Generator sizing guide
Well Pump Starting Watts
Understand running watts, starting watts, locked-rotor amps, and how to select a generator that can reliably start and run a well pump.
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What Are Well Pump Starting Watts?
Well pumps require two different wattage values: the running watts needed once the motor is up to speed, and the starting watts required to overcome initial inertia. Starting watts are typically the larger number and are the main factor in generator sizing.
Starting watts are closely tied to the motor’s locked-rotor amps (LRA). You can often find LRA on the pump’s nameplate or control box. If not, the pump manufacturer may publish typical values for the horsepower and motor type.
Running Watts vs. Starting Watts
Residential well pumps commonly operate at 240 volts. Volts multiplied by amps gives approximate apparent demand:
Apparent demand in VA ≈ Volts × AmpsReal input power in watts also depends on power factor and motor operating conditions. Use a published input-watt rating or measured value when one is available.
If a pump draws 8 amps running at 240 volts:
Running apparent demand ≈ 8 A × 240 V = 1,920 VAA motor with 24 LRA at 240 volts has:
Locked-rotor apparent demand ≈ 24 A × 240 V = 5,760 VA, or 5.76 kVAThis is not the same as exact starting watts. For final sizing, compare the motor’s LRA directly with the generator’s published surge-current capability and allowable voltage dip.
Typical Starting Watts by Pump Horsepower
These ranges align with common residential pump nameplates:
| Pump HP | Running Watts (Approx.) | Starting Watts (Approx.) |
|---|---|---|
| 1/2 HP | 700–1,100 W | 2,000–4,000 W |
| 3/4 HP | 1,000–1,600 W | 3,000–5,000 W |
| 1 HP | 1,500–2,000 W | 4,000–6,000 W |
These values vary by motor design, manufacturer specifications, and wiring distance. Always verify with the pump nameplate when possible.
Submersible vs. Jet Pump Starting Load
Submersible pumps typically have higher starting surge than jet pumps of similar horsepower due to motor type and pumping head. However, both types can require 2–3 times running watts at startup.
For example, a typical 3/4 HP jet pump might require:
- Running: ~1,200–1,600 W
- Starting: ~3,000–4,500 W
Why Starting Surge Varies
Starting surge depends on:
- Motor horsepower
- Locked-rotor amps (LRA)
- Wire length to the pump (voltage drop increases surge VA)
- Motor age and friction
- Control box type (2-wire vs. 3-wire)
- Pressure tank cycling frequency
If your pump is aging or the well is far from your home’s electrical panel, lean toward the upper end of typical generator sizing ranges.
How to Reduce Starting Surge
Some options may help reduce starting demand:
- Soft-start modules (when compatible with the pump model)
- Proper wire sizing to minimize voltage drop
- Pressure tank maintenance to reduce short cycling
- Replacing older motors whose LRA has increased with age
How to Size a Generator for a Well Pump
The safest sizing method is to calculate running and starting watts, then choose a generator where expected peak load is about 70–80% of total rated output. This mirrors best practices for residential optional standby systems under NEC Article 702.
Step 1: Determine Running Amps
Check the nameplate for full-load amps (FLA). Estimate running apparent demand, or use published input watts when available:
Approximate running VA = FLA × voltageStep 2: Determine Starting (Surge) Amps
If you see “LRA,” estimate locked-rotor apparent demand:
Approximate locked-rotor apparent demand in VA = LRA × voltageDo not treat that result as exact starting watts. Compare LRA directly with the generator’s published surge-current capability and allowable voltage dip.
Step 3: Add Your Other Running Loads
Include only the circuits you intend to power during an outage. For a full analysis, try the generator sizing calculator.
Step 4: Add Headroom
Add roughly 20–30% headroom to account for temperature, aging equipment, and measurement uncertainty.
For a step-by-step example, see the Home Generator Sizing Guide.
Common Mistakes to Avoid
- Assuming HP alone determines starting watts
- Ignoring wire length and voltage drop
- Using running watts instead of starting watts for generator sizing
- Trying to start multiple pumps simultaneously without load management
- Backfeeding without a transfer switch (dangerous and illegal)
For safe installation practices, see your local electrical code authority and the National Fire Protection Association, publisher of NEC (NFPA 70).
For portable generator safety, including CO risk, see the U.S. Consumer Product Safety Commission.
Example Calculations
Example for a 3/4 HP submersible pump:
Running current: 6.5 A at 240 V
Approximate running apparent demand: 1,560 VA
Locked-rotor current: 22 A at 240 V
Approximate locked-rotor apparent demand: 5,280 VA
Other running loads during outage: 1,200 W (fridge, lights)Because the example combines motor apparent demand with other loads, the final selection cannot be established by adding those values as if they were all exact watts. Use the pump’s published or measured input watts for the continuous-load calculation, apply headroom to that continuous total, and separately confirm that the generator’s surge-current capability and allowable voltage dip can support the pump’s 22 LRA.
For your exact numbers, enter your pump details into the generator sizing calculator.
Final Thoughts
Well pumps are one of the reasons generator sizing is not purely based on square footage. Their starting surge can be several times running wattage, so a generator must be chosen with real-world peak load in mind.
For broader context, you may also want to read:
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