1. Size it by room square footage, and do not round up
Multiply the room's length by its width, then match that to the model's rated coverage. Roughly speaking, 150 to 250 square feet wants about 6,000 BTU, 250 to 350 square feet about 8,000 BTU, 350 to 450 square feet about 10,000 to 12,000 BTU, and 500 to 700 square feet about 14,000 BTU. Add capacity for a sunny room, a high ceiling, a kitchen or a room with several people in it.
2. Understand why oversizing is the classic mistake here
An air conditioner dehumidifies while it cools, and it only does that while the compressor is running. An oversized unit hits the set temperature in a few minutes, shuts off, and leaves the moisture in the air. The result is a room that is cold and damp rather than comfortable, plus more wear from constant short cycling. In Florida especially, a correctly sized unit running longer cycles feels dramatically better than a bigger one.
3. Choose window, through-the-wall or portable
A window unit is the cheapest and simplest option and fits a standard double-hung window. A through-the-wall unit slides into a sleeve cut through an exterior wall, which suits rooms with no suitable window and leaves the glass clear, but it is a construction job the first time. A portable sits on the floor and vents out through a hose, which is the fallback when the window will not take anything. A ductless mini-split is the step up if you want near-silent operation.
4. Check the electrical supply
Units up to about 12,000 BTU normally run on a standard 115-volt household outlet. Above that, many models need a dedicated 230-volt circuit with a specific plug type, and adding one is an electrician's job. Compare the plug configuration and amp draw on the specification sheet against the outlet in the room, and never use an extension cord or power strip with an air conditioner.
5. Compare efficiency, and look at inverter models
The efficiency rating on the yellow label tells you what the unit costs to run, which matters when the cooling season is nine months long. ENERGY STAR certified models use meaningfully less power than the minimum standard. Inverter or variable-speed units go further: instead of switching the compressor fully on and off, they modulate it, which holds the temperature steadier, dehumidifies better and runs much quieter. If the unit is going in a bedroom, that quietness is usually worth the price difference.
6. Plan the drainage and the seal
In a humid climate an air conditioner pulls a surprising amount of water out of the air. Window units are installed with a slight tilt to the outside so it drains away, and a unit set dead level or tipped inward will drip onto your floor. Through-the-wall sleeves need the same outward pitch. Seal the side panels and the gap above the sash properly as well, since warm humid air leaking back in makes the unit work harder than it should.
A window air conditioner or a portable one?
If the room has a standard double-hung window, take the window unit. At the same BTU rating it cools better, sounds quieter from the bed or the desk, and costs less, because the compressor and the condenser sit outside the glass instead of on your floor. It also deals with condensate for you: pitched slightly outward, the water drains off the back, and you never think about it again, which is worth something in a climate where a working unit pulls gallons a day out of the air.
Take a portable when the opening will not accept anything else. A casement that cranks outward, a horizontal slider, an awning window, a rental that forbids mounting hardware, or a wall of fixed glass are all real reasons to roll a unit in instead. Accept what comes with it: you hear the compressor as well as the fan, the exhaust hose is a hot pipe crossing the room, and a single-hose design exhausts household air outdoors and draws warm humid air back in through the nearest gap, which quietly costs you capacity. If a portable is the answer, pick a dual-hose model, do not undersize it, seal the window kit properly, and set up a continuous drain rather than emptying a tank all summer.
Cooling-only, or a unit that also heats?
Stay cooling-only. For the large majority of rooms here the season runs from March into November, cool-only models cost less at the same capacity, and the money is better spent stepping up to an inverter unit that holds temperature steadier, removes more moisture and runs noticeably quieter in a bedroom. Nine months of better comfort beats three weeks of convenience.
The exception is a room the central heat does not reach on the cold nights, which comes up far more often in metro Atlanta and north Georgia than in Miami: a garage conversion, a room over a carport, an in-law suite, a bedroom at the end of a long duct run. There a single heat and cool window unit saves buying, storing and dragging out a separate heater every winter, and the cabinet size and window fit are essentially the same as the cool-only version. If it really is only a few nights a year, a portable heater costs less than paying for a heating mode you barely use.
