Allison replaced 100-watt fixture with 15-watt fixture

Question

Allison replaced 100-watt fixture with 15-watt fixture. She estimated that fixture will burn 2,000 h per year and that annual savings will be $20.40. What is the cost of electricity per kWh?

Answer 1

Allison expects here light bulb to save 85 watts every hour(100w-15w).  So she expects to save 170 kilowatt hours (85w *2000hours) which she estimates is $20.40 

 

Her cost per kWh is $20.40 /170 Kwh = 12 cents per KWh