Efficiency of an Evaporative Cooler is not calculated like a typical refrigeration cycle.
EER is the standard for performance in evaporative cooling. Others may refer to EUC or I/D effectiveness as indicators. You are theoretically looking for cooling output per energy input. Cooling output is variable based on Tdb and Twb. Energy input is fan motor electicity use.
See 1999 ASHRAE Applications Handbook Chapter 50.4
