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Question

Allison replaced 100 watt fixture with 15 watt fixture.  She estimated that fixture will burn 2000 h per year and that the annual savings will be $20.400 what is the cost of her electricity per kwh? 

Answer 1

This is a multi-step problem.  First calculate the old energy usage by multiplying the old power by the time used  

 

OLD = (100 watts) * (2000 h)  

 

Then calculate the new energy usage with the new power 

 

NEW = (15 watts) * (2000 h)  

 

The energy saved is the difference between these 2 values, OLD - NEW.  But note that the units are in watt-hours, so this is probably a good time to convert the number to kWh by dividing it by 1000.  

 

Money Savings = (Energy Savings) * (Cost per kwh)  

 

so the (cost per kwh) = (Money Savings) / (Energy Savings)  

 

Plug in $20.40 for the Money Savings and the energy saved (in kWh) to find the cost per kwh.  As an order of magnitude check, you should get a number that is measured in cents (not dollars or fractions of a cent).