# Electrical Power - Cost of kW*hr

1. Mar 3, 2009

### calvert11

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?

(Note: sigfigs do not matter)

2. Relevant equations

P = I*$$\Delta$$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. Mar 3, 2009

### mgb_phys

The 9.9 is c/kwh, other than that you are correct, except for the slight typo on the last answer

3. Mar 3, 2009

### Briggs

Looks like you might have made an arithmetic error when calculating in cents.

1017.864*0.099 = 100.768

4. Mar 3, 2009

### 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).

5. Mar 3, 2009

### calvert11

Thank you so much. Removing cents gives the right answer.