Direct-Drive Brushless DC Solar Submersible Well Pump Sizing (Grundfos SQFlex & Lorentz PS2)
Attempting to run a conventional 240V AC single-phase induction well pump off an off-grid inverter requires a massive battery bank because single-phase AC motors demand 5x–7x locked-rotor surge current (35–55 Amps at 240V) during startup. Direct-drive brushless DC / permanent-magnet solar submersible pumps (such as the Grundfos SQFlex, Lorentz PS2, and RPS) wire directly to 2 to 8 solar panels with zero batteries—soft-starting at 8:00 AM with zero surge current and varying RPM dynamically with sunlight.
Conventional Grid/Generator Pumping vs. Solar Hydraulic Engineering
| Hydraulic Parameter | Conventional AC / Generator Mistake | Solar Hydraulic Engineering Spec | Ranch & Homestead Advantage |
|---|---|---|---|
| Motor Starting Surge Current (Inrush) | Standard 1.5 HP 240V AC Pump: 42–55 Amps locked-rotor surge (Trips small inverters) | Brushless Permanent-Magnet MPPT Soft-Start: 0% surge spike; ramps smoothly from 30V–300V DC | Eliminates $6,000+ inverter and lithium battery bank requirement at remote wellheads |
| Cloudy Morning & Late Afternoon Operation | Fixed-speed AC pump stalls or trips low-voltage relay if solar drops below 100% | Wide MPPT Voltage Window (30V–300V DC + 90V–240V AC on Grundfos SQFlex) | Pumps water at reduced GPM even under 25% overcast sky irradiance |
| Backup Generator Dual-Input Compatibility | Requires complex transfer switches and battery chargers | Grundfos CU200 / IO101 controller accepts BOTH 30–300V DC Solar AND 120V/240V AC Generator input | Plug any small 2,000W portable gas generator straight into the well switchbox during multi-day blizzards |
| Motor Lifespan & Brush Maintenance | Cheap $250 brushed DC diaphragm pumps wear out carbon brushes every 9–14 months | 304/316 Stainless Steel Brushless Encapsulated Permanent-Magnet Motor (15–20+ year life) | Avoids pulling 300 feet of water pipe every year to change worn motor brushes |
Total Dynamic Head (TDH), Solar Array Wattage & Daily Gallon Flow Schedule
| Well Depth / Design Scenario | Solar Array (Watts) | Pump / Hardware Spec | Peak Flow / Setting | Daily Output / Result |
|---|---|---|---|---|
| 100 ft TDH (Shallow Well + Tank Lift) | 800W DC (2 x 400W Panels in Series) | Grundfos 11 SQF-2 or Lorentz PS2-600 C-SJ5-8 (Centrifugal) | 11.5 GPM | 4,200 Gallons / Day |
| 250 ft TDH (Medium Depth Well) | 1,600W DC (4 x 400W Panels in Series) | Grundfos 6 SQF-2 or Lorentz PS2-1800 HR-14 (Helical Rotor) | 6.8 GPM | 2,450 Gallons / Day |
| 450 ft TDH (Deep Borehole) | 2,400W DC (6 x 400W Panels in Series) | Grundfos 3 SQF-3 or Lorentz PS2-1800 HR-07 | 3.9 GPM | 1,400 Gallons / Day |
| 700 ft TDH (Ultra-Deep Arid Aquifer) | 3,200W DC (8 x 400W Panels — 2S4P / 4S2P) | Grundfos 2.5 SQF-2 or Lorentz PS2-4000 HR-05H | 2.6 GPM | 950 Gallons / Day |
3-Step Borehole & Ranch Commissioning Checklist
Calculate True Total Dynamic Head (TDH = Pumping Level + Elevation + Friction)
Add Drawdown Pumping Water Level (not static water level!) + Vertical Elevation Rise to Cistern/Trough + Pipe Friction Head (or 2.31 ft per PSI of pressure tank setting).
Series String Voltage Sizing Within Controller Voc Limit
Wire solar panels in series to reach 120V–260V DC Vmp while ensuring cold-morning Open-Circuit Voltage (Voc × 1.18 cold factor) never exceeds the 300V DC Grundfos/Lorentz controller ceiling.
Submersible Splice Heat-Shrink & Stainless Safety Torque Arrestor
Use 3M Scotchcast or heavy-wall adhesive-lined submersible heat-shrink tubing on the 3-wire flat jacketed submersible pump cable (#12 or #10 AWG) and install a stainless steel safety cable only if using poly pipe.
Solar Well Pump & USDA NRCS Engineering Specification Sheet
Copy and customize this hydraulic specification for your well driller quote or USDA NRCS EQIP Practice Code 533 application.
SOLAR SUBMERSIBLE WELL PUMP HYDRAULIC SIZING SPECIFICATION: 1. Borehole & Hydraulic Profile: Well Casing ID = [CASING_IN]" | Static Water Level = [STATIC_FT] ft | Drawdown Pumping Level = [DRAWDOWN_FT] ft | Surface Elevation Lift / Tank Head = [ELEVATION_FT] ft | Pipe Friction Head = [FRICTION_FT] ft -> Total Dynamic Head (TDH) = [TOTAL_TDH_FT] ft. 2. Pump & Array Selection: [PUMP_MODEL] Brushless DC/AC Submersible Pump powered by [PANEL_COUNT] x [PANEL_WATTS]W PV modules ([STRING_VMP]V DC Vmp / [STRING_VOC]V DC Voc) delivering [DAILY_GAL] gallons/day into an atmospheric storage cistern.
Frequently Asked Solar Well Pumping Questions
Why do solar well pumps use "Drawdown Pumping Level" instead of "Static Water Level"?
When a well sits idle overnight, water rises to the "Static Water Level" (say, 80 feet down). Once the pump runs for 2 hours, the water level inside the well casing drops to the "Drawdown Pumping Level" (say, 160 feet down) while water seeps in from the rock formation. You must always size the pump’s Total Dynamic Head (TDH) for the deeper 160-foot drawdown level.
Can a Grundfos SQFlex solar pump run at night off a generator or house inverter if the storage tank runs low?
Yes! One of the best features of the Grundfos SQFlex motor is its internal universal power electronics: it accepts ANY voltage between 30V–300V DC OR 90V–240V AC (50/60 Hz) on the exact same two power wires. Flipping an IO101 generator transfer switch lets you run the pump at midnight from a tiny 2,000W Honda/Predator generator.