An air conditioner uses more energy than an electric heater to heat the same amount of space due to the difference in the underlying principles of how they operate and the laws of thermodynamics.
Heat Transfer Efficiency: An electric heater directly converts electrical energy into heat, which is 100% efficient. All the energy consumed by the electric heater is converted into heat, and there are minimal losses in the process. On the other hand, an air conditioner operates on the principle of heat transfer and uses a refrigeration cycle to transfer heat from one location (indoors) to another (outdoors). This process requires additional energy to move heat against its natural direction, i.e., from a colder region (indoors) to a hotter region (outdoors).
Second Law of Thermodynamics: The second law of thermodynamics states that heat naturally flows from a region of higher temperature to a region of lower temperature. When heating with an electric heater, this law is followed directly as electrical energy is converted into heat, and it naturally stays in the space being heated. In contrast, an air conditioner has to actively "pump" heat from the colder indoor space to the hotter outdoor environment, and this process consumes additional energy.
Efficiency of Energy Conversion: An electric heater has a one-to-one ratio of electrical energy input to heat output (100% efficiency). In comparison, an air conditioner's efficiency is typically expressed as a Coefficient of Performance (COP), which is the ratio of heating or cooling output (heat transferred) to the electrical energy input. The COP for an air conditioner is usually less than 1 for heating, which means it requires more electrical energy input to produce a certain amount of heat output.
Losses in the System: Air conditioners have more components involved in the process, such as compressors, fans, and refrigerant cycles. Each of these components incurs some losses during operation, reducing the overall efficiency of the system and leading to higher energy consumption.
In summary, an electric heater directly converts electrical energy into heat with high efficiency, while an air conditioner has to move heat against its natural flow and involves more complex processes, leading to lower energy efficiency when used for heating. If you want to heat your space efficiently, especially during colder seasons, using an electric heater or other dedicated heating systems is usually more energy-efficient than relying on an air conditioner for heating purposes.