\documentclass{article}
\usepackage{siunitx}
\renewcommand{\familydefault}{\sfdefault} % My document should be in sans serif
\begin{document}%
\qty{5}{\kilo\volt} vs $\qty{5}{\kilo\volt}$.
\end{document}
Likely it is best to adjust how beamer is supported here as it's the main reason.
From https://tex.stackexchange.com/questions/682749/make-siunitx-use-roman-font-in-math-mode-and-sans-font-in-text-in-a-sans-serif/682752#682752
Likely it is best to adjust how
beameris supported here as it's the main reason.