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How to Size a Through-the-Wall AC: Jake’s Guide to BTUs, Square Footage & Efficiency Ratings

Why Sizing Makes or Breaks Comfort

Here’s a truth I’ve learned in years of HVAC work: most people buy the wrong size air conditioner.
They pick one that’s too powerful and think it’ll cool faster. It will—but it’ll also shut off too quickly, never remove humidity, and leave the room clammy.
Go too small, and the unit runs nonstop while the room never gets truly cool.

The goal is steady, balanced comfort—not bursts of cold air. That starts with understanding BTUs and how they relate to your space.

1. What BTUs Really Mean

BTU stands for British Thermal Unit, and it measures how much heat your AC can remove in one hour.
Think of it like horsepower for your cooling system—the higher the number, the more air it can condition.

But bigger isn’t always better. You want enough power to maintain comfort without short cycling (turning on and off too fast).

2. Jake’s Simple BTU Sizing Chart

Start here to find your baseline match between room size and cooling power:

Room Size (sq. ft.)	Recommended BTUs
100–250	5,000–6,000
250–350	8,000–9,000
350–450	10,000–12,000
450–550	12,000–14,000
550–700	14,000–18,000

That’s a general guide for rooms with 8-foot ceilings and standard insulation.
You’ll need to adjust for things like sunlight, ceiling height, and how many people use the room.

3. Adjusting for Real-World Conditions

☀️ Sunlight

South or west-facing rooms: add 10% BTUs.
Shady rooms: subtract 10%.

🏠 Ceiling Height

Add 10% BTUs per foot above 8 feet.

👨👩👧 Occupancy

Add 600 BTUs per person if more than two people regularly occupy the room.

🔥 Appliances

For kitchens or offices with heat-generating equipment, add 2,000–4,000 BTUs.

🧱 Insulation

Poorly insulated rooms need the next BTU size up.
Well-insulated, newer rooms can go slightly lower.

4. Common Sizing Mistakes Jake Sees

❌ Going Too Big

It cools fast but doesn’t dehumidify. You’ll feel cold and sticky.
Fix: Pick a model that runs steady cycles—not quick bursts.

❌ Ignoring Sunlight and Insulation

Two same-sized rooms can need totally different capacities.
A sunny, older room might need 12,000 BTUs while a shaded, newer one only 9,000.

❌ Forgetting Ceiling Height

Vaulted or open layouts mean more air to cool—don’t forget the extra volume.

5. Real-World Examples

🛏️ Small Bedroom (180 sq. ft.)

Shaded, two occupants.
Best fit: 6,000 BTU unit like the LG LT0616CER 6,000 BTU Energy Star.

💻 Home Office (300 sq. ft.)

Sunny exposure, two computers.
Best fit: 9,000 BTU model such as the GE AJCQ09ACG—handles heat load and keeps noise low.

🧰 Garage Workshop (500 sq. ft.)

Poor insulation, west-facing wall.
Best fit: 12,000 BTU heat/cool combo like the GE 12,000 BTU with Electric Heat (AJEQ12DWJ)—a reliable year-round solution.

6. Efficiency Ratings: EER and CEER Explained

Once you’ve got the size right, check efficiency. That’s where EER and CEER come in.

⚙️ EER (Energy Efficiency Ratio)

Measures how efficiently a unit cools under set conditions.
Higher numbers mean lower electricity use.

⚡ CEER (Combined EER)

Includes standby energy use and reflects real-world performance.
Look for CEER 10.5 or higher for Energy Star certification.

A 10,000 BTU unit with a CEER of 11 will use roughly 12% less power than one rated at 9.5—saving you money every summer.

You can check certified products at the Energy Star database .

7. How Efficiency Affects Your Bill

Here’s what the difference looks like in dollars:

Model	CEER	Annual Energy Use	Est. Yearly Cost*
Standard	9.5	930 kWh	$112
Energy Star	11.0	820 kWh	$98

That’s a $14 annual savings, which adds up to $150–$200 over a decade — more than offsetting any price difference.

Jake’s take: “Don’t look for the cheapest unit—look for the one that runs cheapest.”

8. Wall Sleeves & Installation Fit

Even the best AC will underperform if it’s not sealed properly.
A wall sleeve keeps the unit stable, weatherproof, and efficient.

Use one designed for your brand, like the Amana WS900 Wall Sleeve , or a universal sleeve for multi-brand compatibility.
Seal around it with foam insulation and exterior-grade caulk to prevent air leaks.

9. Voltage: 115V vs. 230V

Through-the-wall units come in two voltage types:

115V: Standard plug — perfect for small units up to 10,000 BTUs.
208/230V: Requires a dedicated circuit — for 10,000+ BTUs or models with electric heat.

Check your outlet before buying. Running a 230V system on a 115V line can damage the compressor.
When in doubt, call an electrician—it’s cheaper than a repair bill.

10. Climate Considerations

Different climates change your cooling needs:

Humid regions (Southeast): Go up one size and prioritize models with dehumidify mode.
Dry heat (Southwest): Focus on efficiency—EER matters more than capacity.
Mild climates (Pacific Northwest): Consider a heat/cool combo for year-round comfort.

11. When to Consider a Mini Split Instead

A through-the-wall AC is perfect for single rooms up to about 700 sq. ft.
If you’re cooling multiple connected rooms or open spaces, a ductless mini split may make more sense.

Systems like the MRCOOL DIY Series use less power overall, reaching SEER2 ratings up to 22, though they cost more upfront.

12. The EnergyGuide Label: What to Look For

That yellow tag tells you everything you need to know in seconds.
Focus on these four details:

Cooling capacity: Confirms BTU rating.
CEER value: Higher = more efficient.
Estimated yearly cost: Based on average power rates.
Comparison bar: See how it stacks up to similar units.

If you see the ENERGY STAR® logo, you’re in good hands.

13. Quick Checklist Before Buying

✅ Measure your room’s square footage.
✅ Note ceiling height and sunlight exposure.
✅ Check insulation quality.
✅ Choose between cooling-only or heat/cool model.
✅ Verify outlet voltage.
✅ Confirm wall sleeve compatibility.
✅ Look for CEER 10.5+ or Energy Star label.

If you’ve checked those boxes, your sizing is spot-on.

14. Jake’s Recommended 2025 Models

Here are a few I trust in the field for reliability and performance:

GE AJEQ12DWJ 12,000 BTU Heat/Cool — great for garages and additions.
LG LT0816CER 8,000 BTU Energy Star — quiet and perfect for bedrooms.
Friedrich Uni-Fit 10,000 BTU — fits multiple sleeve sizes and runs whisper-quiet.

All are available on The Furnace Outlet’s Through-the-Wall Collection.

15. Jake’s Golden Rule: Balance Is Everything

“A good AC shouldn’t blast cold air—it should quietly keep you comfortable.”

If it turns on and off too fast, it’s oversized.
If it runs nonstop, it’s undersized.
The perfect unit runs steady cycles and keeps the humidity balanced.