We had these things in San Antonio. The fan had to be very strong to push the air through the mesh.
"However, unlike air conditioners which rely on BTUs as a means of measuring cooling efficiency, the output of evaporative coolers is rated by CFMs. This measurement essentially describes the cubic feet per minute of air that the swamp cooler can blow into your home, and whether you're cooling a single room or entire home, a basic formula exists for determining the proper size swamp cooler you need."
In principle you can use the psychrometric chart to convert to BTU/hr of cooling to CFM of moist air but that's beyond my pay grade.
I guess companies that are in zone B & C on the map in the link know best how to do this since they probably sell both types.
ASHRAE knows this but they're very jealous about lending their costly books to libraries.