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In Star Trek Online Energy Credits are the main form of currency. Players can use these credits to purchase various things ranging from new ships when the player reaches a new rank to new weapons (Space and ground), deflectors and shields. These credits are obtained by various means, including (but not limited to) completing missions and selling items to merchants, such as weapons and armor.
On PC, Energy credits are capped at 2 billion for gold accounts and 15 million for silver accounts (this cap can be raised to 2 billion on all characters on an account for 500 ). Prior to the June 1st, 2017 patch, the cap was 1 billion for gold accounts (or silver accounts with the unlock) and 10 million for silver accounts.
On XBox and PS4, Energy Credits are capped at 2 billion for all accounts. At release, this was 1 billion, but has changed at some point since then.
For a list of NPCs that sell items to buy with credits, see the List of vendors accepting Energy Credits by merchandise type
For a general list of NPCs selling items see the Vendors category list.