What is the Power Consumption Allowed?
Make sure the source of power that is provided for the fan. What is the voltage, and the current allowed. Most of the time the voltages will be 5V, 12V, 24V and 48V. The current depends on the size and the speed of the fan, basically. Multiplying the rated voltage and the rated current you can get the rated power. If you get fans of similar performance, your job to do is to find the fan who consumes power the least. But sometimes you need to make trade-off between power consumption and performance. If all the fans in hand can meet the power consumption requirement, then you need to choose the one with the performance you need.