The cost to run a window AC unit all day will vary depending on the size of the unit, your standard energy tariff, as well as how high you put the settings. However, because modern units are designed to be more energy-efficient, it will probably not cost you more than $2 per day. This doesn’t sound like much, but if you leave it on all day, every day for 30 days, that’s a $60 add-on to your regular utility bills.\r