This simultaneously charged and discharged battery box can hold infinite amount of energy in it. It is going to take a while until you can afford to make one, as the recipe requires for two fusion reactors and some dark matter cells (you can make those using particle accelerator). The beauty of it is that it also allows to output 20MW of power – you better use downgrade transformers if you want to power any machine without exploding it. The capacitor allows to regulate input/output shall the energy be conserved.
Some theories state that breaking the block does not disperse energy into the universe, and that re-placed capacitor will still hold previously stored energy.
Adjustable: Max I/O – 20 MW