Three AC types dominate the residential market in 2026. Window units mount in a double-hung window opening and have been the affordable default for 70 years. Portable units sit on the floor and vent hot air through a hose to a window adapter. Mini-split heat pumps mount on a wall with an outdoor condenser unit and run refrigerant lines through the wall. Each format suits different homes. Choosing the wrong format costs you in either upfront price, ongoing electricity, or comfort. This guide explains the engineering tradeoffs and the right answer for common scenarios.
How window units work
A window AC is a single sealed cabinet that contains both the indoor evaporator coil and the outdoor condenser coil, separated by an insulated wall inside the unit. Half of the cabinet sits outside the window, half sits inside, and the window sash closes down on the top of the cabinet to seal the opening.
The compressor and condenser fan sit on the outdoor side. The evaporator coil and indoor fan sit on the indoor side. Refrigerant cycles between them in a sealed loop. Hot indoor air passes over the evaporator and cools. The heat moves to the condenser and dumps to outdoor air.
Efficiency is rated in CEER (Combined Energy Efficiency Ratio). Modern Energy Star window units rate CEER 12 to 15. Inverter-driven premium window units rate CEER 15 to 18. Older non-inverter units rate CEER 8 to 11.
Installation is simple. Mount the support bracket if required, place the unit in the window, lower the sash, and secure with the included L-brackets. Most window units install in 30 to 60 minutes. The unit can be removed for winter storage.
How portable units work
A portable AC is also a self-contained cabinet, but it sits entirely indoors on the floor. A flexible hose runs from the back of the unit to a window adapter that vents hot exhaust air outside.
Single-hose portables use one hose for exhaust only. Indoor air is pulled across the evaporator (cooling the room) and across the condenser (heating up), and the heated air exits through the hose. The replacement air for what was exhausted comes from infiltration through cracks, doors, and gaps. This infiltration brings hot outdoor air back into the room, which the AC then has to cool. The net efficiency is much lower than the spec sheet.
Dual-hose portables use one hose for outdoor intake (to the condenser) and one hose for exhaust. The cooling loop and the exhaust loop are separate, so no net air is pulled in from outside. Dual-hose is significantly more efficient than single-hose, but most consumer portable AC units still ship single-hose because dual-hose costs more.
Even at their best, portables run at CEER 8 to 11. They are the least efficient AC format on the market. Their advantage is flexibility: no window mounting, no wall penetration, fully portable between rooms (although the 30 to 40 kg weight makes the moving tedious).
How mini-splits work
A mini-split heat pump separates the indoor and outdoor units. The outdoor condenser unit sits on a wall bracket or ground pad outside the house. The indoor air handler mounts on a wall or ceiling inside the room. Two refrigerant lines and a power cable run through a 50 mm hole in the wall between them.
This separation lets the indoor unit be small, quiet, and aesthetically tolerable, while the outdoor condenser can be large and efficient. The compressor runs at variable speed (inverter-driven), matching capacity to demand precisely instead of cycling on and off like a fixed-speed unit. The result is high efficiency, often SEER2 18 to 25 (equivalent to CEER 15 to 22), and quiet operation, often 20 to 30 dB at the indoor unit.
Mini-splits also heat. Running the cycle in reverse turns the indoor coil into a heat exchanger and the outdoor coil into an evaporator. Heat pump efficiency is much higher than resistance electric heating, typically 250 to 350 percent (COP 2.5 to 3.5) at moderate outdoor temperatures.
Installation is the catch. A mini-split requires a refrigeration technician to install (refrigerant handling licenses required), drill a wall hole, mount both units, run the lines, and commission the system. Total installed cost is 2500 to 5000 dollars for a single zone, versus 250 to 600 dollars for a window unit and 400 to 700 dollars for a portable.
Efficiency math
For a 12000 BTU cooling load running 8 hours per day for 90 days (a typical summer in a hot climate):
A SEER 11 window unit: roughly 800 kWh per season at 0.15 dollars per kWh, about 120 dollars per season.
A SEER 14 efficient window unit: 630 kWh, 95 dollars.
A SEER 8 single-hose portable: 1100 kWh, 165 dollars.
A SEER 22 inverter mini-split: 400 kWh, 60 dollars.
