Question

An electric lamp has a resistance of 400ohm. It is connected to a supply of 250v. If the price of electric energy be Rs. 1.20 per unit, Calculate the cost of lighting the lamp for 20 hours.

Answer 1

1 commercial unit = 1 kilo watt hour  (kWh).

kilo watt hour is unit of energy (not power).

E(kWh) = P(W) × t(hr) / 1000

​(1)   as per the above formula 
                 E =  2500*2/1000      = 5 kWh
               therefore,  cost for running the kettle for 2 hours at Rs. 3 per unit = 3*5      =  Rs.15

​(2) as per the above formula 
               E =  2000*0.5/1000   =   1 kWh
              therefore, the cost = 3*1     = Rs, 3

​(3) ​ as per the above formula 
                 E = 3000*0.167/1000     = 0.501 kWh
               therefore,  cost for running the kettle for 2 hours at Rs. 3 per unit = 3*0.501      =  Rs.1.503

​(4)  Power = V2/ R
                     =  250*250/400
                    = 156.25  W
as per the above formula:
         E = 156.25*20/1000
            =3.125 kWh

therefore, cost for lighting the bulb for 20 hours = 1.2*3.125  = Rs. 3.75