Sun Twin heaters can be more energy-efficient than other heaters when used to heat a small area or single person, but are not as efficient as convection heaters for large spaces or many people. They are also not recognized as energy-efficient appliances by the Department of Energy's Energy Star program.
Sun Twin heaters use infrared quartz heating, meaning they emit infrared radiation from a quartz panel that is then directly absorbed by people and objects. In contrast, a convection heater such as a conventional space heater or furnace heats air and then spreads it around a room either through natural convection or a fan. Since Sun Twin heaters heat objects rather than air, they can be more energy-efficient for heating a small space, because the people and objects in the space are heated quickly by the infrared radiation, so the heater can be run for less time.
However, this same quality makes them poor choices for heating a large area or many people, since only objects directly in line-of-sight to the heater are heated by its radiation. A convection heater can spread heated air around to the rest of a large area much more quickly than the Sun Twin heater can do so, meaning the Sun Twin heater must run longer and use more energy to accomplish the same task.