Question

Calculate the cost of operating a heater of 500Watt for 20Hours at the rate of Rs.3.90 per unit.

Answer 1

Given:
Power = 500 W  i.e = .5 kW 
Time   = 20 h
rate     = Rs. 3.90 / unit

According to the formula:

Energy ( in kWh ) = Power ( in kW ) x Time ( in h)

Substituting: 

Energy ( in kWh ) = .5 kW  x  20

Therefore:
Energy = 10 kWh

Using unitary method:

      Energy       |      Rate / unit
______________________________

    1 kWh           |         Rs. 3.90 / unit
so
   10 kWh         |         Rs. x

x = 10 x 3.90

So the cost for operating a heater of 500 W for 20 h at the rate of 3.90 / unit is Rs. 39   | Ans.

Answer 2

Answer:

P=500W=5kw

T=20 hour

We know that

E=P*T

E=5*20

E=10 KWH

Cost=10*3.90

Cost=Rs 39