❄️What does an air conditioner cost to run?

A 900 W window unit running continuously uses 21.6 kWh a day — $3.67 at $0.17, £5.40 at £0.25, €7.56 at €0.35. In practice a thermostat-controlled unit does not run continuously even on a hot day; half to two thirds of the time is more typical once the room is cool.

Turn it off, in almost every case. A house that is allowed to warm up while empty loses heat to the outside more slowly than a cool one does, so less total energy is needed even counting the cooldown when you return. The exception is a genuinely humid climate, where letting the house get damp costs more to fix than it saved.

Only indirectly. BTU is cooling delivered; watts are electricity consumed. The ratio between them is the efficiency (EER or SEER), and a modern unit produces roughly 10 to 12 BTU per watt. So a 12,000 BTU unit is about 1,000–1,200 W, and a more efficient unit of the same BTU rating costs less to run for identical cooling.

For the same cooling delivered into the room, yes — often 30% more. A single-hose portable exhausts room air outside and pulls unconditioned air in through every gap to replace it, so it has to work harder for the same result. Its label may show a similar wattage; the cooling that reaches you is lower.

By a factor of about fifteen. A pedestal or ceiling fan is 20 to 100 W against 900 W for a window unit. It does not lower the air temperature, but moving air across skin feels about 4 °F cooler, which is often enough on a warm evening and lets the thermostat sit higher when it is not.

The rating plate on the side or back, usually printed as watts, or as volts and amps to be multiplied. Failing that, the manual or the model number online. A plug-in energy monitor is better than any of them, because it shows what the unit draws in your room rather than in a test lab.