Why using \mathrm still gives italic text?

Question

I'm writing an equation using the displaymath environment, and I'd like to have the units in non-italic. So I use \mathrm{}, and I write this:

\begin{displaymath}
P_{AVG}=19.5 + \frac{2180}{5 \cdot 60 + 0.2} = 26.77~\mathrm{\mu}W
\end{displaymath}

But the units remain italic:

I tried to use the 'amsmath' package, but the problem remains. What am I doing wrong?

I'm using the book class in pdflatex:\

\documentclass[12pt, a4paper, titlepage, twoside, openright]{book}

EDIT: it took me a while to realize that the W was outside the \mathrm block, but the solutions are both very good.

Answer 1

This is my suggestion:

\documentclass{article}
\usepackage{amsmath}
\usepackage{siunitx}
\begin{document}

\begin{equation*}
P_{\text{AVG}} = \SI{19.5}{\micro\watt} + \frac{\SI{2180}{\micro\joule}}{5 \cdot \SI{60}{\second} + \SI{0.2}{\second}} = \SI{26.77}{\micro\watt}
\end{equation*}

\end{document}

(By loading the amsmath package, I can use text to typeset the "AVG" subscript in roman)

Answer 2

\documentclass{book}
\usepackage{upgreek}
\begin{document}

\[
P_{AVG}=19.5 + \frac{2180}{5 \cdot 60 + 0.2} = 26.77~\upmu W
\]

\end{document}