In order to colour each bar differently and make it appear with its own legend entry, each bar needs to be handled as its own plot, i.e. it needs its own \addplot ... command. Fortunately, you don't have to write \addplot ... seven times, but instead you can use \pgfplotsinvokeforeach {0,...,6} { \addplot ... }.
I would suggest you provide the data in a table, either in an external datafile or in a macro created using \pgfplotstableread. This makes it easier to loop over the data, and it makes your data more maintainable (the coordinate syntax becomes quite tedious for larger datasets).
To provide different colours for the bars, you can define a new cycle list using \pgfplotscreatecyclelist. In the example below, I've defined seven colours according to a Colorbrewer color scheme and used them to define a new cycle list.
To create one legend for all of the plots, you can use the key legend to name=<name>, which stores the legend in a \label, so you can create it somewhere else using \ref{<name>}. In this case, I put it below the center of the second axis, so it aligns nicely with the axes. If it was centered according to the page, it would look misaligned.
By default, the entries in horizontal legends are quite close together. I've inserted a bit more space using the approach described in How can I adjust the horizontal spacing between legend entries in PGFPlots?.
To make the whole plot take up a specified width, you have to do a few calculations, as described in the accepted answer to pgfplots: how can I scale to text width?. You have to specify the plot width with the key scale only axis activated, the space taken up by the y tick labels and the y axis label, and the space between the groupplots, and these dimensions have to add up to your desired overall width. In this case, I set the space between the plots to 0.05\textwidth (so 0.1\texwidth in total, because there are three plots), the space for the leftmost ytick labels to 0.035\textwidth with an offset of 0.005\textwidth, so 0.04\textwidth overall, the space for the y label to 0.02\textwidth, and the width of the axes to 0.74\textwidth/3. This adds up to 0.74\textwidth/3 * 3 + 0.02\textwidth + 0.04\textwidth + 0.1\textwidth = 0.9\textwidth. Try setting the axis width to 0.84\textwidth/3: This will make the whole environment exactly \texwidth wide.
\documentclass{article}
\usepackage{lipsum}
\usepackage{pgfplots,pgfplotstable}
\pgfplotsset{compat=1.4}
\usetikzlibrary{pgfplots.groupplots}
\pgfplotstableread{
Criterion Rapperswil Palermo Jeddah
Turquoise 3620 15749 42754
Yellow 2739 15109 42240
Purple 3710 16146 40533
Red 714 9306 28919
Blue 2997 6840 11614
Orange 1589 4992 11786
Green 2122 11154 28747
}\datatable
\definecolor{RYB1}{RGB}{141, 211, 199}
\definecolor{RYB2}{RGB}{255, 255, 179}
\definecolor{RYB3}{RGB}{190, 186, 218}
\definecolor{RYB4}{RGB}{251, 128, 114}
\definecolor{RYB5}{RGB}{128, 177, 211}
\definecolor{RYB6}{RGB}{253, 180, 98}
\definecolor{RYB7}{RGB}{179, 222, 105}
\pgfplotscreateplotcyclelist{colorbrewer-RYB}{
{RYB1!50!black,fill=RYB1},
{RYB2!50!black,fill=RYB2},
{RYB3!50!black,fill=RYB3},
{RYB4!50!black,fill=RYB4},
{RYB5!50!black,fill=RYB5},
{RYB6!50!black,fill=RYB6},
{RYB7!50!black,fill=RYB7},
}
\pgfplotsset{
select row/.style={
x filter/.code={\ifnum\coordindex=#1\else\def\pgfmathresult{}\fi}
}
}
\begin{document}
\lipsum[1]
\begin{figure}[hbt!]
\centering
\begin{tikzpicture}
\begin{groupplot}
[
group style=
{
columns=3,
xlabels at=edge bottom,
ylabels at=edge left,
horizontal sep=0.05\textwidth, group name=plots
},
ybar=0pt,
/pgf/bar shift=0pt,scale only axis,
width=0.74\textwidth/3,
yticklabel style={text width=0.035\textwidth, align=right, inner xsep=0pt, xshift=-0.005\textwidth},
ylabel=kWh, ylabel style={text height=0.02\textwidth,inner ysep=0pt},
enlarge x limits=0.15,
ymin=0,
xtick=\empty,
tickpos=left,
scaled y ticks=base 10:-3,
cycle list name=colorbrewer-RYB,
legend columns=-1,
legend style={draw=none, /tikz/every even column/.append style={column sep=5pt}},
legend image code/.code={%
\draw[#1] (0cm,-0.1cm) rectangle (0.3cm,0.1cm);
}
]
\nextgroupplot[xlabel=Rapperswil, legend to name=grouplegend,]
\pgfplotsinvokeforeach {0,...,6}{
\addplot table [ x expr=\coordindex, select row=#1, y=Rapperswil] {\datatable};
\addlegendentry {\pgfplotstablegetelem{#1}{Criterion}\of\datatable \pgfplotsretval}
}
\nextgroupplot[xlabel=Palermo]
\pgfplotsinvokeforeach {0,...,6}{
\addplot table [x expr=\coordindex, select row=#1, y=Palermo] {\datatable};
}
\nextgroupplot[xlabel=Jeddah]
\pgfplotsinvokeforeach {0,...,6}{
\addplot table [x expr=\coordindex, select row=#1, y=Jeddah] {\datatable};
}
\end{groupplot}
\node at (plots c2r1.south) [inner sep=0pt,anchor=north, yshift=-5ex] {\ref{grouplegend}};
\end{tikzpicture}
\end{figure}
\lipsum[2]
\end{document}
