\per\kWh produces a wrong output

Question

I want to say 10 cent dollar per kilowatt-hour of electricity. When I compile the following,

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\SI[per-mode=symbol]{0.1}[\$]{\per\kWh}
\end{document}

I get the following result that is not correct.

The correct output must be as follows:

This is a bug or I forgot something to setup?

Answer 1

The problem is that kWh is a combined unit of watt and hours with a kilo prefix. So if you wirte \kWh it expands to \kilo\watt\hour and the \per will be applied to the \watt part only.

Solution 1

Use the sticky-per option to get \per\hour to.

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\SI[per-mode=symbol,sticky-per]{0.1}[\$]{\per\kWh}
\end{document}

Adding bracket-unit-denominator = false will suppress the parens.

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\SI[%
    per-mode=symbol,sticky-per,
    bracket-unit-denominator=false,
]{0.1}[\$]{\per\kWh}
\end{document}

Solution 2

Overwrite the definition of \kWh:

\documentclass{article}
\usepackage{siunitx}

\AtBeginDocument{\DeclareSIUnit{\kWh}{kWh}}

\begin{document}
\SI[per-mode=symbol]{0.1}[\$]{\per\kWh}
\end{document}

Conclusion

Decide wether you think kWh is a combine unit or not. If you think it is, use Solution 1 if you see it as a single unit use solution 2.

Answer 2

Honestly, this idea came from Claudio Fiandrino's comment. I compiled his code and I noticed I got a correct output but with different position of currency sign.

A funny way to solve it, just append \null as follows:

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\SI[per-mode=symbol]{0.1}[\$]{\null\per\kWh}
\end{document}

Because I am still interested in why this way works, I leave this post as is until someone can explain the reason. Of course this post is not regarded as the answer!