Question

How much energy do you save per hour if you replace a 60-watt lightbulb with a 55-watt lightbulb ?

Answer 1

That obviously depends on how much of the hour the light is turned on. Neither of them uses ANY energy when it's turned off.

The difference is (60 - 55) = 5 watts = 5 joules per second.

If the light is on for the entire hour, then the difference in energy is

(5 joule/second) x (3,600sec/hr) = 18,000 joules/hour.

Putting it in a more familiar unit . . .

(5 watts) x (1 hour) = 5 watt-hours = 0.005 Kilowatt-hour.

If your utility company charges you 25¢ per kilowatt-hour (a very high rate), then the more energy-efficient light bulb saves you (0.005) x (25¢) = 1/8 penny per hour of operation.