Glossary by "O"

  • 398

    Obligation to serve

    A regulatory obligation of a utility to provide electric planning services for all customers and to assure adequate supply of electricity now and in the future.

  • 399


    Those hours or other periods defined by NAESB business practices, contract, agreements, or guides as periods of lower electrical demand.

  • 400


    Times when demand for electricity is highest (a.k.a peak demand). Typically on-peak times occur during weekdays during the hottest summer months, when normal demand is high and when air conditioning is operating. Similarly, in some areas on-peak times may be in the winter when high normal demand is combined with high heating-related power use.

  • 401

    Ontario Independent Electricity System Operator

    Acronym(s): OIESO

    A not-for-profit independent (Canadian) corporation that monitors the system and identifies what is required to maintain reliability in the future, reporting on these recommendations through regular publications. The IESO 1. assesses generation and transmission facilities’ adequacy, 2. prepares the Ontario Reliability Outlook, which reports on the progress of interrelated generation, transmission and demand-side projects underway to meet Ontario’s reliability requirements and 3. coordinates emergency preparedness for the province’s electricity system

    Source: ESA Technical Working Group
  • 402

    Open access

    Markets in which energy end-users can pay to have energy that purchased from a third party transmitted via transmission systems owned by other entities. Open access prevents a transmission owning utility from controlling the energy supply. Open access differs from direct access in that direct access refers to acquisition of actual energy, whereas open access refers only to access to transmission systems used to transport it.  

    Source: ESA Technical Working Group
  • 403

    Open access same time information service

    Acronym(s): OASIS

    An electronic posting system that the transmission service provider maintains for transmission access data and that allows all transmission customers to view the data simultaneously.

  • 404

    Open access transmission tariff

    Acronym(s): OATT

    Electronic transmission tariff accepted by the U.S. Federal Energy Regulatory Commission requiring the Transmission Service Provider to furnish to all shippers with non-discriminating service comparable to that provided by Transmission Owners to themselves.

  • 405

    Open circuit

    A battery to which nothing is connected.

    Source: ESA Technical Working Group
  • 406

    Open-circuit voltage

    The difference in electric potential (voltage) between the terminals of a cell or battery when the circuit is open (i.e., nothing is connected).  

    Source: ESA Technical Working Group
  • 407

    Operating expense

    Acronym(s): OPEX

    A category of expenditure that a business incurs as a result of performing its normal business operations. One of the typical responsibilities that management must contend with is determining how low operating expenses can be reduced without significantly affecting the firm’s ability to compete with its competitors. Also known as “OPEX”.

  • 408

    Operating procedure

    A document that identifies specific steps or tasks that should be taken by one or more specific operating positions to achieve specific operating goal(s).  The steps in an operating procedure should be followed in the order in which they are presented, and should be performed by the position(s) identified.  A document that lists the specific steps for a system operator to take in removing a specific transmission line from service is an example of an Operating Procedure.  

  • 409

    Operating voltage

    The voltage level by which an electrical system is designated and to which certain operating characteristics of the system are related; also, the effective (root-mean-square) potential difference between any two conductors or between a conductor and the ground.  The actual voltage of the circuit may vary somewhat above or below this value.  

  • 410

    Operations and maintenance

    Acronym(s): O&M

    Expenses related to operation and maintenance of equipment or facility.

    Source: ESA Technical Working Group
  • 411

    Opportunity cost

    The cost of an alternative that must be forgone in order to pursue a certain action. Put another way, the benefits you could have received by taking an alternative action.

  • 412


    Any interruption of current flow (and thus electric service) in a T&D system. Note that most parts of the electric grid are designed to allow current flow to be routed around affected parts of the grid to reduce or avoid service interruption. See also forced, planned, sustained, scheduled and unplanned outage.

    Source: ESA Technical Working Group
  • 413


    A condition involving generation output that exceeds the system demand. Aside from adverse effects on the grid (stability) overgeneration is undesirable because excess energy has no value if it is not put to productive use. In fact, in some cases “negative prices” are used to encourage use of excess energy.  

    Source: ESA Technical Working Group
  • 414

    Overlap regulation service

    A method of providing regulation service in which the Balancing Authority providing the regulation service incorporates another Balancing Authority  

  • 415


    Chemical reactions resulting in release of electrons by an electrode’s active material.  The opposite of reduction.  

    Source: ESA Technical Working Group