Ok so I have a 5150 BTU air conditioner, I wanna know how much a month it will add to the electric bill?

My uncle is very cheap so i wanna l know how much it will add to the bill. The air conditioner will be used mostly over night but occasionally during the day.. I've been told that it will barely add anything to the bill, but I need more input. Maybe from somebody who really knows. Lol... Well appreciated, thanks.