Answer by JohnJacobs (75). It will depend on the age and size of the ceiling fan you have running. However most home ceiling fans, even when left on 24 hours a day, will hardly reach 5 dollars a month to run.If you find that you can remain cool using a ceiling fan instead of the AC running, you'll end up saving money quickly.t will depend on the age and size of the ceiling fan you have running. However most home ceiling fans, even when left on 24 hours a day, will hardly reach 5 dollars a month to run. If you find that you can remain cool using a ceiling fan instead of the AC running, you'll end up saving money quickly.