The first real heat wave of summer 2026 is rolling across most of the country this week, and if your house doesn't have central air, you're probably eyeing a window or portable unit. Good call. A single room unit running you $150 to $400 will keep a bedroom livable for a fraction of what whole-house cooling costs to run. But the difference between a unit that quietly drops the temperature and one that drips down your wall, trips a breaker, or barely cools at all comes down to a handful of things most people skip on installation day.
I learned this the hard way a few summers back. Bought a window unit on a 95-degree afternoon, slapped it in, and spent the next two weeks listening to it short-cycle and watching condensation pool on the sill. The unit was fine. My install was the problem.
Size the unit before you buy anything
The single biggest mistake is buying based on price instead of room size. Air conditioners are rated in BTUs, and bigger is not better. An oversized unit cools the air fast, shuts off, and never runs long enough to actually pull humidity out — so the room feels cold and clammy at the same time. Undersize it and the thing runs nonstop without ever catching up.
The rough rule from the Department of Energy works fine for most rooms: roughly 20 BTU per square foot. A 200-square-foot bedroom wants about 5,000 BTU. Bump it up if the room gets heavy afternoon sun, has high ceilings, or doubles as a kitchen. Both Home Depot and Lowe's list the BTU and the coverage square footage right on the box, so measure your room first and match it. Don't eyeball it.
Window units: the install steps people skip
A window unit has to tilt slightly toward the outside — about a quarter inch — so condensation drains out the back instead of into your room. Most modern units handle this with the mounting bracket, but if yours doesn't, that backward tilt is on you. Get it level side to side, tilted back front to rear.
Then deal with the gaps. The accordion side panels that come with the unit are thin and leak both air and bugs. Stuff foam weatherstripping or backer rod into the seams, and run a bead of removable seasonal caulk along the top where the sash meets the unit. Here's the part nobody mentions: a window unit is a security weak point. The bottom sash can often be lifted from outside once the unit is in. Drill a small hole through the upper and lower sash frames where they overlap and drop in a nail or a sash lock — five minutes, and the window won't slide open.
Don't overload the circuit — this is a code issue, not a suggestion
Most window and portable units under 12,000 BTU run on a standard 120-volt outlet and pull somewhere between 5 and 12 amps. That sounds harmless until you realize a typical 15-amp bedroom circuit is already carrying lamps, a TV, and phone chargers. Plug a hungry AC into the same circuit and you'll trip the breaker on the hottest day of the year.
The National Electrical Code calls for a window AC to run on a circuit where it draws no more than 50 percent of the breaker rating if other things share the line, or 80 percent if it's a dedicated circuit. In plain terms: give the unit its own outlet when you can, and never run it through an extension cord or a power strip. The unit's startup surge can melt a light-gauge cord, and that's a genuine fire risk, not a hypothetical one. If the only outlet is across the room, the right fix is having an electrician add a receptacle — figure $150 to $300 — not stringing a cord under the rug.
Portable units have their own gotcha
Portable units sit on the floor and vent hot air out a window through a hose, which sounds easier than wrestling a heavy unit onto a sill. The catch most buyers miss: that single hose pulls room air, cools it, and dumps the heat outside — but it also creates negative pressure that sucks hot outside air back in through every gap in the room. A dual-hose model fixes this and cools noticeably better, though it costs more. A few things worth knowing:
- Keep the exhaust hose as short and straight as possible; every kink and loop bleeds efficiency.
- Empty or drain the condensate reservoir on schedule, or the unit shuts itself off mid-afternoon.
- A portable unit is louder than a comparable window unit — worth it for a rental where you can't mount anything, less so for a bedroom you want quiet.
- Self-evaporating models cut down on draining but still need a hose for humid climates.
Whatever you put in, clean the filter every couple of weeks during heavy use. A clogged filter is the number one reason a unit that worked fine in June is barely blowing by August. Pop it out, rinse it under the faucet, let it dry, slide it back. That two-minute job does more for cooling and your power bill than anything else you'll do all summer.