This building you see is called 'Le Bourg', or 'Schalhaus bei Le Bourg', and is one of the rarest structures left over from the occupation. It was a purpose built German electricity substation and was one of 3 built on the island. The other two were at Six Rues and La Fontine, although Le Bourg is the only one left standing.
Le Bourg was constructed by the Organisation Todt in 1943, but the work was overseen by a Swiss firm Brown Boveri and co, who had an office in nearby St Malo. It was built to provide power for the Maltiere stone crusher plant just around the corner (now turned into a block of flats).
The building itself is now a Grade 3 listed building and should be appreciated from outside!