Which air conditioner model has the smallest BTU rating?

Few, if any, air conditioners made as of 2015 have British thermal unit ratings less than 1,000 Btus. One example of such a small air conditioner is the APC2000E produced by Amcor, which is rated at exactly 1,000 Btus of cooling capacity.

Air conditioners with low cooling capacity like the APC2000E are typically portable units intended to cool a very small space or single person only. They are ineffective for cooling large spaces or for cooling over an extended period of time. Modern window-mounted air conditioners are significantly more powerful than these personal air conditioners. Even the least expensive window-mounted units typically have cooling capacities of at least 5,000 Btus.