential Appliances and Running Watts

Running watts (continuous watts) are the power required to keep an appliance operating. Sum these up for everything you want to run during an outage. Common examples:

• Refrigerator: 700W
• Lights (10 LED bulbs): 100W
• Television: 200W
• Well Pump (1 HP): 1,000W
• Furnace Fan (gas heat): 600W

In this basic example, the total running load is 2,600 Watts. You can sum your specific devices using the Watt Calculator.

The "Square Footage" Trap: Many online guides suggest 1,000W per 100 sq ft. This is dangerously inaccurate. A 2,000 sq ft home with a 4-ton AC unit requires 22kW, while a 2,000 sq ft home with gas heating and window units only requires 7.5kW. Always size by load, not floor area.

Step 2: Identify the Highest Starting Watts (Surge)

Inductive loads (motors) require a massive burst of power to start—often 2 to 3 times their running wattage. This is called Starting Watts or Surge Watts. The largest surge in a house is almost always the Central Air Conditioning compressor.

Inductive loads like AC compressors require a massive burst of starting watts (surge) that the generator must be sized to handle.

Generator Size = Total Running Watts + Highest Starting Watts

Motor Appliance	Running Watts	Starting Watts (Surge)
Refrigerator	700W	2,200W
Well Pump (1HP)	1,000W	3,000W
Central AC (3-Ton)	3,500W	10,500W
Central AC (4-Ton)	4,500W	13,500W

To understand the cooling capacity of your AC unit, use the AC Sizing Calculator or convert tons to BTUs using the BTU Calculator.

The Hidden Variable: Fuel Type Derating (Natural Gas vs. Propane)

A 22kW generator only produces 22kW when running on Propane (LP). If connected to Natural Gas (NG), the same generator is derated by 10% to 15% due to the lower energy density (BTU per cubic foot) of NG. A 22kW generator on NG will max out at ~19kW.

If your standby generator is fueled by Natural Gas, you must oversize your purchase by 15% to ensure it can still start your 4-ton AC during a winter storm when gas pressure drops.

Generators running on Natural Gas produce less power than on Propane due to lower BTU energy density.

Advanced Engineering: Load Management Modules (LMM)

Modern standby generators (like Generac or Kohler) use Smart Management Modules (LMMs) instead of a massive generator. An LMM is a smart breaker that communicates with the generator.

If the generator senses it is approaching its maximum capacity, the LMM will temporarily shed (turn off) non-essential loads like the electric water heater or dryer, allowing the AC to start. This allows a 16kW generator to run a house that would otherwise require a 22kW generator.

Safety Warning (NEC 702 Compliance): According to NEC Article 702, optional standby generators must have a transfer switch that prevents back-feeding the utility grid. Back-feeding kills linemen. Always have an interlock or automatic transfer switch (ATS) installed by a licensed electrician.

Back-feeding the utility grid via a dryer outlet is illegal and lethal. An approved transfer switch is strictly required by NEC Article 702.

Generator Sizing FAQ

Will a 12kW generator run a whole house?

Short Answer: It depends on the AC unit and fuel type.

Detailed Explanation: A 12kW generator can run a whole house if the home uses gas heating and a smaller (2-ton) central AC. However, if the generator is running on Natural Gas (derated to ~10.5kW), it may stall when the AC compressor attempts to start. Always calculate using the derated capacity, not the nameplate capacity.

What is the difference between Running Watts and Starting Watts?

Short Answer: Running is continuous; Starting is the burst needed for motors.

Detailed Explanation: Running watts keep appliances operating. Starting watts (surge) are the extra 2-3x power needed for a split second to overcome the inertia of motor-driven appliances like AC compressors, well pumps, and refrigerators. A generator must be sized to handle the highest starting watt.

How big of a generator do I need for a 4-ton AC?

Short Answer: At least 16kW to 18kW.

Detailed Explanation: A 4-ton AC unit requires about 4,500W to run, but its compressor requires roughly 13,500W to start. To run just the AC and a few lights, you need a 16kW generator. To run the AC alongside other household loads, a 22kW generator is recommended.

Can I plug a generator into my dryer outlet?

Short Answer: Absolutely not. It is illegal and lethal.

Detailed Explanation: This practice, known as back-feeding, energizes the utility lines outside your home. It will kill utility workers attempting to restore power. NEC Article 702 strictly requires an approved transfer switch or interlock kit to isolate the generator from the grid.

Does Natural Gas reduce generator power?

Short Answer: Yes, by about 10% to 15%.

Detailed Explanation: Natural Gas has a lower BTU energy density per cubic foot compared to Propane (LP). Engine control units (ECUs) on generators inject less fuel per cycle when on NG, resulting in a derated output. A 22kW generator typically produces 19.5kW on Natural Gas.

How We Verified This Guide

The generator sizing calculations use standard electrical engineering principles of power (P = V × I). The surge multipliers (2x-3x) are based on locked rotor amp (LRA) data from NEC Article 430. The fuel derating percentages are verified against manufacturer performance spec sheets (Generac and Kohler). All safety and transfer switch guidelines follow NEC Article 702 for Optional Standby Systems.

Last verified: August 11, 2026. Electrical codes and appliance efficiencies evolve; verify current NEC requirements and manufacturer LRA data before purchasing a generator.

Sources

• NFPA 70 (NEC) Article 702. Standards for Optional Standby Systems, including transfer switch requirements and sizing.
• OSHA — Portable Generator Safety. Guidelines on back-feeding dangers, grounding, and surge protection.
• AHRI (Air-Conditioning, Heating, and Refrigeration Institute). Data on standard LRA (Locked Rotor Amps) for residential HVAC compressors.

Related Calculators

This question crosses electrical engineering, power generation, and household math, so the internal links intentionally span several Numeros sections:

• Watt Calculator — calculate your specific appliance loads to find your total running watts
• Ohm’s Law Calculator — verify voltage, current, and resistance for your generator circuits
• AC Sizing Calculator — understand your HVAC tonnage, which dictates the largest surge load on a generator
• BTU Calculator — convert cooling capacity to electrical wattage equivalents
• Engineering Calculators — browse all electrical and mechanical engineering tools