An electric bulb draws a current of 0.8 A from 220 V main line. The bulb is used 8 hours daily, calculate the total bill for 30 days, if per unit costs Rs. 7.0.

Solution

Given

Current drawn by electric bulb, I_bulb  =  0.8 A

Voltage supplied to the electric bulb, V_bulb  =  220 V

Time duration of use of bulb per day, T_day  =  8 hours

Time duration of use of bulb in a month, T_month  =  8*30 hours

                    = 240 hours

Power rating of electric bulb, P_bulb  =  I_bulb * V_bulb

            = 0.8 A * 220 V = 176 watts

Energy consumed by bulb in a month, E_bulb  =  P_bulb*T_month

                    =  176 W * 240 hours = 42,240 watt-hours

Energy consumed by bulb in a month in terms of KWh, E_bulb,KWh  =  E_bulb/1000

                    =  42,240 watt-hours/1000  =  42.24 KWh = 42.24 units.    (∵ 1 unit = 1 KWh)


Electricity rate = Rs. 7 per unit.

Total electricity bill = E_bulb,KWh * Electricity rate

                  = 42.24 units * Rs. 7 per unit

                  = Rs. 295.68