AC / DC Backup Power Supply For Solar Irrigation Pump
An interesting use - effectively, grid-replacement when solar is available. 

Minimum requirement is two power diodes and a main supply that can be set to a voltage just below the minimum PV voltage level when operating the pump.

ie Decide some minimum level of voltage that the solar system has to maintain and to supplement this  with AC provided DC when the solar input cannot hold the voltage at the desired level. 

Connect solar system to load via a diode.
Connect mains supply to load via a diode.
Set mains system just below the minimum acceptable loaded solar voltage.

In full sun the PV (solar) system will be able to operate the load.
As PV input drops (cloud or end of day) the PV output will drop below minimum acceptable pump load voltage and the main supply will start to share current provision. As PV output drops lower the mains supply will take over all powering as PV output becomes unable to maintain the required voltage. .



While the above system can be run "batteryless" you may find you get an easiest to manage system using a low capacity 48V battery. This need have minimal capacity - the ability to run the pump for a minute or so would nominally be enough. A 20 A load needs 20 Ah of storage for 1 hour or about 1Ah per 3 minutes. 4 x 12V x 3.5Ah batteries in series will nominally run the system for about 10 minutes. In fact it would be less than that as they would not like the heavy load BUT that is not how you use them. 

When there is adequate PV energy the excess will charge the battery and a regulator will need to stop the batteries from overcharging. Choose a float voltage that you regulate the cells at. As long as PV input is > load current the batteries will be at float voltage. 
Set the mains supply at a DC voltage JUST below float voltage.
 Diode couple mains supply and solar system to load via individual Schottky diodes as before. When the PV voltage is > mains voltage there will be no mains power used.