“What size generator do I need?” is the first question most Birmingham homeowners ask when they start shopping for backup power, and it’s the most important one to get right. An undersized generator can overload or leave key systems without power, while an oversized one costs more to buy, install and run than necessary. Here’s how generator sizing works and what to expect from a professional load calculation.
Start With What You Want to Power
Generator size is measured in kilowatts (kW). The size you need depends on which circuits you want backed up during an outage. Most homeowners land in one of two groups:
- Essential circuits: the refrigerator and freezer, some lights, the furnace blower or a smaller AC system, the garage door, internet and phone charging, and medical equipment. This approach uses a smaller generator.
- Whole-home backup: everything above plus central air conditioning, the electric range, the water heater, the washer and dryer, and the rest of the house, so you barely notice the outage. This requires a larger generator and often load management.
The Systems That Drive Generator Size
A few large loads determine most of a home’s generator size:
- Central air conditioning and heat pumps: the biggest factor in Alabama summers. AC compressors draw extra power for a moment when they start, so the generator must handle that surge.
- Electric heat: electric furnaces and heat strips use a lot of power in winter.
- Electric water heaters, ranges and dryers: each can add several kilowatts.
- Well pumps and sump pumps: smaller, but critical to keep running.
Residential standby generators commonly range from about 10 kW for essential-circuit systems to 26 kW for many whole-home systems, with larger units available for big homes or homes with several AC systems.
Why Rules of Thumb Aren’t Enough
Online calculators and square-footage charts are a starting point, but two homes of the same size can need very different generators. The number and size of your AC units, whether your heat and appliances run on gas or electricity, and how you want the system to handle large loads all change the answer. Load management devices can also let a smaller generator cover more of your home by briefly cycling large appliances instead of running them all at once.
What a Professional Load Calculation Includes
- A review of your electrical panel and the circuits you want backed up
- Nameplate and start-up ratings for your AC, heat pump and other large equipment
- A check of your fuel source: natural gas service or a propane tank
- Placement options that meet clearance requirements around your home
- A recommendation for generator size, transfer switch and any load management
Get Your Generator Sized Correctly in Birmingham
One Source Heating, Cooling, Plumbing & Electrical performs load calculations and installs whole-house standby generators for homes in Birmingham, Hoover, Homewood, Vestavia Hills, Mountain Brook, Pelham, Alabaster and nearby communities. Start with our quick generator quiz, learn more about whole-home generator installation, or compare options in our guide to whole-house vs. portable generators.