The right generator size depends on your home’s electrical load, not just square footage. To determine what size generator you need, calculate the running and starting wattage of the appliances and systems you want to power. HVAC systems, water heaters, pumps, EV chargers and other high-demand equipment can significantly affect sizing. A professional load calculation can help determine the right whole-home generator capacity for your home.
On this page:
Why Generators Are A Good Investment for Homeowners
How to Determine What Size Generator Your Home Needs
What Generator Do You Need Based on Your Home’s Size?
Why Installing the Correct Size Generator Matters
Other Factors That Impact Generator Sizing
Other Considerations to Keep In Mind Before Installing A Generator
Trust Casteel for Home Generator Sizing, Installation, and Repair
Frequently Asked Questions
A power outage can be more than an inconvenience, especially during extreme heat, cold weather or severe storms. A whole-home generator can automatically keep your home's essential systems and appliances running when the power goes out.
But what size generator do you need to run your house? The answer depends on more than just your home's square footage. Your heating and cooling system, appliances, electrical equipment and how much of your home you want to power all play a role.
Why Generators Are A Good Investment for Homeowners
A standby generator is a permanent backup power system that connects directly to your home's electrical system. When an outage occurs, it can automatically turn on and provide electricity when your family needs it most.
For homeowners who want whole-home coverage, a properly sized standby generator can keep important systems running without having to manually connect appliances or extension cords. It can also help maintain heating and cooling, refrigeration, lighting and other everyday electrical needs during prolonged power outages.
Learn more about our Whole-Home Generator Repair and Installation services.
How to Determine What Size Generator Your Home Needs
Generator capacity is measured in watts or kilowatts. One kilowatt (kW) equals 1,000 watts.
The easiest way to estimate your generator needs is to make a list of everything you want to run during an outage and check the running and starting wattage for each item. Appliance labels and owner's manuals are the best sources for this information.
Follow these steps to determine what size generator your home will need:
1. List the Appliances and Systems You Want to Power
Include your HVAC system, refrigerator, water heater, sump or well pump, lighting fixtures, kitchen appliances, EV charger, and other equipment.
2. Add the Running Watts
This is the electricity each appliance needs while operating.
3. Account for the Starting Watts
Household appliances or items with motors and compressors can require significantly more electricity for a few seconds when they start. Refrigerators, air conditioners and sump pumps are common examples.
4. Add the Wattage Loads Together
Your generator needs enough capacity to handle the running load while also accommodating the largest starting load.
5. Leave Some Capacity Available
A generator shouldn't be pushed to its maximum capacity all the time.
For example, a home might use 4,000 watts for central air conditioning, 800 watts for a refrigerator, 4,500 watts for an electric water heater and 1,000 watts for lights and electronics. That's 10,300 running watts before accounting for startup demand. If the air conditioner requires significant starting power, a generator much larger than 10 kW may be needed.
For an accurate whole-home calculation, our team can evaluate your home's electrical load and determine the generator capacity that fits your specific needs.
Call (770) 637-9001 today to speak with a whole-home generator expert.
What Generator Do You Need Based on Your Home’s Size?
Square footage can provide a starting point, but it shouldn't be the only factor used to choose a generator. Two homes with the same square footage can have very different electrical demands.
As a general planning guide, the chart below shows approximate generator ranges based on home size. These are starting points for discussion, not guarantees of whole-home coverage.
Home Size | Estimated Generator Size | General Use |
Up to 1,500 sq. ft. | 10,000 - 14,000 watts | Smaller homes or homes with moderate electrical loads |
1,500-2,500 sq. ft. | 14,000 - 20,000 watts | Most major household systems and appliances |
2,500-3,500 sq. ft. | 20,000 - 26,000 watts | Larger homes with higher electrical demand |
3,500+ sq. ft. | 26,000+ watts | Large homes, multiple HVAC systems or high-demand appliances |
Keep in mind that these ranges can change significantly depending on your home's equipment. For example, an electric furnace, electric water heater, multiple air conditioners or an EV charger can increase your generator requirements.
Not sure what size generator is right for your home? Book a whole-home generator consultation online today.
