latex units in equation

%In preamble, add the following command:

\makeatletter
\providecommand\add@text{}
\newcommand\tagaddtext[1]{%
  \gdef\add@text{#1\gdef\add@text{}}}% 
\renewcommand\tagform@[1]{%
  \maketag@@@{\llap{\add@text\quad}(\ignorespaces#1\unskip\@@italiccorr)}%
}
\makeatother

% In your equation, add this: 
\begin{equation}
    E = mc^2 \tagaddtext{[your unit]} 
\end{equation}