In the first step, the aircraft will be equipped with one electric motor (2 MW) that will replace one of the four gas turbine engines. It will be powered from the 2 MW generator and 2 MW battery pack (the capacity wasn't disclosed).
In the second stage, the second engine will be replaced with an electric motor, but still the demonstrator will have two gas turbine engines.
We're not sure if we'll ever see a plug-in hybrid commercial project like this (with electric-only motors) actually become a thing, but maybe the aircraft industry will see a breakthrough entrant like (Tesla did in the automotive industry) and embrace electric flight.
"The three companies together announced the groundbreaking collaboration, bringing together some of the world's foremost experts in electrical and propulsion technologies, at the Royal Aeronautical Society in London.
The E-Fan X hybrid-electric technology demonstrator is anticipated to fly in 2020 following a comprehensive ground test campaign, provisionally on a BAe 146 flying testbed, with one of the aircraft's four gas turbine engines replaced by a two megawatt electric motor. Provisions will be made to replace a second gas turbine with an electric motor once system maturity has been proven."