The Nemo Link interconnector between the UK and Belgium has now entered commercial operation, National Grid has announced.
The 1GW subsea cable stretches for 80 miles from Richborough in Kent to Herdersbrug on the Belgian coast.
Costing £560 million to build, Nemo Link is Britain's first new electricity interconnector in more than half a decade.
To connect the cable to the transmission network, a 20km overhead power line was installed between Richborough and Canterbury at a cost of £200 million.
The project was undertaken by a joint venture between National Grid and the Belgian transmission system operator Elia.
National Grid chief executive John Pettigrew, said: "Interconnectors like Nemo Link are the perfect tool to move renewable energy from where it is produced to where it is needed most.
"By connecting the UK and Belgian electricity markets, we will also ensure customers have access to different sources of generation and lower priced electricity. This will mean that customers pay less for their energy."
National Grid is planning to build in three new interconnectors over the next five years. The North Sea Link to Norway (1.4GW) and IFA2 to France (1GW) are already under construction and the company gave financial approval in November to the Viking Link to Denmark (1GW).
Nemo Link is the UK's third electricity interconnector to continental Europe and its first to Belgium.