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Direct coupling between the PV array and the pump is only possible with DC motor pumps. The simplified electrical layout is the following:

 

The following figure shows a typical pump behavior, superimposed on the I/V array characteristic. An equivalent figure - with your real components - is available in the "System" definition dialog in PVsyst, when you are choosing your Pumps and PV modules.

 

Such a configuration implies in a very careful optimization. At any time, the operating point is the intersection of the two characteristics: PV production and pump consumption. If the pump curve is too high (array current undersized), the pumping threshold will be high, penalizing the low irradiances (low season, bad days and morning/evening). If it is too low, the full potential power of the array is not used during bright hours. The optimal sizing is therefore depending either on the irradiance distribution (i.e. location, orientation, meteo), or on the periods at which the water needs are the more important.

Moreover, the pump characteristic is strongly dependent on the head, displacing the curve parallel to itself. Therefore, the sizing will also be dependent on the conditions of use, impeding using simple "thumb rules" valid for any system at any place.