Air Conditioners: How to Buy the Right One
Buy an air conditioner on price and you'll likely get the size wrong — and size is the one thing that decides whether the room actually gets comfortable. The job is to match the type and cooling capacity to your room and how you live in it, then pay attention to efficiency because this is an appliance you run for hours on the hottest days.
Key takeaways
- BTU sizing is the most important number — too big is as bad as too small.
- The type (window / portable / mini-split) follows your window, your rules and your budget.
- Efficiency (SEER / CEER) decides your running cost on the days you use it most.
- Measure the room and the window before you shop — most AC returns are fit problems.
Start by sizing, because an air conditioner that's wrong for the room can't be fixed with settings. Too small and it runs flat out and never cools; too big and it blasts cold air, shuts off before removing humidity, and leaves the room clammy and cold. Cooling capacity is measured in BTUs, and it should match your room's square footage (with adjustments for sun, ceilings and occupancy). Getting this right is more important than brand or features.
Then choose the type that fits your home and rules. A window unit is the cheapest and most efficient way to cool one room if you have a suitable window and are allowed to fit one. A portable rolls anywhere and needs only a vent hose, but costs more to run and cools less effectively. A mini-split is the quietest, most efficient option and cools best, but it's a permanent, professionally installed fixture. Match the type to your constraints first, then compare models within it.
What actually matters when buying an air conditioner
Getting the BTU size right
As a rough guide, a room needs about 20 BTU per square foot, so a 300 sq ft room wants roughly 6,000 BTU. Add capacity for rooms with lots of sun, high ceilings, a kitchen or several people; subtract a little for heavily shaded rooms. Resist rounding up 'to be safe' — an oversized unit short-cycles, fails to dehumidify and wastes energy. Sizing it correctly is the single most valuable thing you can do.
Type: window, portable or mini-split
Window units are cheapest, most efficient and quietest of the cheap options, but need a compatible window and permission. Portables are flexible and renter-friendly but louder, less efficient and partly defeated by the hot air in the exhaust hose. Mini-splits cool best, run quietest and cost least to run, but require professional installation and a bigger upfront spend. Decide the type before comparing specs.
Energy efficiency and running cost
Because you run an AC hardest on the most expensive-to-cool days, efficiency matters. Look for the efficiency rating (SEER for split systems, CEER/EER for window and portable units) and the Energy Star label. A more efficient unit costs a little more upfront and repays it every summer — our cost-per-use calculator helps weigh that trade-off.
Noise, and where you'll feel it
An AC you sleep or work next to needs to be quiet. Portables put the compressor in the room with you and are the loudest; window units are moderate; mini-splits are near-silent indoors because the compressor lives outside. Check the decibel rating if the unit will run in a bedroom or office.
Dehumidification and modes
Cooling and drying go together: a right-sized unit removes humidity as it cools, which is half of what makes a room feel comfortable. A dedicated 'dry' mode is useful in muggy climates. An oversized unit, by contrast, cools the air fast but leaves it damp. If humidity is your real problem more than heat, consider a dehumidifier instead.
The jargon, decoded
Air-conditioner labels are an alphabet soup. Here's what the terms that matter mean.
| Term | What it means |
|---|---|
| BTU | British Thermal Units — the cooling capacity. Match it to room size; bigger is not better. |
| SEER | Seasonal Energy Efficiency Ratio — the efficiency measure for split systems over a whole season. Higher is cheaper to run. |
| CEER / EER | The efficiency rating for window and portable units. Higher means more cooling per watt. |
| Mini-split | A wall-mounted indoor unit linked to an outdoor compressor. Quiet, efficient, permanent and professionally installed. |
| Inverter | A compressor that varies its speed instead of switching on and off. Quieter, steadier temperatures and lower running cost. |
| Single vs dual hose | For portables: dual-hose units are noticeably more efficient because they don't pull already-cooled room air out through the exhaust. |
How much should you spend? Budget tiers
Price is driven mainly by type and capacity. Here's what each tier buys.
