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