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The biggest challenges for solar flight up until now has been how to store enough energy from its array of solar panels to keep the slow & lightweight aircraft flying in darkness. In the case of the The Solar Impulse, engineers are relying on the battery capacity of the plane - which will store the energy generated during the day to keep the plane alight at night. The ultra-thin solar cells integrated into the wings will be used to collect sunlight and convert it into energy to power the four electric motors.
Sources 1 & 2