| Tier | Typical price | What you get |
|---|---|---|
| Budget | $150 – $350 | A basic window unit or entry portable that cools one room. Fine for occasional use; watch the efficiency rating, as cheap units can cost more to run. |
| Mid-range | $400 – $700 | Efficient inverter window units or a dual-hose portable with quieter operation, a good dry mode and smart controls. The sweet spot for regular summer use in one or two rooms. |
| Premium | $1,200 + installed | A mini-split system: quietest, most efficient, cools best and often heats too. Worth it if you cool the same space every summer and can install permanently. |
Browse current air conditioner listings on Amazon →
Which type fits your situation?
If you own your home and cool the same room every summer, a mini-split pays back in comfort, quiet and running cost. If you rent or can't alter the building, a window unit is the cheapest effective choice where a suitable window exists. If you have no usable window, move the unit between rooms, or face strict rules, a portable is the flexible compromise — accept that it's louder and less efficient. Let your window, your tenancy and your budget make the call before you compare brands.
Tip: a dual-hose portable is meaningfully more efficient than a cheaper single-hose one. If a portable is your only option, the dual-hose design is the upgrade worth paying for.
Common mistakes (and how to avoid them)
1. Oversizing the unit
Buying more BTUs than the room needs is the classic error. An oversized AC cools fast then shuts off before drying the air, leaving the room cold and clammy and wearing out the compressor. Size to the room, not your anxiety.
2. Ignoring the window or venting
Window units need a compatible window; portables need somewhere to vent the hose. Measure first — most AC returns are because the unit physically didn't fit the window or the room layout.
3. Overlooking running cost
The cheapest unit can be the most expensive to own if it's inefficient and you run it daily all summer. Check the efficiency rating and Energy Star label, not just the sticker price.
4. Treating a humidity problem as a heat problem
If a room feels muggy rather than hot, an oversized AC makes it worse. A right-sized unit, or a dedicated dehumidifier, is the better fix.
When is the best time to buy?
Air conditioners are cheapest off-season — early spring and especially autumn, once the summer rush is over and retailers clear stock. Buying in a heatwave means peak prices and thin availability. If you can anticipate next summer's need and buy in September or October, you'll pay noticeably less.
Tip: our seasonal sale calendar maps the cheapest months for every major category, and the discount calculator tells you what a sale price really works out to.
Frequently asked questions
What size air conditioner do I need?
As a starting point, allow about 20 BTU per square foot of floor area, so a 300-square-foot room needs roughly 6,000 BTU. Add capacity for sunny rooms, high ceilings, kitchens or several occupants, and trim a little for shaded rooms. Avoid the temptation to oversize: a unit that's too big cools the air quickly but switches off before it removes humidity, leaving the room cold and damp.
Is a window or portable air conditioner better?
A window unit is cheaper to buy, cheaper to run, quieter and cools more effectively, so it's the better choice when you have a suitable window and permission to fit one. A portable is the answer when you can't use a window, need to move the unit between rooms, or face rules against window units, but it is louder and less efficient. If you go portable, choose a dual-hose model for better efficiency.
How much does it cost to run an air conditioner?
Running cost depends on the unit's capacity, its efficiency rating and your electricity price, but it's significant because you use an AC hardest on the hottest, most expensive days. A more efficient unit (higher SEER or CEER, with an Energy Star label) costs a little more upfront and saves money every summer. Sizing the unit correctly also keeps running costs down, since an oversized unit cycles wastefully.
Do I need an air conditioner or a dehumidifier?
If the room feels hot, you need cooling. If it feels muggy and damp but not especially hot, a dehumidifier may solve the real problem at lower cost. A correctly sized air conditioner does both jobs as it cools, but an oversized one cools without drying, which can make a humid room feel worse. Diagnose whether your discomfort is heat or humidity before buying.