How much does it cost to leave a light on for an hour?

A “normal” incandescent bulb costs about 0.75cents per hour, and LEDs or CFLs cost only one-sixth of that – so leaving the lights on (either overnight or while you're at work for the day, say both are about 8 hours) costs you roughly 6 cents for a normal light and a bit over 1 cent for modern bulbs.