For any pump, whether it's a solar water pump or ordinary centrifugal pump, it has a fluid inlet which is charging pipe and a fluid outlet which is discharge pipe. Assume that you will be able to move discharge pipe and extend it in the vertical direction, then simply put, the head is the height in which pump can raise the fluid up inside that pipe.
How can we measure the head?
There are 2 different cases in measuring pump head based on from where the pump pulling water as following:
1. If the pump pulls water from a suction tank in the same level: Pump manufacturer will provide total head in their specs (pump datasheet) to be represented by the head of horizontal centrifugal pump minus a suction head, as per fig 1. So, it does not matter what will be the suction tank level.
2. If the pump pulls water from a lake: Pump manufacturer will provide total head in their specs (pump datasheet) to be represented by vertical pump head plus suction head, as per fig 2.
The total head aforementioned is the sum of the static head and friction head. Static head refers to how high you need to get the water, and of course, it based on a suction tank, also the static head is the main component of total head. Friction head refers to how resistance the water will face during going up, and of course, it based on the discharge pipe diameter and the length.
How to buy the correct pump?
In order to buy the correct pump, you need to know what flow rate and total head you will need.
Generally speaking, when the pump output pressure increases, the pump head increases, and when the pump output flow increases, the head will decrease. That is why some few pumps manufacturers use the pressure term instead of head to characterize the pump specs, but the head is still more useful and clearer term to assist you in the pump job evaluation.
In a certain application, if you need to pump a liquid for 40ft, this means that you need to select a centrifugal pump with at least 40ft total head plus the friction losses, so it is better to select a bit little higher head to substitute for the friction to get required flow. For example, 1 hp horizontal centrifugal pump with maximum head 68ft is a good choice.
Pump head will not be affected by pumped liquid type as long as the liquid viscosity is low like water, so the head achieved by the pump will be almost the same.