The running cost of electricity depends only upon the number of units generated.
The factors contributing to the running cost of electricity include the annual cost of fuel, lubricating oil, maintenance, repairs, and salaries of operating staff.
Since the above-mentioned charges are contingent on the energy output, the running cost is directly proportional to the number of units generated by the station.
In simple words, if the number of units generated by the power station are more, it will have a higher running cost and vice-versa.
Also see: Electrical Power Generation