Questions people actually ask

A 12 by 12 room is 144 square feet, which falls in the ENERGY STAR table's 100-to-150 sq ft band: 5,000 BTU per hour. Add 10 percent if the room is very sunny, subtract 10 percent if it is heavily shaded, and add 600 BTU for each person beyond two who regularly occupies it. Full sizing walkthrough.

Roughly 20 BTU per square foot is the rule of thumb, but the ENERGY STAR table is better because it is not linear: it gives 5,000 BTU for 150 sq ft (about 33 BTU per sq ft) and 34,000 BTU for 2,500 sq ft (about 14 BTU per sq ft). Small rooms need proportionally more because the walls, windows and ceiling do not shrink as fast as the floor area. The full chart.

SACC is Seasonally Adjusted Cooling Capacity, the DOE-rated capacity for portable air conditioners. It subtracts heat that leaks back into the room through the exhaust duct and heat carried in by outside air infiltrating to replace what the unit exhausted. The box number usually comes from a different standard that ignores both, so SACC is the number that describes what actually reaches your room. How big the gap gets.

Take its nameplate wattage, divide by 1,000, and multiply by your electricity rate. A 1,300 W unit at 18.34¢/kWh costs about 24 cents an hour, roughly $1.91 for an eight-hour night, or about $57 over thirty nights of overnight running. Worked example with a real unit.

A fan, by one to two orders of magnitude. A ceiling fan on low can draw under 2 watts and a good DC tower fan runs at 19 watts flat out, against 1,300 watts for a large portable air conditioner. But they do different jobs: a fan moves air and makes you feel cooler, while an air conditioner actually removes heat from the room. The fan is not a cheaper air conditioner, it is a different tool. The wattage-to-dollars table.

A window unit, when you have a window you can use. Window units reject heat entirely outside the room, are ENERGY STAR certified with published CEER and annual kWh figures, and cost less to buy and run. A portable's exhaust duct passes back through the room it is cooling, and single-hose portables also pull replacement air in from outside. Portables win only when a window unit cannot physically go in. The full comparison.

Not as air conditioners. Anything sold as a ventless or hose-free air conditioner is either an evaporative cooler, which adds humidity and only helps in dry air, or a fan. A refrigeration cycle has to put its waste heat somewhere, and with no duct that somewhere is the room you are standing in — the net effect is slightly warmer air. What to do in a windowless room.

No, and the drier your air the better they work. An evaporative cooler cools by evaporating water into the air passing through it, so it trades humidity for temperature. In Phoenix in June that trade is excellent. In Houston or Miami the air is already close to saturated, so there is little evaporation available and you end up with air that is barely cooler and noticeably damper. The humidity threshold.

Only in a room with real airflow to the outside — an open window or door. An evaporative cooler continuously adds moisture, so in a closed room the humidity climbs until evaporation slows and the cooling stops. The units designed for closed rooms are small and comparatively inefficient; the big ones assume you are cooling a garage with the door up. Indoor versus outdoor units.

You move the heat somewhere else, which without a window means one of three things: a portable unit ducted through a drop ceiling, a wall vent or an adjacent room; a through-the-wall or mini-split installation; or accepting that you are only going to move air, not cool it. A fan in a windowless room makes you feel cooler without lowering the temperature, and its motor adds a little heat. All three options, honestly.

In order of how much they actually do: a U-shaped window unit, which many leases permit because it needs no brackets and no drilling; a portable unit vented through a window with the supplied kit; or a good fan plus blackout blinds. Check the lease and the building rules before buying, because the restriction is almost always about exterior appearance or brackets, not cooling. What each option needs.

Almost always some combination of three things: heat radiating down from an unventilated attic, hot air rising and collecting upstairs while the thermostat sits downstairs, and duct runs that are longer and leakier by the time they reach the top floor. The fix depends which one dominates, and they are diagnosable in about ten minutes. How to tell which one it is.

Slightly, briefly, and only for the person sitting directly in the airstream. Melting ice absorbs real heat, but a bowl of it holds a trivial amount of cooling compared with a room's heat load, and if the ice came from a freezer inside the same house the freezer dumped more heat into that house than the ice removes. It is a comfort trick, not cooling. What actually works instead.

Counterclockwise when you look up at it, which pushes air straight down and creates the breeze you feel. Most fans have a small switch on the motor housing to reverse it. If your ceiling fan seems to be doing nothing on a hot day, this is the first thing to check and it takes ten seconds. How to check yours.

Only if your nights get genuinely cool. A whole-house fan pulls outside air in through open windows and exhausts it through the attic, so it can only ever bring your house down toward the outdoor temperature. Where nights drop into the 60s it can replace air conditioning for much of the season. Where they stay in the 80s it is an expensive attic fan. The temperature test.

A tower fan, usually — it has a smaller footprint, the airflow is spread over a tall column rather than concentrated in one stream, and the DC-motor models are dramatically quieter and cheaper to run. A pedestal fan wins when you need to aim air at a specific height, such as across a bed from the far side of the room. Side by side.

No, and we say so on every page. We have no lab and own none of these units. What we do instead is compile the published manufacturer specifications, use the federal ENERGY STAR and DOE certification data, compute the running costs at a cited electricity rate, and show every formula so you can reproduce any number on the site. The method in full.

Because we do not currently hold a verified price for that product. Our prices come from a live data feed and expire after 48 hours; once a figure is that old it disappears rather than sitting on the page looking authoritative while being wrong. A stale price is worse than no price. How our price data works.