Plenty of US apartments, especially older walk-ups and prewar buildings, come with no central air and a lease clause about what you may put in the windows. That leaves most renters choosing between two machines: a window unit that sits in the sash with its hot half hanging outside, or a portable that stands on the floor and pushes hot air out through a hose. Both cool a bedroom. Which one you can actually use often comes down to the building, not the spec sheet.
Before comparing models, it helps to know three things about your place: what kind of windows it has (double-hung, sliding or casement), whether the lease or building rules say anything about window units, and how many other things share the outlet you plan to use. The sections below go through the real differences between the two types, ENERGY STAR sizing, noise, condensate and running cost, and end with a list of questions worth sending your landlord before you order.
🪟Window unit vs. portable: the real difference
A window ac unit sits in an open window, held in place by the sash and a couple of side panels, with the hot half of the machine hanging outside. Because it dumps heat directly outdoors, it's generally cheaper to run and cools more efficiently for the same amount of cooling power. The trade-offs are physical and practical: it needs a window it actually fits (most are made for standard double-hung sash windows and don't play nicely with casement or sliding ones), it blocks the view and the light, and it's heavy and a little nerve-wracking to install a couple of floors up.
A portable air conditioner is the more flexible option. The whole machine stays on the floor, and a wide exhaust hose runs to a plastic kit that fits into a cracked-open window. Nothing hangs outside, nothing bolts to the frame, and you can wheel it from the living room to the bedroom as your day moves around. The catch is that a portable is usually a bit less efficient and often louder than a comparable window unit, because the noisy compressor lives inside the room with you rather than outside it.
For a renter, the deciding factor is often the building itself. Which brings us to the part people skip.
📋Check the lease before you buy
Plenty of leases and buildings restrict window units. Some cities and HOAs limit them for safety (a unit falling from a window is a real hazard), some buildings ban them on the street-facing side for looks, and many landlords simply want to approve how anything gets mounted in their window frames. In New York City, for example, the Department of Buildings expects window units to be supported by metal brackets or mounting rails, and many buildings insist their own staff do the install — sometimes for a fee. Rules like this are local, so ask your landlord or check your city's building department before you buy. A portable sidesteps most of that, since it doesn't attach to the building — but even then it's worth a quick, friendly message to your landlord if you'll be venting through a window with a kit.
None of this is meant to scare you off. It's just the classic renter move: check the lease, ask before you install anything semi-permanent, and keep the original window hardware so you can put everything back the way it was when you leave. If your building rules out a window unit, a portable is almost always the answer for an apartment without central air.
📏Sizing by BTU (without the math headache)
Cooling power is measured in BTU (British Thermal Units per hour). Too few and the unit runs flat-out and never quite cools the room; too many and it blasts cold air, shuts off, and cycles on and off so fast it never pulls the humidity out — leaving the room cold and clammy. The goal is to match the unit to the room.
The standard way to think about it is BTU relative to floor area. ENERGY STAR publishes a simple chart for room air conditioners, based on an 8-foot ceiling:
- 100–150 sq ft: 5,000 BTU
- 150–250 sq ft: 6,000 BTU
- 250–300 sq ft: 7,000 BTU
- 300–350 sq ft: 8,000 BTU
- 350–400 sq ft: 9,000 BTU
- 400–450 sq ft: 10,000 BTU
- 450–550 sq ft: 12,000 BTU
- 550–700 sq ft: 14,000 BTU
- 700–1,000 sq ft: 18,000 BTU
Then adjust for the room you actually have:
- Very sunny room: add about 10%. Heavily shaded: subtract about 10%.
- More than two people regularly in the room: add 600 BTU for each extra person.
- Kitchen: add 4,000 BTU, because cooking throws off a lot of heat.
- Don't oversize "just to be safe." An overpowered unit is worse at removing humidity, so the room feels cold and damp rather than cool and dry.
Example: a 12 × 15 ft bedroom is 180 sq ft, so the chart says 6,000 BTU; if it faces west and bakes in the afternoon, about 6,600 BTU, which in practice means the next size up. Figures as of October 2026 — check the current chart on ENERGY STAR's room air conditioner page.
💨Single-hose vs. dual-hose portables
If you go portable, you'll run into two designs. A single-hose unit has one hose that carries hot air out. It's cheaper and simpler, and it's fine for a small-to-medium room. The quirk is that as it blows air outside, it creates a slight negative pressure indoors, so warm air gets pulled in through gaps around doors and windows — which nibbles at its efficiency in a bigger space.
