Canada’s most populous province won’t have enough electricity to feed demand in five years and is considering increased imports from Quebec and the U.S., more adept management of the province’s nuclear facilities, and new power plants to make up for the shortfall.

Electricity demand in Ontario will outstrip generation by 1,400 megawatts during peak summer hours in 2023, and the Independent Electricity Systems Operator (IESO), the provincial agency that manages Ontario’s power grid, is floating several options to make...