A 120 watt light bulb uses about 0.1 kilowatt of electricity per hour. if electricity costs $0.20 per kilowatt hour how much does it cost to have a bulb on for an hour?

A 120 watt light-bulb is a 0.120 kilowatt light bulb.

In one hour, it uses 0.120 kilowatt-hour of energy.

If electrical energy costs $0.20 per kilowatt hour, then that light bulb costs

(0.120 kilowatt-hour / hour) x (0.20 dollar / kilowatt-hour) =

(0.120 x $0.20 / hour) =

$0.024 / hour = 2.4¢ per hour