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Electrical Power - Cost of kW*hr

  1. Mar 3, 2009 #1
    1. The problem statement, all variables and given/known data
    An electric air conditioning unit draws 15 amps of direct current from a 134 V direct voltage source, and is used 24 hours a day during 21.1 days in July. How much will the electricity cost for the month if the local electrical rate is
    9.9 /kW· hr?

    Answer in units of dollars.
    (Note: sigfigs do not matter)

    2. Relevant equations

    P = I*[tex]\Delta[/tex]V

    3. The attempt at a solution
    This seems like a straight forward question:

    P = 15*134 = 2010 W = 2.01 kW
    t = 21.1 days * 24 hours = 506.4 hours

    P*t = 1017.864 kW*hr

    Since units aren't given for 9.9, I tried for both dollars and cents:
    1017.864* 9.9 = $10076.85
    1017.864*.099 = $100.785

    Both answers are wrong. I don't know where I'm making the mistake.
     
    Last edited: Mar 3, 2009
  2. jcsd
  3. Mar 3, 2009 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    The 9.9 is c/kwh, other than that you are correct, except for the slight typo on the last answer
     
  4. Mar 3, 2009 #3
    Looks like you might have made an arithmetic error when calculating in cents.

    1017.864*0.099 = 100.768
     
  5. Mar 3, 2009 #4

    berkeman

    User Avatar

    Staff: Mentor

    Your last answer is correct. They want the answer of dolllars, with no cents. That's why they had such strange numbers (like 21.1 days).
     
  6. Mar 3, 2009 #5
    Thank you so much. Removing cents gives the right answer.
     
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