How does lowering altitude conserve energy in Solar Impulse 2? In this article the author explains that the aircraft flies at a lower altitude to conserve energy. 


  During the day, it climbs to its maximum cruising altitude of 8,500 metres to capture the most rays. To conserve energy, it works its way down to 1,500 metres in the evening, and stays there overnight.


How is this possible?
Generally, best range is at high altitude(thin air, less drag), and best endurance is at low attitude(thick air, more lift) and low power. 

It makes sense that at night they would have to descend to a best endurance altitude to conserve battery power. 


Source: solarimpulse.com