Expert: David Hemmer - 6/15/2008

Question
Hello:

Here is my question:

Find the kilowatt-hours of electricity used by a light bulb with a rating of 75 watts if the light is used for 130 hours.

I want to know how the different units cancel from the calculation. Here is my solution, but it may not be totally accurate:

(130 hours X 75 watts/hour) = 9750 watt hours
9750 watt hours divided by 1000 watts/kilowatt = 9.75 kilowatt hours.

I know that the answer is correct, but I am not sure if the solution is correct for how the units cancel for kilowatt hours remains as the only unit in the answer.

I thank you for your explanation.

Answer
just write everything as fractions qand cancel the units if it helps you.

130 hours *  75watts          = 9750 watt hours
          
            



9750 watt hours * 1 kilowatt         =9.75  kw*hour
              ------------
              1000 watts


it is not 75 watts/hour just 75 watts. I think you need to understand what a watt is to understand why watts/hours does not make sense.