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 10^{3} x 60 x 60 = 3.6 x 10^{6} 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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