The mini-split saves 60 dollars per season versus a basic window unit and 105 dollars per season versus a portable. Over 10 years, the savings are 600 to 1050 dollars, which offsets a substantial portion of the higher installation cost.
In milder climates with shorter cooling seasons, the math tilts toward window units. In hot climates with long cooling seasons (Phoenix, Houston, Atlanta, much of the southern US), mini-splits pay back within 5 to 7 years.
When to choose window
Rental apartments where you cannot install a wall-mounted system. Cabin or vacation properties where the AC runs only seasonally and the install cost would not amortize. Single rooms in a larger home where central AC is impractical. Climates with short cooling seasons under 60 days per year. Budgets under 600 dollars.
Window units are also the right choice when the window orientation works. South-facing or west-facing windows with no shade are bad locations because the outdoor portion of the unit faces direct sun, reducing condenser efficiency. North-facing or east-facing shaded windows are ideal.
When to choose portable
Buildings that prohibit window units. Renters who move frequently and want to take the AC with them. Rooms without a window that can accept a window unit (basement windows, slider windows, hopper windows). Temporary cooling needs (a few weeks per year).
Avoid portables for primary daily cooling in hot climates. The high energy consumption adds up. If you find yourself running a portable 8 hours a day all summer, the dollar difference versus a mini-split installation amortizes within 2 to 3 years.
When to choose mini-split
Homes you own or have long lease commitments on. Hot climates with 90 plus cooling days per year. Homes that also need heating, where the heat pump function eliminates separate furnace or baseboard heating costs. Multi-room cooling where a multi-zone mini-split system serves the whole house from one outdoor condenser.
Avoid mini-splits when the property is short-term, when the installation logistics are impractical (historic buildings, condos with strict facade rules), or when the cooling load is small and seasonal.
Combined recommendations
Most homeowners in moderate to hot climates benefit from mini-splits as the primary cooling system, supplemented by portable units in rare-use spaces (guest rooms, garages).
Most renters benefit from window units in primary spaces and portable units in spaces without compatible windows.
Most cabins and seasonal properties benefit from a single window unit per main living space.
Avoid the trap of buying an oversized unit. An AC that is too large cycles on and off rapidly, removes less humidity than a properly-sized unit, and produces a cold-but-clammy room. Match BTU to the room.
For more on whole-house climate strategy see our heat pump vs furnace guide and our review methodology at /methodology.
Frequently asked questions
Is a mini-split worth the extra installation cost?+
In most climates with more than 60 cooling days per year, yes. A 12000 BTU mini-split with SEER2 rating of 22 to 25 uses roughly 40 to 50 percent less electricity than a 12000 BTU window unit at SEER 11. Over a 10 year lifespan in a hot climate, the energy savings exceed the higher installation cost. In mild climates with under 40 cooling days, the payback period stretches beyond 10 years and a window unit is fine.
Why are portable AC units so inefficient?+
Portable units exhaust hot air through a window hose but draw replacement air from inside the room. That replacement air comes from somewhere, usually pulled through cracks and door gaps from outside, which means you are also pulling in hot outdoor air to replace what you exhausted. The net cooling work is much higher than the spec sheet implies. Dual-hose portable units fix this but cost more and most consumer portables still ship single-hose.
Can a mini-split heat in winter too?+
Yes, most modern mini-splits are heat pumps that run in reverse for heating. Cold-climate variants (often labeled Hyper-Heating or Whisper-Heat) maintain rated capacity down to about minus 15 degrees Celsius. Below that, supplemental heat is needed. In moderate climates, a single mini-split system handles both summer cooling and winter heating for a single room or zone.
What size BTU do I need?+
Roughly 20 BTU per square foot of room area, adjusted up for sun exposure, high ceilings, or many occupants, and down for shaded or well-insulated rooms. A 150 square foot bedroom needs about 5000 BTU. A 300 square foot living room needs about 7000 to 8000 BTU. A 600 square foot great room needs 12000 BTU or more. Oversizing wastes electricity and causes humidity problems.
Are window units allowed in apartments?+
Depends on the building. Many newer apartment buildings prohibit window units for liability and aesthetic reasons. Some allow them on specific window types only. Check your lease. If window units are prohibited, portable units are usually allowed because they sit inside the room and only require a hose vent, which most landlords accept for a single sash window.
