An 18,000 BTU air conditioner is generally sufficient for a room that is 150 square feet in size. A rule of thumb for determining the appropriate cooling capacity for an air conditioner is to have around 20 BTUs per square foot of living space. For a 150 sq ft room, that would be:
20 BTU/sq ft x 150 sq ft = 3,000 BTU
Since the 18,000 BTU air conditioner provides much more cooling capacity (18,000 BTU) than the required 3,000 BTU for the room, it should be more than enough to cool the space efficiently.
A 24,000 BTU (2-ton) air conditioner might be excessive for a room of this size, but it would still be able to cool the room effectively. However, it might lead to more frequent cycling on and off, potentially causing some discomfort or inefficient energy usage.
In summary, an 18,000 BTU (1.5-ton) air conditioner should be sufficient and efficient for a 150 sq ft room, while a 24,000 BTU (2-ton) air conditioner would also work but might be considered overkill for this room size.