The stored energy can be significant, but it is an electrostatic energy source, not an electrochemical one. For example, a fully-charged 12" x 12" x 1" charged specimen can store almost 1 kJ of electrostatic energy. The Perspex (Plexiglas) acts like a "plateless" capacitor, with negative charges trapped in a layer within the interior and a nearly matching number of positive charges on all the outside surfaces of the specimen. The effective voltage between the inner and outer charge regions can exceed 2.5 million volts.
Unlike a regular capacitor or a battery, it cannot be slowly discharged, nor can it be recharged since, after discharging, the dielectric layer is severely compromised. Unless chilled (say with dry ice) the injected charge leaks away within minutes to hours. Once the discharge process begins, it progresses to completion within tens or hundreds of nanoseconds depending on the size of the specimen. So, a charged specimen could potentially serve as a compact multimillion-volt HV source in a pulsed power application. It is otherwise a rather poor capacitor, and would definitely not serve as a battery.
: Is the energy stored in an acrylic block significant before discharge? Can a
: charged block be used as battery?
: Ian Tresman