Alectra Utilities says more than 4,500 customers are still without power in Richmond Hill.
The outage, caused by equipment failure, is affecting homes and businesses.
The latest outage map update says over 12,000 customers were initially affected, with 4,606 still without power.
The outage comes at an inopportune time with temperatures soaring into the mid 30s on Thursday.
Richmond Hill is currently under a heat warning that extends into Friday.
Alectra says the estimated time for having power restored is between 8 and 10 p.m. tonight.
