How to Size a Through-the-Wall AC: BTUs, Square Footage & Efficiency

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Size matters. Whether you're cooling a bedroom, office, or your whole apartment, picking the right size through-the-wall air conditioner can make or break your comfort—and your energy bill.

❄️ Why Sizing Your AC Matters

Installing an air conditioner that’s too small or too big can leave you:

🥵 Uncomfortable due to underperformance
💸 Paying more on energy bills
🔧 Dealing with breakdowns and short cycling

💡 Too Small = Constant Running

The unit never gets a chance to shut off, eating electricity and wearing out faster.

💨 Too Big = Quick Cool, Poor Dehumidification

An oversized unit cools air quickly but doesn't run long enough to remove humidity—leaving you clammy and sticky.

🧰 Tony says: “Sizing isn’t just square footage. Sunlight, insulation, ceiling height—all that stuff matters.”

🔢 Understanding BTUs

BTU stands for British Thermal Unit. It's the amount of energy needed to cool or heat one pound of water by one degree Fahrenheit.

When it comes to ACs, BTU ratings tell you how much cooling power the unit can deliver per hour.

BTU Rating	Cooling Area (approx.)
6,000 BTU	150–250 sq. ft.
8,000 BTU	250–350 sq. ft.
10,000 BTU	350–450 sq. ft.
12,000 BTU	450–550 sq. ft.
14,000 BTU	550–700 sq. ft.
18,000 BTU	700–1,000 sq. ft.

👉 Energy Star Room AC sizing chart

🧠 Step-by-Step: How to Size a Through-the-Wall AC

Let’s walk through Tony’s trusted process.

🪟 1. Measure Your Room (Square Footage)

Use a tape measure and multiply:

📐 Length x Width = Sq. Ft.

Example:
12 ft (length) × 15 ft (width) = 180 sq. ft.
You’ll likely need a 6,000–8,000 BTU unit.

If your room is oddly shaped, divide it into rectangles, calculate each, and add them together.

🏠 2. Factor in Ceiling Height

Standard ceilings are around 8 ft. If you’ve got vaulted ceilings or anything over 9 ft, increase BTUs by 10%–20%.

🧰 Tony says: “Cooling tall air columns takes juice. If it feels like a church ceiling, pad your number.”

🌞 3. Count Sun Exposure

Rooms that get direct sunlight for most of the day (south or west-facing) need 10% more BTUs.

🌤️ Bright sunroom at 400 sq. ft = don’t stick with 10,000 BTU—bump to 12,000.

👪 4. Add Occupant Load

Every extra person beyond two in a room adds heat. Add 600 BTUs per person.

Example: A home office for 3 people needs 600 BTUs more than the base square footage would suggest.

🍳 5. Watch for Heat Sources

📍 If the room includes:

Kitchen appliances
Large electronics
Servers or AV racks

👉 Add another 1,000–2,000 BTUs, depending on intensity.

🚪 6. Open vs. Closed Floor Plans

If your room isn’t closed off:

Open concept = more space = more BTUs
Add 10–20% BTU capacity to handle connected areas

📊 Real-Life Room Sizing Scenarios

Room Type	Size (sq. ft.)	Sun?	Occupants	Suggested BTUs
Small Bedroom	150	No	1	6,000
Large Kitchen	300	Yes	2	10,000
Studio Apartment	500	Yes	2+	12,000–14,000
Home Office	225	Partial	3	9,000
Garage Gym	400	Yes	1	12,000–14,000

⚙️ Efficiency Ratings Explained

🔋 EER vs. SEER vs. SEER2

Metric	Meaning	Best Use
EER (Energy Efficiency Ratio)	BTUs / Watts at 95°F	Great for comparing through-the-wall and window units
SEER (Seasonal EER)	Measures seasonal efficiency	Used in central and ductless systems
SEER2	Newer 2023+ U.S. standard for better real-world testing	Applicable to newer AC models

👉 DOE’s SEER2 update

🧰 Tony says: “If it’s got a SEER2 label and Energy Star sticker, you’re on the right track.”

🌎 Look for ENERGY STAR Certified Units

These units meet strict energy efficiency guidelines and can save up to 15% more energy than standard models.

✅ Features to Look For:

Digital thermostat
Smart scheduling or WiFi
Adjustable fan speeds
Sleep mode

👉 ENERGY STAR product search

⚡ Sizing for Voltage & Power Supply

Most TTW AC units fall into one of two categories:

Voltage	BTU Range	Plug Type	Circuit Needs
115V	≤ 12,000	Standard	15–20 amp
230V	≥ 12,000	Special	Dedicated 20–30 amp

Don’t assume your wall outlet supports a larger unit. Always check your:

Electrical panel
Breaker capacity
Outlet type

🧱 Matching Size to Wall Sleeve

Through-the-wall units are installed in sleeves that act like a cradle in your wall. Not all units are the same size.

🔩 Check:

Sleeve dimensions
Airflow direction (rear vent vs. side vent)
Brand compatibility

👉 Friedrich’s wall sleeve guide

💰 Don’t Just Size for Cooling—Size for Cost

A well-sized unit pays off in:

🕒 Longer lifespan
💸 Lower energy bills
😌 Fewer repairs
🔇 Quieter operation

Bigger isn’t better—balanced is better.

🧰 Tony says: “Treat your AC like your shoes—fit matters. Too big or too small, and you’re gonna regret it.”

🧼 Maintaining Your Efficiency

After choosing the right size, protect your investment:

🧽 Maintenance Tips:

Clean the filter monthly
Check and seal around the sleeve annually
Ensure drainage lines are clear
Don’t block airflow with furniture

📈 Upgrading? Consider Smart Efficiency

Modern TTW ACs come with:

WiFi control
Programmable settings
Energy reporting

This helps you track usage and adapt cooling based on daily patterns.

👉 GE Smart Through-the-Wall Units

✅ Final Checklist: How to Pick the Right Size

✔️ Measure your room’s square footage
✔️ Check ceiling height
✔️ Factor in sun, appliances, and people
✔️ Decide on BTU range
✔️ Confirm wall sleeve dimensions
✔️ Verify electrical supply (115V vs. 230V)
✔️ Choose Energy Star when possible
✔️ Think ahead—how often will you use it?

📌 Conclusion: Sizing Isn’t Optional—It’s Essential

Right size = better comfort, longer lifespan, lower bills.
Don’t leave it to guesswork—follow the numbers and make sure your wall unit works with your space, not against it.