The California Energy Commission states that swamp coolers are 75 percent more efficient than air conditioners. This is because swamp coolers evaporate water into the air, a physical process that subtracts heat.
A swamp cooler only uses electricity to power its blower, which takes in dry air and emits cooler, more humid air. If there is a natural source of wind, it is possible to create an evaporative cooling mechanism that uses no electricity whatsoever. Any form of evaporative cooling, however, is only effective when the humidity is low. When the air is saturated with water, more water cannot evaporate into it.