Operating expenses are the day-to-day costs of running a business such as power generation (as distinct from capex, which are expenses used to build the power plant and other infrastructure). Opex is deducted from generated revenues to calculate the operating profit.

There can be a wide range of operating costs, some fixed (not varying according to output) such as wages, leases, administration, operations & maintenance and so on; and some variable (in particular fuel costs, in thermal power generation).