How to Choose a Window Air Conditioner

A window air conditioner is one of the few appliances where two numbers on the box tell you nearly everything that matters: BTU/hr (how big a room it can cool) and CEER (how much electricity it burns doing it). Get the first one right and the unit will actually hold its set temperature and dehumidify properly; get the second one right and you’ll pay noticeably less every summer for the same cooling. This guide covers both, with the cost math done from real certification data — every efficiency and running-cost figure below is computed from our table of all 420 ENERGY STAR-certified room air conditioners, transcribed from the ENERGY STAR registry.

Step 1: Size the unit to the room

Cooling capacity is rated in BTU/hr, and the long-standing rule of thumb — used by DOE’s Energy Saver guidance and ENERGY STAR alike — is roughly 20 BTU per square foot of floor area:

Then adjust for how the room is actually used, per DOE guidance:

• Heavily shaded room: reduce capacity by about 10%.
• Very sunny room: increase capacity by about 10%.
• More than two people regularly in the room: add ~600 BTU per extra person.
• Kitchen: add ~4,000 BTU.
• High ceilings, poor insulation, and hot upper floors all push you toward the larger end of a range.

Oversizing is the classic mistake. A too-big unit blasts the air cold and shuts off before it has run long enough to pull moisture out of the air, leaving the room clammy and cycling on and off. A correctly sized unit runs longer, steadier cycles and dehumidifies as it cools. When in doubt between two adjacent sizes for a normal room, pick the smaller one.

To shop by size class directly, the data table has pre-filtered views: up to 8,000 BTU (117 certified models — single rooms up to ~350 sq ft), 8,001–12,000 BTU (200 models — the most common class, medium bedrooms and living rooms), and over 12,000 BTU (103 models, up to 34,100 BTU for large and open-plan spaces).

Step 2: What CEER means, and why it’s the number to compare

CEER — Combined Energy Efficiency Ratio — is the federal efficiency metric on every room AC’s yellow EnergyGuide label. It’s the cooling output (BTU/hr) divided by total electrical input in watts, where “combined” means the test also counts standby power — what the unit draws while plugged in but not actively cooling. That’s what distinguishes it from the older EER metric it replaced in 2014; CEER numbers run slightly lower than EER for the same hardware, so don’t compare a CEER spec against an old EER spec directly.

Higher is better, and because capacity is already in the numerator, CEER is comparable across sizes: a 15.0-CEER unit delivers the same cooling on less electricity than a 13.0-CEER unit, whatever the BTU rating.

Across the current certified registry, CEER runs from 12.0 to 17.6, with a median of 15.0. That top figure belongs to Midea’s 9,500 BTU U-shaped inverter unit (MAH09B1AGR) — more on why below.

How much the efficiency spread actually costs you

The registry lists each model’s tested annual energy use (kWh/yr, based on a standard usage assumption); our table multiplies that by a national- average-style electricity rate of $0.16/kWh to get an estimated annual running cost. Computed across all 420 certified models:

• All sizes: estimated running cost spans $44 to $335 a year (median $85) — but most of that spread is capacity, not efficiency. A 34,100 BTU monster costs more to run than a 5,000 BTU bedroom unit no matter how efficient either is.
• Within the mid-size 8,001–12,000 BTU class (200 models), where capacity is roughly comparable, estimated cost still ranges $61 to $108 a year — the least efficient certified models in the class cost about 75% more per year to run than the most efficient.
• Holding size nearly constant — the 91 certified models rated within ±200 BTU of 8,000 — cost runs $60 to $73 a year (CEER 13.2 to 16.0), a ~$13/yr spread on identical cooling.

And remember: this is the spread inside the certified list. Every model in the table already beats the federal minimum, so the gap between an efficient certified unit and a bargain-bin non-certified one is larger still. Over a typical 8–10 year service life, choosing well within your size class is worth on the order of a few hundred dollars — often more than the sticker-price difference. If your electricity rate is above $0.16/kWh (much of the Northeast and California), scale the numbers up proportionally.

U-shaped and inverter designs: why the efficiency leaders look different

The top of the CEER table is dominated by two related design ideas:

Inverter (variable-speed) compressors. A traditional window AC compressor is either fully on or fully off. An inverter compressor varies its speed continuously, throttling down to a low hum once the room reaches temperature instead of cycling. That means steadier temperatures, far less of the jarring compressor-kick noise, and better real-world efficiency — partial-load operation is where these designs shine.

U-shaped (“saddle”) chassis. Pioneered by Midea’s U-series, the unit wraps around the window sill so the sash closes into the gap between the indoor and outdoor halves. The compressor ends up outside the closed window, which cuts indoor noise dramatically, and the window itself remains openable. The highest-CEER unit in the entire certified registry — 17.6 CEER, at an estimated $65/yr to run for 9,500 BTU — is exactly this type. For comparison, plenty of conventional certified units of similar capacity land in the low-13s and cost $100+ per year.

The trade-offs: U-shaped and inverter units cost more upfront, installation brackets are more involved, and the U-shape only fits standard double-hung windows. But if the unit runs all summer in a bedroom you sleep in, the noise reduction alone is usually worth it, and the efficiency premium pays itself back.

The ENERGY STAR Most Efficient shortcut

Beyond base certification, ENERGY STAR annually designates a stricter “Most Efficient” tier. In the current registry, 136 of the 420 certified models carry the Most Efficient 2025 mark. The CEER bar varies by capacity class (larger units face physics-driven lower ceilings — the Most Efficient floor is around 14 CEER in the small and mid classes but lower for the biggest units), so the designation is a convenient size-adjusted filter: it means “top tier for its capacity,” which raw CEER sorting alone doesn’t tell you.

If you want to skip the spreadsheet work, start from the Most Efficient view of the table, narrow to your BTU band, and pick from what’s left.

A note on window vs. through-the-wall

The certified table covers both true window units (388 models) and through-the-wall models (32), tested under the same ENERGY STAR room-AC program — the Type column distinguishes them. Through-the-wall units slide into a permanent wall sleeve rather than a window; if you’re replacing one, match the sleeve dimensions first and shop efficiency second.

The short version

1. Measure the room, apply ~20 BTU/sq ft, adjust for sun, kitchen, and occupancy — and don’t oversize.
2. Open the matching size view of the certified data table: ≤8,000 BTU, 8,001–12,000 BTU, or over 12,000 BTU.
3. Sort by CEER (or start from the Most Efficient list).
4. For a bedroom or any room where the unit runs long hours, strongly consider a U-shaped inverter model — quietest and most efficient, and the running-cost math covers much of the price premium.