What AC Size Do You Really Need? Stop Guessing by Square Footage

Improper sizing is one of the single biggest reasons a home feels uncomfortable. Many contractors still use a “rule of thumb” based solely on square footage, but that is just the tip of the iceberg. To get a system that lasts 15 to 20 years and keeps your utility bills low, you need to understand the science of tonnage and heat load.

1. Understanding Tonnage and BTUs

When we talk about “tonnage,” we aren’t talking about the weight of the unit. Tonnage refers to the cooling capacity of the system measured in BTUs (British Thermal Units).

• 1 Ton = 12,000 BTUs per hour.
• A 2-ton system has the capacity to move 24,000 BTUs of heat out of your home every hour.

The goal of a professional AC Sizing Guide is to match the tonnage of the system to the specific heat load of your home.

2. The Dangers of an Undersized System

If your system is too small for the heat load, it will struggle to remove energy as fast as your home gains it.

• Symptoms: The system runs 24/7 without ever reaching the set point, leading to outlandish utility bills and “hot spots” in the house.
• The Reality Check: Even if you increase the size (and thus the electrical draw), a correctly sized system often saves hundreds of dollars because it runs for the appropriate amount of time to cool the house and then shuts off.

3. The “Bigger is Better” Myth: Oversized Systems

Back in the day, contractors would often “bump you up half a ton” just to be safe. This is a terrible idea for two main reasons:

1. Short Cycling: An oversized unit cools the air so fast that it shuts off before it can finish a full cycle. This prevents the oil in the compressor from circulating properly, leading to premature breakdowns.
2. Humidity Issues: Cooling the air is only half the job. A system needs long run times to effectively dehumidify the air. An oversized system leaves the air cold but “clammy” because the humidity was never removed.

4. The 4 Variables of a Real Heat Load Calculation

A pro doesn’t just look at the floor plan. We look at the environment.

Variable A: Construction and Age

A home built in 1950 has vastly different energy requirements than a home built in 2026. Modern construction standards for insulation and window seals mean a newer home can often use a much smaller, more efficient unit than an older home of the same size.

Variable B: Regional Temperature Averages

In Central Oklahoma, we design for the average high, not the extreme exception. If you size a system for the hottest day ever recorded, it will be oversized for the other 99% of the summer, leading to the short-cycling issues mentioned above.

Variable C: Precipitation and Humidity

Humidity dictates how the system should be sized. High-precipitation areas require systems designed for longer run times to ensure the home doesn’t become a breeding ground for mold or dry out adhesives and wood in the winter.

Variable D: Your Improvements

Have you added window tinting? Upgraded to R-50 insulation? These improvements drop your effective heat load, allowing you to install a smaller, less expensive, and more efficient unit.

Conclusion: Ask Your Contractor the Right Questions

If a contractor walks into your home and gives you a quote without asking about your comfort, your utility bills, or your insulation, they are guessing. A professional should ask:

• “Does the system struggle to keep up in the afternoon?”
• “Do you have rooms that are always hotter than the rest of the house?”
• “How often do you hear the system kick on and off?”

At Yarbrough & Sons, we don’t just swap boxes; we solve problems. We use climate maps and construction data to ensure your system is sized perfectly for your home’s unique load.Are you ready to stop guessing? Click the link below to get an Instant Estimate on a system sized correctly for your home.