It is a myth that fans reduce the temperature of rooms, so leaving them on while you are away will have no affect on the room’s temperature. 

Myth-busting fans facts

1. “Fans are cheap to run” - WRONG! A large ceiling fan uses 60-80 Watts. If never switched off this adds up to $136-181 a year for one ceiling fan. Six ceiling fans that are always left on will use between $816- $1086 a year! Fans are cheaper to run than air conditioners but make sure you turn them off when you leave the room. Smaller fans and lower speeds use less energy.
2. “Fans keep the house cool" - WRONG! Fans cool people as the moving air evaporates moisture off bare skin. Fans do not have a significant impact on inside temperatures.
3. “You have to leave the fans running to stop mould growth” - Mostly WRONG! Many people have told COOLmob that when they first moved to Darwin people told them to keep their fans running to reduce mould growth during the wet season. However many Darwinians keep their fans turned off unless they are using rooms. They find that, as long as they keep the windows open while they are out, and their house is dry and clean (mould needs dust or dirt to grow), they do not have problems with mould growth.

More information on environmentally-friendly ways of keeping mould at bay are in COOLmob’s Greenhouse-Friendly Habits for Top End Households booklet.