Short Answer:
The right size AC unit depends on your home’s square footage, insulation, window exposure, and our famously humid Charleston climate. A professional load calculation is imperative for correct sizing. Too small of a system will never maintain target temp and run endlessly. Too big, and the system will short cycle “not run enough” and cause high humidity which could lead to mold issues in home.
Why AC Size Matters
1. Too Small = Underpowered
- Constant running
- Never hits target temps
- High humidity indoors (bad news in Charleston)
2. Too Large = Overpowered (Yes, that’s bad too)
- Short cycling = higher wear and tear
- Inconsistent cooling
- Poor humidity control
Factors That Affect Sizing in Charleston Homes
Climate
- Charleston’s humid subtropical climate means your AC must cool and dehumidify. Bigger isn’t always better…balance is key.
Insulation & Windows
- Poorly insulated homes or single-pane windows can bump up your needed capacity.
Sun Exposure
- East/west-facing homes or rooms with lots of windows need more cooling power.
Occupants
- More people = more heat generation. Add BTUs for crowded households.
Ceiling Height
- Vaulted ceilings? You’ve got more air to cool so factor that in.
What’s a Manual J Load Calculation?
It’s the gold standard for HVAC sizing. Our pros measure:
- Square footage
- Insulation type
- Air leakage
- Number of windows and direction they face
- Occupancy and appliance heat load
We use this data to calculate the exact size system your home needs – not just guess from square footage.
Semantic Cluster: Related Questions
What happens if my AC is too big?
Short cycles, poor dehumidification, and higher energy bills.
Can I just replace my old unit with the same size?
Not always. Insulation, ductwork, and home usage may have changed.
What size AC do I need for a 1,500 sq ft house in Charleston?
Probably 2.5–3 tons (30,000–36,000 BTUs), but you’ll need a pro to assess.
Case Study: James Island Right-Size Rescue
Client: Retired couple downsized to 1,200 sq ft cottage
Problem: New AC (3.5-ton) ran for short bursts and left humidity high
Solution: Replaced with 2-ton high-efficiency system after load calc
Result: Steady cooling, lower humidity, and energy bill dropped 22%
Talk to a Charleston Pro Before You Buy
At Passion Heating & Air, we specialize in new and existing home load calculation and system design. We have performed thousands of designs for homes 13k square foot down to 1200 square foot homes.
We factor in everything from attic insulation to whether your porch fan runs 24/7. No shortcuts, no upsells…just real numbers.
Ready to Find the Right Fit?
Book a Load Calculation or AC Quote Today:
- Call us at (843) 834-0607
- Book online here
About the Author
John Gradick is a certified HVAC technician with 25+ years of experience serving the Charleston area. He’s the lead tech at Passion Heating & Air and lives in Summerville with his wife and two dogs. When he’s not fixing ACs, he’s fishing on the Ashley River.
