# Electrical Power - Cost of kW*hr

## Homework Statement

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)

## Homework Equations

P = I*$$\Delta$$V

## 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:

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

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

1017.864*0.099 = 100.768

berkeman
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).

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