When deciding what size generator you need for your house, square footage won’t tell you much. Sizing depends on your peak electrical demand, starting surges for large appliances, and your specific backup goals.
Common Residential Generator Sizes
Residential generators are rated in kilowatts, or kW. One kilowatt equals 1,000 watts.
General examples:
- 5-7.5 kW: Refrigerator, lighting, internet equipment, and a few outlets
- 10-14 kW: Selected circuits and possibly a smaller air conditioner
- 17-22 kW: Many homes with central air conditioning, kitchen appliances, lighting, and refrigeration
- 24-26 kW: Larger homes with several major appliances and one central air conditioning system
- 30 kW or more: Homes with multiple air conditioners, electric heat, electric water heaters, pools, workshops, or extensive whole home backup
These ranges can help you start thinking about generator size, but they are not a final recommendation. The generator still needs to match the home’s actual electrical demand and the starting requirements of motor driven equipment.
How Running and Starting Watts Affect Generator Size
Generator sizing starts with a list of the circuits and appliances you want to run during an outage. Running watts describe the power an appliance uses after it starts. Starting watts describe the short surge needed to start a motor or compressor.
For example, a homeowner may want to run…
| Appliance / circuit | Approximate running watts |
| Refrigerator | 700 watts |
| Lighting | 500 watts |
| Internet and security equipment | 200 watts |
| Garage door opener | 500 watts |
| Microwave | 1,200 watts |
| Sump pump | 1,000 watts |
| Air conditioner | 3,000 watts |
| Estimated running load | 7,100 watts |
At first glance, this looks like a 7.1 kW load. That does not mean a 7.1 kW generator is large enough.
A refrigerator may use 700 watts while running but need much more power for a moment when the compressor starts. Air conditioners, pumps, furnaces, and other motor-driven equipment can have large starting demands. If the generator cannot handle that surge, the appliance may not start, the generator may overload, or voltage may drop.
And this is where we see homeowners get into trouble. They add up the running watts, choose a generator that matches the total, and then find out it cannot start the largest motor in the house.
Houston homeowners also need to account for air conditioning. A generator that can operate lights, a refrigerator, and a few outlets may not have enough capacity to start a central air conditioning system. Some newer units use variable speed compressors or soft start equipment and may have lower starting requirements, but the equipment specifications still need to be checked.
Portable generators must be operated outdoors and at least 20 feet away from the home, with exhaust directed away from windows, doors, and vents. Review the portable generator safety guidance from the U.S. Consumer Product Safety Commission before using one.
Whole Home Backup vs. Selected Circuits
A whole home generator connects through an automatic transfer switch and can supply power to most or all of the home’s electrical system. This may be useful for homes with central air conditioning, electric heating, medical equipment, pumps, electric water heaters, home offices, pool equipment, or other large loads.
Some homeowners do not need a generator large enough to run every appliance. In that case, an electrician can install a system that supplies power only to selected circuits. This may reduce generator cost and fuel use, but you need to decide in advance what you want available during an outage.
Load management equipment can also allow a smaller generator to supply more of the home. The system may temporarily delay a lower priority load, such as an electric water heater or pool pump, while the air conditioner operates. What that means is the generator does not have to power every large appliance at the same time.
Generator sizing should account for the electrical panel, air conditioning and heating equipment, water heater, pumps, pool equipment, workshop loads, EV charger, and any medical equipment. The generator size depends on the home’s actual electrical loads, not simply its square footage or service rating.
A 200 amp electrical service does not automatically mean the home needs a 48 kW generator. The service rating tells you how much electricity the home can receive from the utility. It does not tell you how much generator capacity is required during an outage.
Choose Electrical Works of Houston for Generator Installation
Choosing a generator is only the first part of preparing your home for backup power. Electrical Works of Houston can review your electrical system, identify the loads you want to operate, calculate the required generator capacity, and verify that the available natural gas or propane supply can support the generator.
Our electricians can help you decide between selected circuit backup and whole home coverage, then install the transfer switch and related equipment according to applicable electrical requirements.
Contact our team today to discuss generator sizing, installation, and Houston electrical services.