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\documentclass[12pt]{article}
\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{arrows}
\begin{document}
\begin{figure}[hpt!]
\centering
\tikzstyle{block} = [thick,draw,rectangle,minimum width=1.4cm, minimum height=1.4cm] 
\begin{tikzpicture}[>=latex,node distance=2.8cm,font=\footnotesize] 
\node[block,label={$\eta_\text{Bat}=\SI{97}\percent$}](A) at (0,0) {}; 
\node[block, right of=A,label={$\eta_\text{inv}=\SI{95}\percent$}](B) {}; 
\node[block, right of=B,label={$\eta_\text{EM}=\SI{88}\percent$}](C) {\normalsize\sffamily M}; 
\node[block, right of=C,label={$\eta_\text{mech}=\SI{95}\percent$}](D) {}; 
\node[block, right of=D](E) {}; 
\draw[thick] (B.north east) -- (B.south west);%schrägstrich 
\draw[thick] (B) (2.6,0.3) node {\Huge =};%Gleichzeichen 
\draw[thick] (B) (3.1,-0.3) node {\Huge $\sim$}; %Zeichen 
\draw (C) circle (.6); 
\draw[thick] (C)(8,0) circle (0.15); %Kleiner zahnrat 
\draw[thick] (C) (8.65,0) circle (0.30);% Großerzahnrat 
\foreach \Phi in {0, 45, ..., 325} %Um den Zahnrat die striche 
{\draw [line width=0.7mm] (8,0) ++(\Phi:0.15) -- ++(\Phi:0.10);} 
\foreach \Phi in {15, 60, ..., 335} 
{\draw [line width=0.7mm] (8.65,0) ++(\Phi:0.30) -- ++(\Phi:0.10);}% 
\draw[thick] (E) node [rotate=45] {\Huge $\approx$}; 
\draw (-.5,0) to [battery] (0.5,0); 
\draw[->] (A.east) -- (B.west) node[midway,above] {$P(t)_\text{el}$}; 
\draw[->] (B.east) -- (C.west) node[midway,above] {$P(t)_\text{el}$}; 
\draw[->] (C.east) -- (D.west) node[midway,above] {$P(t)_\text{mech}$}; 
\draw[->] (D.east) -- (E.west) node[midway,above] {$P(t)_\text{mech}$};   
\end{tikzpicture}
\end{figure}
\end{document}

./test.tex:6: LaTeX Error: Missing \begin{document}.

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l.6 \begin{figure}[hpt!]