In a household electric circuit the following devices are operated -

a) 3 bulbs rated 40W for 5 hrs

b) A heater rated 1500W for 3hrs

c) A fridge rated 400W

What is the cost of energy to operate them for 30 days at Rs. 3 per unit?

a) 3 bulbs rated 40W for 5 hrs : 3*40*5 = 600 Watt hours

b) A heater rated 1500W for 3hrs 3 * 1500 = 4500 Watt hours

c) A fridge rated 400W

Would assume the Fridge would run for 24 hours a day : 400*24 = 9600 Watt hours

Total Watt hours for 1 day:

600 + 4500 + 9600 = 14700 Watt hours

Unit of Electricity is in kWh

14 700 wh = 14700/1000 = 14.7kWh

But 1kWh = Rs. 3

14.7 kWh = 3*14.7 = 44.1

Total cost = Rs. 44.10