You should be able to use EnergyManagementSystem to do this:

• Create an exterior equipment. I think you'll need to use Exterior:Lights here, because it has an actuator called "Electric Power", Exterior:FuelEquipment doesn't look it has one. See source: ExteriorEnergyUse.cc#L316^[1] . Set the Exterior:Lights End-Use Subcategory to "Water Pump" or whatever you want in order to have the right reporting in your eplustbl.htm.

• Add an EMS Sensor for your flow rate, probably capturing the WaterUse:Storage's output variable Water System Storage Tank Inlet Volume Flow Rate [m3/s] (ref here)

• Declare an EMS Actuator for your chosen Exterior equipment's Electric Power.
• Write an EMS program that will compute the power used by the pump based on the flow rate given by the EMS Sensor.

Debug, look at outputs.

I hope it helps.

^[1] You could make sure: try with Exterior:FuelEquipment, request the EMS actuator dictionary (see here), check if there is indeed an actuator for this object or not.

Here's a completely untested, written on the fly and manually (problems/typos expected), EMS snippet. I think the calling point should be about right, but you may have to change it.

Exterior:Lights,
  RainWaterPump,
  [...],
  Rain Water Pump; !- End Use Subcategory


EnergyManagementSystem:Actuator,
  RainWaterPump_ElecPower_Override,     !- Name
  RainWaterPump,                        !- Actuated Component Unique Name
  Exterior:Lights,                      ! Actuated Component Type
  Electric Power;                       ! Actuated Component Control Type

WaterUse:Storage,
   RainWaterTank, !- Name
   [....]

EnergyManagementSystem:Sensor,
  RainWaterTank_OutletFlowRate_Sensor, !- Name
  RainWaterTank,                       !- Output:Variable or Output:Meter Index Key Name
  Water System Storage Tank Inlet Volume Flow Rate; !- Output:Variable or Output:Meter Name

EnergyManagementSystem:ProgramCallingManager,
  Model_RainWaterPump_CallingManager,                         !- Name
  AfterPredictorAfterHVACManagers ,       !- EnergyPlus Model Calling Point
  Model_RainWaterPump;                        !- Program Name 1

EnergyManagementSystem:Program,
  Model_RainWaterPump ,         ! Name
  IF RainWaterTank_OutletFlowRate_Sensor > 0,
    ! ### IMPLEMENT PUMP POWER CALC HERE, assuming 30 ft of head and 0.5 total pump eff ###
    SET RainWaterPump_ElecPower_Override =  RainWaterTank_OutletFlowRate_Sensor * 89,672 / 0.5,
  ! This IF/ELSE statement is useless here (if flow is zero, power is zero), but I wanted to show the syntax
  ELSE,
     SET RainWaterPump_ElecPower_Override = 0.0,
  ENDIF;