Dryers that use 110 volts tend to be much smaller than 220-volt dryers, making direct efficiency comparisons difficult. However, 220-volt dryers generally create a warmer heating environment, which leads to better efficiency.
Most home electrical systems are not capable of supplying sufficient wattage through a 120-volt outlet due to amperage limitations. Since the efficiency of a dryer scales with its operating temperature, manufacturers generally make 110-volt dryers smaller than 220-volt units. However, smaller dryers require more drying cycles to dry the same amount of clothing, and dryers cool off when the warm clothes they hold are replaced. As a result, smaller dryers end up wasting a bit of energy that is retained in larger dryers.