The question of energy defines the entire project. At midday, each square metre of land surface, in the form of light energy, receives the equivalent of 1000 watts, or 1.3 horsepower of light power. Over 24 hours, this averages out at just 250W/m². With 200m² of photovoltaic cells and a 12 % total efficiency of the propulsion chain, the plane’s motors achieve an average power of no more than 8 HP or 6kW.
...
The 12 000 or so photovoltaic cells in 145 micron monocrystalline silicon combine lightness and efficiency. Their efficiency could have been even higher, like the panels used in space, but their weight would then have penalized the plane during night flight. This phase being the most critical, the main constraint is the one imposed by the batteries. Still heavy, they require a drastic reduction of the weight of the rest of the plane, so as to optimize the whole energy chain and to maximize the aerodynamic performance provided by a large wing span and a wing profile designed for low speeds. With an energy density of 240Wh/kg, the accumulators weigh 400kg, or more than ¼ of the total mass of the plane.