For which unit do we pay our electricity bill?

How can you check just by seeing the meter whether electricity is in use or not?

One day the meter reading was 7643 units while on the following day, it was 7657 units. What is the consumption of electricity in a day?

A source of potential difference V volt sends current I ampere in a circuit for time t second. Write expressions for

(a) electrical energy supplied by the source, and

(b) electrical power spent by the source.