An Energy Crystal can hold up to 100,000 EU. When first crafted, they have no charge. They must first be charged in an MFE Unit, MFS Unit, MV Solar Array, HV Solar Array, or Charging Bench (MK2 or MK3). The more EU it's holding, the brighter it looks.
Important: When you use a partially or fully charged Energy Crystal, it will completely lose its charge when used to create a HV Transformer or an MFE Unit. When used to create a Lapotron Crystal or Mining Laser, the created item will be charged according to the Energy Crystal's original charge.
Powering Machines: The Energy Crystal doesn't charge many of the machines, but it does power the Induction Furnace.