High School Physics

# Numericals on electrical energy (kWh consumption)

We will now solve a set of numerical problems based on electrical energy consumption using the kWh calculation formula. Basically, these numericals are related to the calculation of the consumption of electrical energy (part of the procedure of electric meter readings to produce electric bills).

Formulas used

Energy consumed= Power x Time taken

1 kWh electrical energy = 1 kW power x 1 hour duration of running

1 kWh of energy = 3.6 × 106 J

Derivation of kWh and joule relationship – see this for details.

Numerical problems on electrical energy consumption or kWh calculations [solved]

Numerical Problem 1] A radio set of 60 watts runs for 50 hours. How many ‘units’ (kWh) of electrical energy are consumed ?

Solution.

We want to calculate the electrical energy in kilowatt-hours.
So first we should convert the power of 60 watts into kilowatts by dividing it by 1000. That is :
Power, P = 60 watts= 60/1000 kilowatts = 0.06 kW
And, Time, t = 50 h

Now, putting P = 0.06 kW and t = 50 h in the formula given below:
Energy consumed= Power x Time taken
So, Energy consumed =0.06 kW × 50 h = 3 kWh
Thus, the electrical energy consumed is 3 kWh or 3 units

Numerical Problem 2] A family uses 250 units of electrical energy during a month. Calculate this electrical energy in joules.

Solution.

250 units of electrical energy means 250 kWh of electrical energy.
Now, we know that: 1 kWh of energy = 3.6 × 106 J
So, 250 kWh of energy = 3.6 × 106 × 250 J = 9 × 108 J
Thus, 250 units of electrical energy = 9 × 108 joules.

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