Energy Crystals can hold up to 100,000 EU. When first crafted, they have no charge and before use they must first be charged in an MFE Unit, MFS Unit, MV Solar Array, HV Solar Array, or Charging Bench (MK2 or MK3). It is also an essential part of making Nano Armor.
It can also be used as an RE-Battery with 10 times more storage. However, it can only generally be used in more advanced machines (not in the tier 1 machines such as the Macerator, Extractor, etc). However, you can use Transformer Upgrades on them, and then it will work, just put it in the battery slot.
Unlike RE Batteries, Energy Crystals cannot be stacked no matter how much power they have, where as uncharged batteries can be stacked. Also, you can not right-click while holding them to charge machines in your inventory.
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 a 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, as the induction furnace is one of the few tier 1 or 2 machines that accepts a 128 EUp voltage. However, all tier 1 & 2 machines will work with energy crystals if you give them a Transformer Upgrade