I just installed my first Aircon 3 weeks ago. A 9000btu inverter in my bedroom. For health reasons it's best I maintain temperature of 21c. My bedroom is 18sqm so 9000 btu is perfect. My bill is in a Thai name and she is currently unable to be contacted..However to work out my monthly cost what could be easier simply deduct last months bill for my new reading and it should give me a good idea of how much extra I can expect to pay per month....only one problem for the first time in 2years my electric bill has not turned up and it was due in my post box a week ago. Now I have no way of contacting them to complain because I don't have any old bills and I don't know my house address... luckily I can pay it next month when my next bill arrives...if it does arrive...only in Thailand๐ธ