I installed portable AC units in the room as a very fast solution and also because mine is a centuries house of character and I do not want any unsightly pipes and cables ruining the facade. The first month I had them, I let guests use them as they pleased. The amount of sheer waste of electricity verging on vandalism was incredible in that first month.There were people who set the AC on 16 degrees then went out. From July I changed the system and installed radio controlled on and off switches with every AC unit, I just have one controller which can control several outlets. I now tell guests that even though the temperature during the day is in the 30s (Celcius), at night it falls into the low 20s and if they open a window and a balcony they will have a refreshing breeze helped with an electric fan.But if they still want AC they have to pay 5 Euros. I chose a really low sum - which I calculated is what it would cost me if they left it on for 24 hours. It is amazing how for 5 Euros guests have suddenly found that the temperature is pleasant after all. It is pertinent to say that Euro 5 is about 7% of the total price of a room per night. I am sill getting positive review and only one guest who left a positive review even mentioned that they had to pay 5 Euros ( They made the mistake of gauging the temperature of the room just after they had come in from a long walk in the noonday sun). Moreover, money aside I feel that I am doing my bit for the environment as after all I do not make any money if they choose to pay me for the use of ACs.