Ultrasonic and evaporative humidifiers are the most energy-efficient humidifers on the market. Ultrasonic humidifiers contain oscillating plates that create mist by sending constant vibrations through the stored water until the water evaporates. Evaporative humidifiers simply use a fan to blow on the stored water, which sends moisture into the air.
Evaporative humidifiers are considered the safer option for people with breathing problems, as lower-end and older versions of ultrasonic humidifiers produce a by-product of white dust that can be consequential when inhaled. However, these issues have been eliminated in newer models. In terms of the type of mist produced, cool mist humidifiers conserve more energy than warm mist humidifiers, as they don't have to use electricity to heat the water up.