A window air conditioner is the most direct way to add cooling to a single room — and the cheapest way to do it. The decision matters because window ACs are some of the most energy-intensive appliances in a typical home, and a 20 percent efficiency difference compounds across a cooling season. This guide covers what to look for.
Step 1: size the unit to the room
Window AC is measured in BTU per hour. Undersized = the unit runs continuously and never cools the room; oversized = the unit cycles on briefly and off, never dehumidifying the air, which leaves the room cold and clammy.
Rough BTU sizing:
- 150 sq ft: ~5,000 BTU
- 350 sq ft: ~8,000 BTU
- 450 sq ft: ~10,000 BTU
- 700 sq ft: ~14,000 BTU
- 1,000 sq ft: ~18,000 BTU
- 1,400 sq ft: ~24,000 BTU
Adjustments:
- Heavily shaded room: -10 percent
- Very sunny room (top floor, west-facing): +10 percent
- Kitchen (heat from cooking): +4,000 BTU
- Room regularly occupied by more than 2 people: +600 BTU per person
The actual sizes available are standardized, so round up to the nearest standard increment rather than over-buying.
Step 2: read the efficiency rating
For room air conditioners, the standard efficiency metric is the Combined Energy Efficiency Ratio (CEER):
CEER = BTU/hr / watts input
Higher CEER = less electricity for the same cooling. CEER ratings for window ACs typically fall between 10 and 15. Energy Star qualification starts at 12+ for most size classes. The most efficient models reach 14 to 15.
If you're comparing two similarly-sized units, the one with 1 point higher CEER uses about 7 percent less electricity per season.
For central ACs and heat pumps, the relevant metric is SEER2 — see our guide to understanding SEER, EER, and BTU ratings for details.
Step 3: inverter vs conventional
Recent window ACs increasingly use inverter-driven compressors rather than fixed-speed compressors. An inverter compressor runs at variable speed instead of cycling on and off; it ramps down when the room is near setpoint rather than overshooting and stopping.
Real-world savings from inverter window ACs are typically 20 to 40 percent per cooling season vs conventional on/off units. They're also substantially quieter — the compressor isn't running full-out, and the fan isn't cycling.
Look for the term 'inverter' in the product description. The marketing language varies ('Smart Inverter', 'Variable-Speed Compressor') but mechanically they're the same idea.
Step 4: smart / connected features
Many modern window ACs include Wi-Fi connectivity, app control, and integration with Alexa or Google Home. The practical energy benefit:
- Schedule the unit to run only when the room is occupied
- Pre-cool before arriving home so the unit isn't running full-blast when you walk in
- More precise temperature setting than mechanical dials (one-degree increments vs. 1 to 5 degrees analog dials)
- Energy usage reports in the app to find surprising patterns
The features vary widely by model. If you're going to control the unit from an app, check whether the manufacturer's app gets consistent negative reviews — a poorly-built app can make smart features useless.
Step 5: install and seal
A surprising amount of window-AC inefficiency comes from installation, not the unit itself:
- Gaps around the unit let hot air in and cooled air out. Use the included foam weatherstripping; add more if the gaps are larger than 1/4 inch.
- A poorly-fitted side panel accordion extension leaks air. Use rigid foam board to fill the gap rather than the standard thin plastic accordion.
- A unit mounted without slight outward tilt (so condensate drains outside) can leak inside.
- A poorly-supported unit vibrates against the window frame, transferring noise and letting the pane rattle.
Spend an extra 30 minutes on install — sealing gaps, leveling, supporting the external overhang — and the unit will both cool better and use less power. Read our weatherstripping and air sealing basics guide for the broader principle.
Step 6: practical operation
Once it's installed, the operational choices matter as much as the unit:
- Set it to the highest comfortable temperature (75 to 78°F is reasonable)
- Use it for a single room with the door closed rather than expecting it to cool the whole house
- Pair with a ceiling or floor fan to mix the air — fans let you raise the AC setpoint a few degrees with no comfort penalty
- Clean the filter every 4 to 6 weeks during cooling season
Where the calculator fits
We don't yet have a dedicated AC calculator, but pairing the AC sizing rules above with the insulation calculator on the home page gives you a two-step plan: an efficient AC plus good attic insulation means the AC spends less energy maintaining the room temperature you want.