The regulator has said it will continue to take tough action
EDF Energy Networks has been fined £2m for poor customer service, industry regulator Ofgem has said.
It said that the company had failed to meet the required timescales for connecting customers.
EDF had taken steps to improve its connections service but this should have been done earlier, Ofgem said.
Electricity companies have to provide an offer to property developers about the cost and details of connecting new premises to the network within 90 days.
The company, which completed more than 10,000 connection projects last year, said it "greatly regretted" the delays.
EDF Energy Networks has three electricity distribution networks and is the monopoly provider in certain areas of the country.
If new homes or business premises are built, the developer makes a request to the company for these properties to be connected to the electricity network.
As part of its licence, EDF Energy Networks is required by the regulator to make an offer on the cost and details of the connection to the developer within three months.
However, Ofgem's investigation found that since 2006, the company had failed to meet this deadline more than 100 cases between April 2006 and November 2008.
"All 108 customers concerned were contacted and we have made goodwill payments to them all," a statement from EDF Energy Networks said.
"Since these historic cases, we have made significant changes to ensure such issues do not arise again and that we deliver quotes for new connections within the regulatory timeframe of three months."
A spokesman for the regulator said that the delays could cause "inconvenience" and lead to extra costs for property developers.
The company would have faced an even higher fine if not for the £450,000 in compensation it had already paid out to affected customers.
"Customers should not have to accept poor service in any part of the energy market," said Sarah Harrison, Ofgem's managing director.
"All energy companies should be in no doubt that if they are failing to offer good service, Ofgem will take tough regulatory action," she added.
EDF distributes electricity to 7.8 million customers in the south and east of England, where it operates and maintains the electrical supply system.