They are doubling the power of the ERS system (60kw to 120kw), but the amount it can be used per lap is going up ten fold (400KJ to 4MJ total energy output).
So in the terms used by the commentators, they are going from having an extra 80bhp available for 7 seconds per lap to having an additional 160bhp available for pretty much the whole lap (comfortably all non traction limited acceleration zones at most tracks).
Not to mention the potential for ERS powered anti lag on the turbos.
It is not simply 'double KERS'.
So in the terms used by the commentators, they are going from having an extra 80bhp available for 7 seconds per lap to having an additional 160bhp available for pretty much the whole lap (comfortably all non traction limited acceleration zones at most tracks).
Not to mention the potential for ERS powered anti lag on the turbos.
It is not simply 'double KERS'.