What is the best 15,000 BTU air conditioner?

Up to about 700 sq ft. ENERGY STAR's room air conditioner sizing chart has no 15,000 BTU size: it recommends 14,000 BTU for a room of 550 to 700 sq ft and 18,000 BTU for 700 to 1,000 sq ft, so a 15,000 BTU unit covers a room of up to about 700 sq ft. The chart assumes an ordinary room: add 10% for a very sunny room, take 10% off for a heavily shaded one, add 600 BTU for each person beyond two who is regularly in it, add 4,000 BTU for a kitchen, and size up for a ceiling above about 8 feet. The compare.ac size calculator applies all of these to your room. Each model on this page also shows the room its own maker or listing states, which can differ from the chart.

A common US rule of thumb for a standard, reasonably insulated room with an 8-foot ceiling is about 20 BTU per hour per square foot, so a 300 sq ft room is often quoted around 6,000 BTU and a 500 sq ft room around 10,000 BTU. compare.ac instead sizes against the official ENERGY STAR room air conditioner chart, which is not a flat rate: it recommends about 7,000 BTU for that same 300 sq ft room and about 12,000 BTU for a 500 sq ft room, then adjusts up for a hot or humid climate and down for a mild one once you pick your region. Add capacity for a sunny west-facing or top-floor room, a kitchen, or extra people who are regularly in it. Undersizing means the unit runs flat out and never cools; oversizing short-cycles and leaves the room cool but clammy.

Yes, more than almost anything except the size of the room. compare.ac's base sizing, before any climate adjustment, follows the official ENERGY STAR room air conditioner chart, which assumes a temperate summer. A hot, dry region like Arizona, or a hot, humid one like Florida and the Texas Gulf coast, realistically needs on the order of 20 to 30 percent more cooling per square foot, because the outdoor design temperature is higher and, in humid places, the unit also spends energy removing moisture (the latent load). A mild, marine climate such as the Pacific Northwest or coastal New England needs a little less. compare.ac applies this adjustment once you pick your region in the tool, using ASHRAE Standard 169 design temperatures and ACCA Manual-J regional sizing practice, and always shows both your real room size and the adjusted figure it sizes against. For a final purchase, a professional Manual-J load calculation for your specific home is the accurate method.

Running cost is driven by efficiency: CEER on window and portable units, and SEER2 on mini-splits. Higher is better on both, but they are separate scales and a SEER2 number cannot be read as a ratio against a CEER number. Directionally, an efficient inverter mini-split is the cheapest type to run, while a single-hose portable can cost roughly two to three times as much for the same comfort, because much of its capacity is lost back through the exhaust hose. For window and portable units that ENERGY STAR certifies, we go a step further and show an estimated dollars-per-year figure, taken from the model's published annual energy use (a DOE test that assumes about 750 hours of cooling a year) priced at the average US residential electricity rate. It is a standardized figure for comparing units, not a personal bill: your real cost depends on how much you run it, your local electricity rate, and your climate. Mini-splits are usually the most efficient of all but are rated on SEER2, so we do not convert those to a dollar figure, and a missing running cost is a data gap, never a sign a unit is worse.

Noise is measured in dB(A). A quiet split indoor unit on a low setting can be around 19 to 25 dB(A), which is barely audible and fine for a bedroom. Around 30 to 35 dB(A) is a soft hum that most people accept in a living room. Portable monobloc units are usually 52 to 58 dB(A) because the compressor sits in the room with you, which is closer to a dishwasher and can disturb sleep. If quiet matters, a split almost always wins; if you only cool occasionally, a portable may be acceptable.