A well pump can be one of the hardest loads in an off-grid system, not because it necessarily uses the most energy each day, but because the motor may demand a large amount of power for a short time when it starts.
That makes a pump a system problem: motor, inverter, battery, cable run, controls, water level, pressure, and total dynamic head all interact.
1. Start with the motor data, not a horsepower shortcut
Two pumps with the same horsepower can have different running current, starting current, control equipment, and hydraulic performance.
Before sizing the inverter, collect:
- exact pump and motor model,
- supply voltage and phase,
- service-factor or maximum current where provided,
- locked-rotor or starting-current information if published,
- 2-wire or 3-wire control arrangement,
- well depth and static/dynamic water level,
- required flow and discharge pressure.
Franklin Electric’s AIM material publishes motor-specific application and electrical data. That is a much better basis than a generic table that says every 1HP pump needs a certain inverter wattage.
2-wire versus 3-wire
In common residential submersible systems, a 2-wire motor keeps its starting components with the motor assembly, while a 3-wire design uses an above-ground control box for at least part of the starting circuit.
A 3-wire arrangement can make some control components easier to service, but that does not make it automatically superior for every off-grid installation. Pump availability, motor design, controls, cable length, service practices, and inverter compatibility all matter.
2. Starting the motor: inverter, battery, and starter have to agree
A pump that runs comfortably at 1–2kW can still trip an inverter during startup if the inverter cannot supply the required surge for long enough or if the DC bus sags.
Check four limits together:
- motor starting requirement,
- inverter surge power and surge duration,
- battery/BMS peak-current capability,
- DC conductor and connection voltage drop.
A soft starter or variable-speed drive can reduce starting stress in some systems, but it is not a universal add-on. The device has to be approved for the motor type and application, and the pump manufacturer may specify particular controls.
Do not assume that a generic soft starter will turn any 1HP pump into an easy load for a 5kW inverter.
If motor-starting behavior is uncertain, use the motor manufacturer, drive manufacturer, or a qualified pump/electrical professional to verify the combination before buying the inverter.
3. Sometimes it is easier to store water than electricity
A second architecture is to pump when solar energy is available and store water in a cistern or elevated tank.
[PV] → [pump controller] → [well pump] → [water storage]
│
└→ later household/irrigation useThis can reduce the need to start a large AC pump from the house battery at night. Depending on the design, a small pressure/booster pump may still need battery power.
Direct-solar pump systems are not sized by panel count alone. The selected pump must deliver the required flow at the site’s total dynamic head, and the controller must accept the array voltage/current over the expected temperature range.
| Question | Inverter-fed AC pump | Solar/direct pump + storage |
|---|---|---|
| Existing well equipment | Often easier to retain | May require pump replacement |
| Night pumping | Available if battery/inverter support it | Usually relies on stored water |
| Starting load on house inverter | Can be significant | Can be reduced or eliminated |
| Water storage required | Pressure tank may be enough | Larger cistern/tank often central |
| Design driver | Motor + inverter + battery | Head + flow + daily water demand |
The better option depends on the existing well, water demand, service access, climate, and cost—not on a blanket preference for AC or DC.
Use our Solar + Battery Sizing Calculator for the electrical energy side after the pump’s real operating data is known.
For a broader off-grid household load budget, add the pump’s known daily electrical use to the Build My Off-Grid System planner. This planner does not calculate total dynamic head, select the pump, validate motor-starting current or certify inverter surge capability; use the manufacturer’s data and appropriate professional review for those checks.