The portion of the Ontario transmission system that is controlled by the IESO is called the “IESO-controlled grid”. It includes all transmission lines equal to or greater than 50 kiloVolts. These are the high voltage transmission lines that provide wholesale electricity to large industrial consumers, and to distributors, who then provide electricity at the retail level.

The IESO is responsible for balancing the supply and demand of energy so that, on the IESO-controlled grid, supply always equals demand.

• Energy Supply - Energy is supplied to the market by generators located within Ontario and by imports from neighbouring jurisdictions.
  
  
• Energy Demand - Consumers of energy in Ontario are called loads. There is also demand from neighbouring jurisdictions for energy produced in Ontario.
  
  
• Measuring the Flow of Energy - Meters are used to measure the flow of energy at any point where energy flows into or out of the IESO-controlled grid. The meters must be capable of measuring energy at specific time intervals. The resulting meter readings are the basis of settling energy charges.