Here is the electric bill for our shcool district for one month.  Pretty costly eh?  If we weren't a public school, the bill would actaully be higher (we get a break from Uncle Sam).  Anyway, our cost on this bill is $6297.67.  Our total KwH used is 91200.  This means that for each KwH, we pay $6297.67 / 91200 = $.06905, or 6.9¢.

So if you left a 1500w hair dryer running for 4 hours, you would pay...
Kw used= 1.5
time= 4hours
KwH= 1.5*4 = 6
6KwH*6.9¢ = 41.4¢.

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HERE is a cool Canadian site where you can turn on appliances.  You tell it how long you use these, and it shows you your power bill.