A dual-hose unit uses one hose to draw outdoor air in for cooling the machine and another to push heat out, so it doesn't rob the room of conditioned air the same way. It tends to cool a larger room faster and more efficiently, at a somewhat higher price and with a slightly bulkier setup. For a compact bedroom a single-hose is often all you need; for a larger living area or a very hot climate, the extra cost of dual-hose usually pays off in faster cooling.
Read the right BTU number on a portable. Portables now carry two ratings. The bigger "ASHRAE" figure is the old lab number; the SACC (Seasonally Adjusted Cooling Capacity) shown on the yellow EnergyGuide label is the DOE rating, and it accounts for heat leaking back in around the hose and for hot days. SACC typically runs about 25–40% lower — a unit sold as "14,000 BTU (ASHRAE)" may be only around 9,000–10,000 BTU SACC, especially if it's single-hose. Size a portable with the SACC number against the chart above, and you won't end up with a unit that can't keep up.
🔇Noise — especially in a bedroom
This is the spec renters underestimate. In a studio or a bedroom the AC is only a few feet from your head, and a portable's compressor runs inside the room, so it's typically louder than a window unit whose noisy half is outside. If you plan to sleep with it running, look for a quiet portable air conditioner for bedroom use: check the decibel rating on the spec sheet, favor models with a sleep or night mode that dials the fan down, and read a few reviews specifically for comments about sleeping through it. A unit you switch off at 2 a.m. because it's too loud isn't cooling anything.
💧Drainage and condensate
Cooling air also pulls water out of it, and that water has to go somewhere. Many modern portables are "auto-evaporative," meaning they push most of the moisture out with the exhaust air so you rarely empty anything — though in a very humid room you may still need to drain a small reservoir or attach a hose now and then. Window units usually drip their condensate out the back, outside, which is one less chore. When you compare models, check how each one handles water: a unit that needs frequent manual emptying is a hassle you'll notice fast in muggy weather. Don't rig any permanent drain into a rental's plumbing — a self-evaporating model or a simple gravity hose to a nearby drain is the renter-friendly route.
💰Energy cost, since you pay the bill
An AC can run for hours a day in summer, and in most rentals you're the one paying the electricity. That makes efficiency real money, not a rounding error. Look for the ENERGY STAR label, and compare efficiency between units of the same cooling power — for window units that's the CEER number on the EnergyGuide label (higher is better). A built-in thermostat and an eco or auto mode help a lot: they let the unit hold a set temperature and idle instead of running full blast the whole time. Between a window unit and a portable of similar size, the window unit is usually the cheaper one to run — worth weighing if your building allows it.
Check the outlet, too. Most rental bedrooms and living rooms run on 15-amp, 120-volt circuits, which top out at 1,800 watts — and the electrical code's 80% rule means a device running for hours should stay under about 1,440 watts. Typical 5,000–10,000 BTU window units draw roughly 450–1,000 watts, so they're fine on their own circuit, but a big portable sharing that circuit with a space heater, a gaming PC or a hair dryer can trip the breaker. Plug the AC straight into the wall (no extension cord or power strip, which most manuals forbid), and note that larger units — often those above about 15,000 BTU — may need a 230-volt outlet that most apartments don't have.
📦The renter angle: nothing permanent, it moves with you
Neither type changes the apartment. A portable rolls away and its window kit lifts out in a few minutes. A window unit comes out of the sash, the side panels fold back, and the window closes normally again. Keep the box, the side panels and every screw from the original install, because those are what you need to put the unit in at the next apartment. If the building's staff installed it on brackets, ask who removes the brackets at move-out and who pays for it, before the last week of your lease.
✉️What to ask your landlord before you order
A two-minute message now saves you from owning an AC the building won't let you install. Something like this covers it:
- Are window units allowed in my unit? If yes, are any windows off-limits (street side, fire escape window)?
- Who has to install it? Some buildings require their own staff or brackets, and some charge a fee.
- Is there a size or wattage limit? Older buildings sometimes cap units at a certain BTU or ask you not to run more than one per circuit.
- Can I vent a portable through this window? Mention the window type (sliding, casement, double-hung) so they know a kit will sit in it.
- Which outlets share a circuit? The super or landlord often knows which rooms are on the same breaker.
- Does the building have to provide cooling? In a few places, such as Montgomery County, Maryland, landlords must supply working AC in summer, which changes whether you should buy one at all.
Save the reply. If a window unit is approved in writing, that note is useful if anyone questions it at move-out.