10 tube lights of 20 W each are used in a house for 5 hours daily, calculate the bill of one month for the electricity with the rate of Rs. 7.30 per unit.

Solution

Given

Numbers of tube lights, N_tube  =  10

Power rating of each tube light, P_tube  =  20 W

Time duration of use of each tube light per day, T_day  =  5 hours

Time duration of use of each tube light in a month, T_month  =  5*30 hours

                    = 150 hours

Energy consumed by each tube light in a month, E_tube  =  P_tube*T_month

                    =  20 W * 150 hours = 3000 watt-hours

Energy consumed by each tube light in a month in terms of KWh, E_tube,KWh  =  E_tube/1000

                    =  3000 watt-hours/1000  =  3 KWh = 3 units.    (∵ 1 unit = 1 KWh)


Total KWh consumed by 10 tube lights in 1 month, E_Total  =  N*E_tube,KWh  =  10*3 units = 30 units.


Electricity rate = Rs. 7.3 per unit.

Total electricity bill = E_Total,KWh * Electricity rate

                  = 30 units * Rs. 7.3 per unit

                  = Rs. 219