Problem with a relay decoupling voltage
Another option is using ORing diodes instead of the relay.  







simulate this circuit &ndash; Schematic created using CircuitLab

In the circuit above the transient between the solar panel and the 12V DC power source will be smooth and at about 11.0&nbsp;V (assuming a 0.5&nbsp;V voltage drop). You could play with the transition point by adding more Schottky diodes in series with D2 and D3.  

A drawback is that each Schottky will dissipate power. If possible, you could use an ORing configuration for each pump. This will reduce the power dissipation a little.  

Illustration the right circuit dissipates less:
The current per pump is 30&nbsp;W&nbsp;/&nbsp;12&nbsp;V&nbsp;=&nbsp;2.5&nbsp;A.
Using for example the VS-12TQ035-M3 for every Schottky diode, the voltage drop per diode in the left circuit will be about 0.45&nbsp;V @ 5&nbsp;A, so a 2.25&nbsp;W dissipation. In case only D1 is conduction, the wasted power is thus 2.25&nbsp;W.
Using the right circuit, the voltage drop 0.35&nbsp;V @ 2.5&nbsp;A, so a 0.875&nbsp;W dissipation per diode. In case only D4 and D5 are conduction, the wasted power is a little less: 2&nbsp;*&nbsp;0.875&nbsp;W&nbsp;=&nbsp;1.75&nbsp;W.