Air conditioners are not only installed in offices and retail stores but also installed at almost all private residences throughout Japan except in Hokkaido, where the temperature is not so high as in Honshu during summer.
Then, not only used during summer, AC is used in winter too, as the heater. However, most families purchase the new AC in May or June which is the peak season for installers. AC also has a dry function mode to dehumidify the air.
AC are installed in hotel rooms too. It is very hard to find hotels which are not with AC, therefore you do not have to worry when booking your accommodation in Japan. Then, there is no supplement for rooms with AC as like in Mediterranean resort hotels.
If AC are at home, you have to clean your AC regularly. The filters will be full of dusts and it can be greasy too. Cleaning spray is sold at drug stores or you can call the professionals to clean your AC.