Question

if 5 bulb of 100 watt light up for 40 hours the energy consumption will be?

Answer 1

This is deceptively simple to answer, since electrical Wattage is measured in time units of one hour. 
5x100 Watts = 500 Watts per hour
500 Watts x 40 hours = 20,000 Watts (20 Kilowatts)

Answer 2

power of five bulbs=500watt
energy consumed=power*time
= 500*40=20000wh
=20000/1000kwh=20 kwh
hope it help u