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Demand Overview

Ontario's supply and demand of electricity must be kept in balance every minute of the day. The IESO forecasts hourly demand to ensure that sufficient generation will be available to meet that peak. The level of demand is primarily influenced by factors like weather, hours of daylight, business hours, school holidays, and daily consumption patterns. At certain times of the year demand in the middle of the night can go lower than 11,000 megawatts (MW) and then rise by more than 10,000 MW later the same day.

Here are some historical demand numbers that illustrate the variety of demand conditions experienced in Ontario.

Forecasting Demand - Peak and Energy

The IESO forecasts demand for electricity in the province for the next 18-months and assesses whether the existing and proposed generation and transmission facilities are adequate to meet those needs. Normal Weather forecasts represent a typical peak for that time of year, while Extreme Weather peaks reflect severe weather conditions, such as humidex of 50°C.


​Normal Weather Peak (MW)

​Extreme Weather Peak (MW)

Winter 2016-1721,91423,170
Summer 2017​22,512​24,851
Winter 2017-18​​21,811​​23,017

Total Ontario Energy Demand, Year

​​Normal Weather Energy (TWh)

​%Growth in Energy

​2016 (forecast)​136.70.4%
​2017 (forecast)137.10.3%​


Numbers shown here are as of December 2016. Read the 18-Month Outlook to find out more about the demand forecast. 

Total Annual Ontario Energy Demand


Total (TWh)

Increase Over
Previous Year


A Megawatt (MW) is a unit of electrical power equal to one million watts. (A typical light bulb uses 100 W of power).        A Megawatt-hour (MWh) is the measure of one million watts produced or consumed in one hour. For example, one MWh is enough electricity to power 10,000 light bulbs for an hour. A Gigawatt-hour (GWh) is equal to 1,000 MWh. A Terawatt-hour (TWh) is equal to 1,000 GWh.

Electricity Demand Records

The all-time record for Ontario demand was set on Tuesday, August 01, 2006, when peak demand for electricity reached
27,005 megawatts.

Here are the top twenty record demand days for the Province of Ontario:



Ontario Demand (MW)

1Tuesday, August 01, 200627,005
2Wednesday, July 13, 200526,160
3Monday, June 27, 200526,157
4Monday, July 31, 200626,092
5Monday, July 17, 200625,898
6Tuesday, June 28, 200525,861
7Monday, July 18, 200525,857
8Wednesday, August 02, 200625,816
9Tuesday, August 09, 200525,816
10Tuesday, July 12, 200525,808
11Tuesday, June 26, 200725,737
12Thursday, August 02, 200725,584
13Monday, July 11, 200525,506
14Wednesday, June 27, 200725,467
15Thursday, July 21, 201125,450
16Tuesday, August 13, 200225,414
17Wednesday, August 01, 200725,402
18Thursday, July 21, 200525,383
19Thursday, July 14, 200525,362
20Monday, August 12, 200225,349

More Demand Records

The record minimum electricity demand set on April 4, 2010 was 10,618 MW.

The winter peak for electricity demand set on December 20, 2004 was 24,979 MW.

The summer peak for electricity demand set on August 1, 2006 was       27, 005 MW.