Solar pump inverter tank level control should answer two separate questions: is there enough water at the source, and is there room for more water at the destination? Confusing those questions is how a system ends up pumping from a dry well, overflowing a storage tank, or restarting every time a float moves in choppy water.
The solar array is only the energy source. Reliable automatic operation comes from a control story that makes sense from the well to the tank: permission to pump, request for water, stop condition, delay, and recovery.
This guide uses the WLD280 input arrangement to show how installers can build and test that story without treating every float switch as interchangeable.
Table of Contents
- Draw the source and destination separately
- Define the four level states
- Select the contact logic
- Prevent false starts and stops
- Coordinate level control with sunlight
- Commission without running dry or overflowing
- Leave a fault-finding record
Solar Pump Inverter Tank Level Control Has Two Sides
Stand at the controller and imagine two conversations arriving through the signal wires.
The first comes from the well, pond or source tank:
> “Water is available. The pump may run.”
The second comes from the receiving reservoir:
> “There is space. Please send water.”
The pump should normally run only when both statements are true. If the source falls below its safe level, the system stops to protect the pump. If the destination reaches its high level, the system stops to prevent overflow. The two stops have different causes and should be recorded differently during commissioning.
The BEDFORD WLD280 Solar Pump Inverter supports solar pump operation, automatic sleep and recovery, water-shortage protection and water-level control. In the reviewed WLD280 manual, DI1 and DI2 are assigned to source-level detection, while DI3 and DI4 are assigned to water-tower or destination-level detection. The exact terminal logic must still be checked against the manual version supplied with the unit.
Build a Four-State Truth Table Before Wiring
Before touching a terminal, write the intended result in plain language.
| Source condition | Destination condition | Pump request | Reason |
|---|---|---|---|
| Source safe | Tank low | Run when solar input permits | Water is available and needed |
| Source safe | Tank full | Stop | Prevent overflow |
| Source low | Tank low | Stop | Protect pump and wait for recovery |
| Source low | Tank full | Stop | Neither permission nor demand exists |
This table is simple, but it prevents a common commissioning mistake: testing the tank-full switch while the source protection is bypassed, then forgetting to restore the bypass.
For long transfer lines or slow-recovery wells, add timing states:
- source recovery delay before retry;
- tank-low confirmation time before restart;
- maximum retries after a water-shortage event;
- minimum run time where appropriate;
- weak-light sleep and recovery behavior.
The purpose of a delay is to reject a temporary change. It should not conceal a badly positioned float or a source that cannot support the required daily volume.
Normally Open and Normally Closed Are Not Labels for “Right” and “Wrong”
A float switch can provide a normally open or normally closed contact. The words describe the electrical state in the reference condition, not which option is universally safer.
Schneider Electric’s contact explanation defines a normally open contact as open when not actuated and a normally closed contact as closed when not actuated. The installer must then ask what happens if the cable is cut, the float sticks, or the sensor is removed.
The reviewed WLD280 manual uses parameter F0-15 to match the selected input logic. It shows source-level connections through DI1/DI2 and destination-level connections through DI3/DI4 for supported float or electrode arrangements. Because manuals, control boards and field devices can differ, verify:
- the exact WLD280 model and manual revision;
- whether the field contact is NO or NC;
- whether the input expects a dry contact;
- the common terminal arrangement;
- whether one or two level points are being used;
- the correct F0-15 setting for that wiring;
- the safe response to an open circuit.
Do not determine the contact type by wire color alone. Use the switch documentation and a continuity test with power isolated.
Level Sensors Fail Mechanically as Often as They Fail Electrically
A perfect wiring diagram cannot help a float trapped against the tank wall.
At the source, verify that the float or electrodes represent usable pump submergence, not merely the presence of a thin layer of water. Account for drawdown while the pump runs. At the destination, place the high-level point below the overflow and allow enough free volume for water already travelling through a long pipe after shutdown.
Check these physical details:
- free movement across the complete float travel;
- clearance from ladders, walls, pipes and incoming jets;
- no cable slack that can wrap around the float;
- electrode material suitable for the water;
- stable mounting that will not shift during cleaning;
- accessible position for inspection;
- protected signal cable route away from power conductors.
Grundfos support guidance lists blocked movement, contamination, incorrect installation height and switch failure among common level-control problems. Those basic mechanical checks belong in a solar project too.
The Sun Creates a Third Operating Condition
Now add changing solar power.
At dawn, the tank may be low and the well may be safe, yet the PV input may still be insufficient for useful pumping. At noon, energy may be strong but the tank may already be full. Late in the day, cloud changes can make the pump enter and leave weak-light operation.
Solar pump inverter tank level control should therefore distinguish:
- No demand: destination tank is full.
- No permission: source water is too low.
- No usable energy: sunlight is temporarily insufficient.
- Ready to pump: source safe, tank requests water and solar input is usable.
The WLD280 can enter sleep when light is weak and recover when solar conditions improve. That function should not be mistaken for source-level protection. A weak-light event and an empty well may both stop the pump, but the recovery logic and service response are different.
Grundfos describes solar pumping with a reservoir as a way to store energy in the form of water and shows a level switch stopping the pump when the tank is full. See its official SQFlex solar pumping data booklet. The hardware is different, but the system principle is useful: storage and level control are part of the pumping design, not optional decorations.
For the PV electrical side, use the BEDFORD solar pump inverter DC voltage guide before commissioning. Correct level logic cannot compensate for a PV string outside the controller’s approved voltage range.
Commission the Signals Before Pumping Water
Start with a dry electrical signal test while the pump is safely prevented from running.
- Confirm the normal state of each input on the display or status page.
- Move the source low-level device and confirm the intended input changes.
- Move the source high-level device where used and confirm recovery logic.
- Operate the destination low-level device and confirm a water request appears.
- Operate the destination high-level device and confirm the run request disappears.
- Disconnect one signal wire temporarily under the approved test method and confirm the defined fail-safe response.
- Restore every connection and photograph the final terminal arrangement.
Then perform a wet test:
| Test | Expected result | Actual result |
|---|---|---|
| Tank low, source safe, good sunlight | Pump starts after approved delay | |
| Tank reaches high level | Pump stops without overflow | |
| Source reaches low level | Pump stops before unsafe operation | |
| Source recovers | Pump waits for recovery delay | |
| Cloud reduces PV power | Weak-light behavior follows setup | |
| Signal cable is disturbed | No unstable repeated restart |
Do not test source protection by deliberately allowing the real pump to run dry. Simulate or operate the approved sensing device, then verify the full chain from input state to motor stop.
The BEDFORD pump inverter dry-run protection guide provides a broader acceptance-test method for water-shortage protection.
Give the Distributor a Diagram That Matches the Site
A useful handover diagram should show more than four terminal numbers. Mark:
- source type and normal water range;
- pump intake elevation;
- source low and recovery points;
- destination low and high points;
- cable length and route;
- NO/NC state for each device;
- WLD280 input assignment;
- delay and retry settings;
- PV and AC source arrangement, if backup input is used;
- person responsible for cleaning and inspection.
This record helps a distributor answer a future call such as “the pump does not start at noon.” The technician can ask whether the tank is full, the well input is open, the sunlight status is normal, or the signal cable is damaged before requesting a parameter reset.
Good solar pump inverter tank level control is quiet when it works. The tank fills, the pump stops, the source recovers and the next cycle begins without someone standing beside the controller. That apparent simplicity is earned by separating the two water levels and testing every state deliberately.