What would be the stall speed of Solar Impulse? In most of the tables, only the take off (22 mph) and cruise (43) speed of the Solar Impulse are listed. It is an experimental plane, slow, with very huge wing for solar batteries.

The take off speed may not be the slowest speed the aircraft can fly, for instance, when landing. Is it possible to infer somehow the expected slowest speed of this airplane?
Solar Impulse has no flaps and a high aspect ratio wing. Therefore, the c$_{l_{max}}$ of the wing should be no higher than 1.8, most likely 1.6. While a rigid airfoil can be designed with a higher c$_{l_{max}}$, it will not have an attractive L/D and a shorter wing would have a c$_{l_{max}}$ below that of the airfoil. Since no fuel is consumed, mass is constant, which makes it very easy to give a lower bound for the minimum speed.

Solar Impulse 2 has 269.5 m² wing area to carry its 2.3 tons of mass. This is a wing loading of just 8.53 kg/m². If we assume standard day conditions at sea level (density $\rho$ = 1.225 kg/m³), we find vor v$_{min}$:

$$v_{min} = \sqrt{\frac{2\cdot m\cdot g}{c_{l_{max}}\cdot S\cdot \rho}} = 9.24 \;\small{\frac{m}{s}} \;\text{or}\; 33.27\; \small{\frac{km}{h}}$$

For those of you from metrically challenged cultures: This is 18 knots or 20.67 mph.