Q.In a house, four bulbs each of 60 watt glow for 5 hrs and an electric heater of 1200 watt is used 2 hrs every day. If the minimum charge up to 20 units is Rs. 80 and the charge for 21 units to 250 units is Rs. 7.30/unit, calculate the cost of electricity consumed in a month.

Soln:

For 4 bulbs:

Power (p) = 4 * 60 = 240 watt = 0.24 kw

Time (t) = 5 hrs

Power consumption (PC1) = p * t = 0.24 * 5 = 1.2 kwhrs

For electric heater:

Power = 1200 watts = 1.2 kw

Time = 2 hrs

Power consumption (PC2) = 1.2 * 2 = 2.4 kwhrs

Total power consumption in a day  = PC1 + PC2 = 1.2 + 2.4 = 3.6 kwhrs

Total power consumption in a month (30 days) = 3.6 * 30 = 108 kwhrs (unit)

The charge up to 20 units = Rs. 80

No. of extra units = 108 - 20 = 88

The cost of 88 units = 88 * 7.30 = Rs. 642.4

Total cost of electricity consumed in month = Rs. 642.4 + Rs. 80 = Rs. 722.4 Ans.