Why Installing the Correct Size Generator Matters
Choosing a generator that's too small can leave you without enough power for the systems you expected to use. It can also cause the generator to overload when several high-demand appliances operate at the same time.
Going too large isn't necessarily better, either. A generator that is significantly larger than your home's actual needs can be less efficient and may cost more to purchase and install.
The goal is to find the right capacity for your home's realistic electrical demand, with enough room to handle normal starting loads. Whether you live in Atlanta, Marietta, Alpharetta, or surrounding Metro Atlanta communities, local climate factors like heavy summer storms and winter freeze risks make proper generator sizing crucial for reliable backup power.
Other Factors That Impact Generator Sizing
Beyond square footage, consider other appliances and factors that may change the electrical demand of your home, such as:
Heating and cooling: Central air conditioners, heat pumps and electric furnaces can require substantial power.
Number of residents: More people generally means more appliances and electrical equipment running throughout the day.
Major appliances: Electric water heaters, ovens, dryers and ranges can add significant demand.
Well or sump pumps: These require additional power and may have higher starting wattage.
EV chargers: Charging an electric vehicle can add a large electrical load.
Backup goals: Powering only the essential appliances and lights requires less capacity than powering your entire home. Determining the level of backup power your family would feel comfortable with can help you better determine what size generator you need.
Other Considerations to Keep In Mind Before Installing A Generator
Generator size isn't the only decision you'll need to make. You'll also want to consider the generator's fuel source, such as natural gas or propane, along with its transfer switch, installation requirements and maintenance needs.
If you're considering a portable generator, safe operation and proper storage are also important. Portable generators should never be operated inside a home, garage or other enclosed space due to the risk of carbon monoxide exposure.
Trust Casteel for Home Generator Sizing, Installation, and Repair
Choosing the right generator for your home doesn’t have to be a guessing game. From the unique power demands of historic homes in Metro Atlanta to the intense seasonal weather patterns across Georgia, our technicians understand the specific needs of our local community. Casteel’s licensed experts can help you determine the exact type and capacity of the generator that makes sense for your specific property.
Navigating Georgia’s building codes, electrical permitting, and utility requirements, including those for Georgia Power and local municipal electric providers, is a critical part of a safe standby generator installation. Our licensed electricians handle every detail, from initial permit acquisition to the final inspection, ensuring your system fully complies with all Metro Atlanta municipal safety standards.
Whether you're installing a new backup generator or need expert service for an existing system, Casteel is here to help you stay prepared for the next outage. Contact us today to schedule an appointment and receive professional guidance for your home generator needs across the Greater Atlanta area.
Frequently Asked Questions
What size generator do I need to run my whole house?
Most homes need a standby generator between 10,000 and 26,000 watts, but the right size depends on your home's electrical load. HVAC systems, water heaters, pumps, EV chargers and other high-demand equipment can increase the generator capacity required.
How many watts does it take to run a house?
The wattage needed to run a house varies based on the appliances and electrical systems in use. A home may require several thousand watts for basic needs, while whole-home backup can require 10,000 watts or more. A load calculation provides the most accurate estimate.
Is a 22kW generator big enough to run a house?
A 22kW generator can provide whole-home backup for many homes, but it may not be sufficient for every property. The home's HVAC systems, water heater, appliances and other electrical loads should be evaluated to determine if 22kW provides enough capacity.
Does a generator power the whole house?
A properly sized whole-home standby generator can power the entire home during an outage. The generator connects to the home's electrical system through a transfer switch and automatically supplies power when utility electricity is interrupted.
How do I calculate what size generator I need?
To calculate generator size, list the appliances and systems you want to operate, add their running wattage, account for starting wattage and combine the loads. A professional electrical load calculation can provide a more accurate generator size recommendation.
Does square footage determine what size generator I need?
Square footage can provide a general starting point, but it does not determine generator size by itself. Two homes with the same square footage can have very different electrical demands based on their HVAC systems, appliances, pumps, water heaters and other equipment.
What happens if my generator is too small?
An undersized generator may overload when multiple high-demand appliances operate at the same time. It may fail to provide enough power for the systems you expect to use and can experience performance issues when electrical demand exceeds its capacity.
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