An accumulator in a hydraulic motor environment can be a PITA.... As the bulk of hydraulic motors off the shelf are rev/flow coupled, an increase in speed might be a problem as the accumulator unloads through the motor, particularly if motor torque varies, such as used on a power saw. This would require a somewhat more upmarket motor with a built in "governor" or a flow control valve directly before the motor....
An easier solution might be to have two generators, the smaller one being a prime, the second, larger being an induction generator permanently coupled to the network. The induction could be on a slip/one way drive, allowing it to sync and freewheel with the mains when it is out of use... As net load increases, the second generator can come up to speed with only a small bit of current sensing circuitry to up the governor, thereby assisting the prime mover....
My .0000001c worth....