Question

Calculate the cost of using two LED bulbs each of power 20W for 10 hrs a day @ Rs.5 per unit.​

Answer 1

Given:-

• Power of the LED bulbs = 20 W

• Time for which they are operated per

day = 10 hours

• Cost of per unit = Rs.5

To find:-

• Cost of using the two LED bulbs

Solution:-

We know that :-

Energy = Power × Time

Energy consumed by the two bulbs

in a day :-

= (20 × 10)×2

= 200×2

= 400 Wh

Now, let's convert the energy from Wh to kWh :-

=> 1 Wh = 0.001 kWh

=> 400 Wh = 400(0.001)

=> 0.4 kWh

Hence, cost of using the bulbs :-

= 0.4 × 5

= Rs.2

Thus, the required cost is Rs.2 .

Some Extra Information:-

• The unit kWh is the commercial unit of

energy used by electricity boards.

• If power is in kW (kilowatt) and time is in

hour, then unit of energy is kWh.

• 1 kWh = 1 kW × 1h

= 1000 W × 3600 s

= 3.6×10⁶ J

Answer 2

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★ given :-

• power of each bulb, P = 20 W
• time taken, t = 10hrs per day
• cost of 1 unit of electricity = Rs.5

___________________

★ to find :-

• cost of using two LED bulbs ?

___________________

★ solution :-

☆ Calculation of Electrical Energy Consumed :-

・The electrical energy consumed in kilowatt-hours can be calculated by using the formula:

Energy = Power × time

here,

• power, P = 20W

➪ p = 20/1000 = 0.02 kilowatt-hours

and

• time, t = 10 hours

so, energy consumed by 1 bulb = 0.02 × 10

➪ E = 0.2 kilowatt-hours

And, Energy consumed by 2 bulbs = 2 × 0.2

➪ E' = 0.4 kilowatt-hours

Thus, the total electrical energy consumed is "0.4 kilowatt-hours" or "0.4 units"

_____________________

✩ Calculation of Cost of Electrical Energy :

we have been given that:

・cost of 1 unit of electricity = Rs.5

so, cost of 0.4 units of electricity = Rs.5 × 0.4

➪ Cost = Rs.2

therefore, the cost of using two LED bulbs is Rs.2