By a Solar Engineer Who's Designed 75+ Off-Grid Water Systems
If you're on well water, losing power means losing water. No showers, no toilet flushing, no livestock watering.
After designing 75+ solar-powered well pump systems, I'll tell you what most installers get wrong: they size for running power, not starting surge.
The critical number: A typical deep well pump needs 2-4x its running power just to start.
The Quick Answer
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Pump Type
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Running Power
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Starting Surge
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Minimum Inverter
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Shallow well (1/2 HP)
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600W
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1,800W
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2kW
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Deep well (1/2 HP)
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750W
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2,500W
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3kW
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Deep well (1 HP)
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1,000W
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4,000W
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5kW
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Deep well (3 HP)
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2,000W
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8,000W+
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8kW+
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Why Well Pumps Are the Hardest Load
The Surge Problem
When a well pump starts, the motor is essentially a dead short for the first fraction of a second. It needs massive current to overcome inertia.
What happens with undersized inverters:
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Pump tries to start
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Inverter sees massive current draw
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Inverter trips into overload protection
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Pump never starts, no water
What I see constantly: Homeowner buys a 3kW inverter for a 1 HP well pump (1,000W running), thinking "I've got 3x headroom." They don't. The pump needs 4,000W+ surge, and the inverter shuts down instantly.
The Solution: Surge Rating Matters
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Inverter Type
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Typical Surge Rating
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Cheap modified sine wave
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1.2x for 1 second
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Standard pure sine wave
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1.5x for 3 seconds
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SolarInverterUS hybrid
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2x for 10 seconds
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The difference: 2x for 10 seconds means a 5kW inverter can deliver 10kW surge—enough to start most residential well pumps.
Real Case: Texas Ranch Well Pump
A rural Texas homeowner called me after two failed attempts to solar-power his well pump.
Setup: 3HP submersible well pump (380ft depth), two previous 5kW inverters had tripped on startup
Problem: Standard 5kW inverters only provide 1.5x surge (7.5kW) for 3 seconds. His pump needed 8,000W+ for 5-8 seconds.
Solution: SolarInverterUS 5kW hybrid with 2x surge for 10 seconds. First try, pump started instantly.
Sizing Your Inverter for Well Pump
Step 1: Identify Your Pump
Step 2: Calculate Surge Requirement
Rule of thumb:
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1/2 HP: 3-4x running power surge
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1 HP: 3-4x running power surge
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3 HP: 4-5x running power surge
Step 3: Check Inverter Surge Specs
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Surge multiplier (1.5x, 2x, etc.)
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Surge duration (seconds)
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Recovery time between surges
Step 4: Verify 240V Output
Most deep well pumps run on 240V. All SolarInverterUS hybrid inverters output native 120V/240V split-phase—no autotransformer needed.
Battery Sizing
Daily Pump Runtime:
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1-2 people: 30-60 minutes
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3-4 people: 60-90 minutes
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5+ people: 90-120 minutes
Example: 1 HP pump (1,000W), 1 hour daily = 1 kWh/day
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Autonomy Days
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Battery Size
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1 day
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2-3 kWh
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2 days
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5 kWh
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3 days
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8-10 kWh
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Cost Estimates
Basic Well Pump Backup: $4,500-6,500
Full Off-Grid Well Pump System: $10,500-15,500
