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What size solar panel?

15 Watts is the nominal panel size required. Smaller than 5 Watts will not be enough power. Larger panels work so long as voltage is less than 24 V.  If a larger panel is used, a charge controller and battery would can limit the voltage. For example, to run the pump all night, perhaps a 45 amp hour battery, 100 W panel and controller could be used.  In this case, even though the panel is rated at over 24 volts, the charge controller limits the output to 12 V or 24 V depending on the battery used.

On the scanned specification sheet, notice that voltage varies from 8 to 24 V DC and the maximum head (zero flow) is around 3 meters. The flow rate is zero at maximum head. Look at the bottom of the flow curve graph-there is no flow at maximum head; this just shows what the pump would do with maximum power in, pushing a water column up as far as it could, say up a pipe that was open at the top. The water column would reach 3 meters. No water would spill out the top.

12 Volt is nominal voltage. The PV direct pump adjusts output based on a varying solar input. Solar panel output varies in both current and voltage. To achieve maximum power point (MPP) tracking, the pump adjusts both torque and speed. The pump can run fast, high RPM with a high flow, at low head: this works with a lower voltage and high current. Torque required is small at lower head. Even at a fairly low voltage, the pump could absorb full power; the power is all going to high flow. The converse is true as well: to get the maximum head, which requires maximum torque, the voltage would have to be at a maximum value, closer to 24 V, and the current would be low, just enough to turn the pump fast enough to prevent back flow. But nobody does this because flow rate is zero.

Any panel with an voltage output greater than 24 Volts will need a charge controller or battery in the system to avoid running the pump too hard.  Any solar panel around 15 Watts will work fine for both voltage and power.