Air conditioner shopping mostly comes down to two decisions: which type of unit fits your space, and whether it's actually sized correctly for the room.
The three main types
Window units are the most affordable and energy-efficient option per BTU, but need a compatible window and take up part of it while installed.
Split systems separate the noisy compressor into an outdoor unit, connected to a quiet indoor unit by refrigerant lines through a small wall opening. Quieter indoors and often more efficient than a window unit, but require professional installation and cost significantly more upfront.
Portable units roll anywhere in a room and vent hot air out through a window using a hose, needing no permanent installation. The most flexible option, but generally the least efficient of the three for the same cooling output, since the whole unit — including its own compressor — sits inside the room it's trying to cool.
The number that actually matters: BTU
Cooling capacity is measured in BTUs (British Thermal Units per hour). Bigger isn't automatically better — an oversized unit cools the room quickly but shuts off before it properly removes humidity, leaving the room feeling cold and clammy rather than comfortable. As a rough starting point, many sizing guides suggest around 20 BTU per square foot of room space, adjusted upward for high ceilings, lots of direct sun, or a kitchen nearby, and downward for a shaded or well-insulated room. Manufacturers and utility companies often publish more detailed sizing charts — it's worth checking one specific to your unit rather than relying on a single rule of thumb.
Efficiency rating
Look for an efficiency rating appropriate to your region (such as an Energy Star label or a listed SEER/EER rating) and compare it between models you're actually considering — a slightly pricier, more efficient unit often pays for itself over a few cooling seasons through lower running costs.
The one thing people forget
Check the actual noise rating (in decibels) if the unit will be anywhere near a bedroom or workspace — window and portable units in particular can vary a lot in how loud they run, and that's not something you can tell from the box or a photo.