In every tariff-2 major part is there.

1)Fixed part-which is a nominal amount.Whether  you use or not use the power you have to pay it upto a certain limit of usagage.

2)Varriable Part-Beyond the limit you cross,it depends on the usage of power.

Suppose tarrif T=a+b(x)

a=fixed part upto the usage of n watt per month(Whether you use or not use you have to pay it.

b(x)=Varriable part,beyond n watt usage per month  you have to pay it.

Suppose in a month m watt is used

for mn

T=a+b(m-n)