erc-default-coding-system

erc-default-coding-system is a variable defined in `erc.el'.
Its value is

(utf-8 . undecided)

• This variable is an alias for `erc-server-coding-system'.
• This variable is obsolete since ERC 5.1;
  use `erc-server-coding-system' instead.

Documentation:
The default coding system for incoming and outgoing text.
This is either a coding system, a cons, a function, or nil.

If a cons, the encoding system for outgoing text is in the car
and the decoding system for incoming text is in the cdr. The most
interesting use for this is to put `undecided' in the cdr. This
means that `erc-coding-system-precedence' will be consulted, and the
first match there will be used.

If a function, it is called with the argument `target' and should
return a coding system or a cons as described above.

If you need to send non-ASCII text to people not using a client that
does decoding on its own, you must tell ERC what encoding to use.
Emacs cannot guess it, since it does not know what the people on the
other end of the line are using.

You can customize this variable.