Question

An electric refrigerator rated at 400 W operates 8 hours/day. What is the cost of the energy to operate it for 30 days at Rs. 3.00 per kWh?

Answer 1

Hint: In this question, you need to first find out the energy consumption per day and then calculate what the energy consumption would be for 30 days. Then all you need to do is multiply it with the cost of consumption for your answer.

Formulas used:
Energy consumption is power rating multiplied with the time of operation.
 $ E = P \times T $
Cost is energy consumption multiplied by cost per unit of energy consumed.
 $ \operatorname{Cos} t = E \times \operatorname{Cos} {t_{unit}} $

Complete step by step answer:
The refrigerator has a power rating, as has every other electronic device that we use and as such, when they run, they consume electric energy. The energy consumption is measured in Kilowatt hour. For this question, first, we need to find out how much energy will the device with a power rating of 400W consume for one day if it runs for 8 hours.
Energy consumption is power rating multiplied by the time of operation.
 $ E = P \times T $
 $ {E_{for 1 day}} = 400 \times 8 \\
= 3200Wh \\
= 3.2KWh \\
 $
To convert any quantity into its kilo form, just divide it by thousand.
Moving forward, we need to calculate the energy consumption for 30 days. So just multiply the energy consumption for 1 day by 30 and that will be the total energy consumption.
 $ {E_{net}} = {E_{for\,1\,day}} \times 30 \\
= 3.2 \times 30 \\
= 96KWh \\
 $
Now the cost is energy consumption multiplied by the cost per unit of energy consumed.
 $ {Cos} t = E \times \operatorname{Cos} {t_{unit}} \\
= 96 \times 3 \\
= Rs.\,288 \\
\ $

Note: The electricity bill that we pay also works on this same concept as the number of units on the meter means that many kWh of energy is used. This multiplied with the local electric cost is the electricity bill that we pay.