This can be easily done by indicating the required max water flow for your application in cubic meters per hour, and the max pump head in meters.
The DC solar water pump head can be gotten from knowing the distance between the pump and well & the distance between the well and water tank as shown in this picture:

h1 is the head above water (water to tank), h2 is the head under water (pump to water), and d is the horizontal distance between tank and pump. Therefore, the actual pump head: H=h1+h2+d/10.
For example:
h1=50m, h2=30m, d=20m.
Then H=50+30+(20/10)=82m.
The actual pump head is 82m.
According to the estimated flow and head, refer to the performance curves and select the solar water pump with the appropriate flow and head. Then confirm which working voltage 12V/24V/36V/48V/72V/110V is suitable for the solar pumping system.
If you are not sure, you can send all your requirements to our customer service, and then we will help you to choose the right pump.