An electric heater of rating 1000 W is used for 5 hrs per day for 20 days. The electrical energy utilized is : 

100 kWh

When some of the electrical appliances are used then the energy utilised is given by the product of the power of the appliances and the time period.

We have given the power of the electric heater : 1000 W = 1 kW

and the time period is 20 × 5 = 100 hr

Electrical energy = Power × Time

Electrical energy = 1 × 100

Electrical energy = 100 kWH