A device for saving power consumption of heating and air conditioning system is provided. The energy saving device is adapted to automatically stop an air conditioner when outdoor temperature is lower than a set temperature or automatically stop a heater when outdoor temperature is higher than a set temperature. The energy saving device is provided with an outdoor temperature sensor and converter for measuring an outdoor temperature and converting the measured temperature into a first voltage value, a conversion circuit for converting a set temperature of stopping the air conditioner/heater into a second voltage value, a control circuit for controlling the air conditioner/heater, and a comparison circuit for comparing the first voltage value with the second voltage value and transmitting the result to the control circuit.