It will be interesting to see what, if anything, this is ultimately used for. It has a giant footprint (required for the solar panels to generate enough energy), but a small payload.  It's possible something like this could be used for surveillance or training, but it has severe limitations. Via Rare  (emphasis added):

A solar-powered plane that has wowed aviation fans in Europe took to the skies Friday over the San Francisco Bay area in a successful test flight.

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The Solar Impulse is powered by about 12,000 photovoltaic cells that cover massive wings and charge its batteries, allowing it to fly day and night without jet fuel. It has the wing span of a commercial airplane but the weight of the average family car, making it vulnerable to bad weather.

Its creators say the Solar Impulse is designed to showcase the potential of solar power and will never replace fuel-powered commercial flights. The delicate, single-seat plane cruises around 40 mph and can’t fly through clouds.