With a larger Fresnel lens, it was possible to distill water with Si and b-Si (Fig. ). Compared to Si, the distillation was more efficient with b-Si (Fig. ), obviously due to stronger heating and evaporation. Even for the smaller Fresnel lens (open markers in Fig. ), b-Si was producing more condensate (distilled water). The amount of distilled water following ∝ √ t dependence on exposure time (Fig. ) for the large Fresnel lens in case of direct heating of water and using Si for photothermal conversion. Such temporal evolution is expected for a heat diffusion-defined evaporation of the target water. Since the evaporation step is the most energy costly, the non-reflective surface of b-Si and stron absorption of all solar spectrum by Si was essential to energy deposition and heating. Since sunlight has different incidence angles depending on the date and time, a mechanical positioning system to trace Sun is needed to efficiently collect sunlight with a lens for the normal incidence (smallest reflective losses). By using a solar cell and electrical storage, e.g., a lithium-ion battery, an all-weather, space-saving solar distillation unit can be realized by water electrolysis combined with solar heater.