Info about day operation:

During the daytime the electricity gained from the photovoltaic system or the wind turbine (on a windy but sunny day *both* deliver energy!) is used to power the water pump, the seawater distiller (or RO - reverse osmosis filter, whichever is used to create clean water) and the electrolyzer which splits water into H2 (hydrogen) and O2 (oxygen).

Other possible ways (not in the picture) to store daytime energy would be to either:
a) pump groundwater into an elevated storage tank (this way the pumps don't have to able to work at night)
or
b) store the el. energy in batteries (not recommended for most environments)

Why is it necessary to store energy?
Because energy is usually more abundant during one part of the day (usually daytime). To use the expensive machinery such as desalinators most efficiently, it is best to try to run them 24h/day (except for service time, of course). So by hooking a 10kW desalinator system to a 15 or 20kW power source I can assure exess power during part of the day - this excess power can be used when the power source delivers insufficient energy, say during a calm, wind-less night.