The power plant, which has a thermal efficiency of around 58%, was bought by the company from Welsh Power in 2009.
First Minister of Wales Carwyn Jones said that this new power station is an important addition to the diverse energy mix of technologies that are putting Wales at the cutting edge of UK energy production.
The Severn Power Station uses combined-cycle gas turbine technology and was built at a cost of £600m.
The facility is expected to generate enough electricity for around 1.5 million homes.
