It’s true that a ceiling fan consumes 90% less power than air conditioning. That’s what the NGO WWF says, and they made the calculations. But it’s also obvious, as the fan does nothing but move the air around. The only relief it gives is when you sit in the right place on the sofa, right opposite the draught it generates. But its effect isn’t accumulative. To understand why, we have to talk about body temperature. Generally, our skin is at a temperature of 32ºC. If the room temperature is, say, 28ºC or 30ºC, the air blown by the fan moves away the mass of hot air next to our skin and replaces it with air at room temperature. But leaving the fan switched on when we’re not there doesn’t mean that the room will be cooler when we get back; we’ll only notice the difference when the electricity bill arrives at the end of the month. But take care. If you start to sweat and the fan is switched on, turn it off right away. The air that it’s moving is warmer that the air next to your skin, so you’ll feel even hotter.