When selecting a Portable Air Conditioner, there are other factors to take into consideration other than the BTU and the suggested square footage. If the space you're looking to cool has one or more of the following factors, it is strongly recommended to go one or two size up:
1) Heat source such as computer servers, major appliance, skylight, direct sunlight, etc.
2) Higher floors.
3) Lots of windows, especially those facing the West.
4) Poor insulation, such as garage, sunroom, mobile home, etc.