What is the cheapest air conditioner to run?

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.

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.

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.