In this post, I will solve problems on how to calculate the cost of electricity per kWh. And the formula for how to calculate the cost of electricity is:
One kilowatt-hour (1kWh) = 1 x kW x 1hr = 1 x 103 x 60 x 60 = 3.6 x 106 J
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Examples of How to Calculate the Cost of Electricity Per kWh
Here are a few examples to guide you on calculate the cost of electricity per kWh
Example 1
A household refrigerator consumes electrical energy at the rate of 200W. If electricity cost 5k per kWh. Calculate the cost of operating the appliance for 30 days.
Solution
Data:
Power, P = 200W
Time, t = 30 days = 30 x 24hrs
And to change power into kilowatt-hour, we simply apply the formula below
P = (200/1000)kW x 30 days x 24hrs = 0.2 x 30 x 24 = 144 kWh
Therefore, our power in kilowatt-hour is 144kWh
But the electricity cost 5k in every kilowatt-hour (kWh)
Hence, we have 5k x 144kWh = 720k
And we can equally write the cost of operating the appliance for 30 days is $7.20
Example 2
A lamp marked 100W, 250V is lit for 10 hours. If it operates normally and 1 kWh of electrical energy cost 2k. What is the cost of lighting the lamp?
Solution
Data:
Power, P = 100W
Voltage, V = 250V
Time, t = 10 hours
1kWh = 2k
Power in kWh x Time = (100/1000)kW x 10 = 0.1 x 10 = 1kWh
We can now say 2k x 1kWh = 2k
Thus, the cost of lighting the lamp is 2k
Example 3
A transformer supplies 15V from a 220V mains. If the transformer takes 0.7A from mains when used to light three lamps connected in parallel and each rated 15V, 40W. Calculate the cost of using it for for 24hrs at 30k per kWh.
Solution
Data:
Current, I = 0.7A
Voltage, v = 220V
Power = 154W
The power in kWh which is consumed in 24hrs is
Power x Time = (154/1000)kW x 24hrs = 3.696kWh
The cost of power consumed for 24hrs at 30k per kWh is 3.696kWh x (30/100) = N 1.1
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