It is a fact that conventional air conditioners are a much more economical option than inverters, but the latter are much more economical in their operation. After all, most of the brand names have eliminated conventional devices and are now throwing their entire production into the inverter.
The inverters operate according to the conditions prevailing in the room. When you open them, they work at a maximum so that space can get the conditions you set. When they reach these conditions, they continue to operate, but at a considerably reduced rate in order to maintain them. Conventional air conditioners, on the contrary, either operate at a fixed rate or cease operating, depending on the temperature prevailing in the space. In any case, inverters achieve lower consumption than conventional ones and despite their increased cost, damping is fast.
What energy class?
As Mr. says, anything above Beta energy class is good. Energy-class A air conditioners are much more expensive and you can choose them if you feel that the use you make in your new air conditioner will be continuous. The greater the use of the air conditioner, the more efficient and economical it is to choose a larger class.
Tip: It is very important, regardless of the energy class, that the inverter air conditioner does not work in the room when the outside temperature is too high, since it does not work to the desired level. Inverter air conditioners ideally deliver an average ambient temperature of 27-30 degrees Celsius.
How much it will cost me;
A conventional 9000BTU air conditioner will cost you, for example, about 200 € without the installation cost, and an inverter of equivalent power around 400 €.