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GeneratorChecker

LRA to Surge Watts Calculator

Estimate startup demand from Locked Rotor Amps and convert it into realistic inverter peak requirements using the same logic as our compatibility engine.

1

Inputs & Assumptions

Interactive: change any field and results update instantly.

Three-phase is an advanced industrial case. Most portable power stations are single-phase.

Quick Presets

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Calculated Result

Within common high-capacity range

Surge Apparent Power

18,000 VA

Low PF (0.75)

13,500 W

Typical PF (0.85)

15,300 W

Recommended Peak

17,595 W (with buffer)

PF-adjusted watts are shown for reference. Inverter peak sizing uses surge apparent power (VA) with your selected safety buffer.

Surge Apparent (VA basis) 15,300 W
Recommended Peak (with buffer) 17,595 W

Formula: Surge VA = LRA × Voltage (single-phase). Recommended peak uses VA basis + buffer.

Common Inverter Tiers (quick sanity check)

Small Tier

~2,000W peak

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Mid Tier

~3,600W peak

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Large Tier

~7,200W peak

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The Number That Actually Trips Your Power Station

Running watts get the attention. Surge watts trip the inverter.

When a motor or compressor starts, it draws a short burst called Locked Rotor Amps (LRA). During that startup window, demand is often 3x to 6x higher than normal running load. A well pump that runs around 1,000W can still spike toward 4,500W at startup. A 3-ton central AC with 75 LRA on 240V can hit about 18,000 VA of apparent power at compressor engagement.

If your portable power station inverter cannot absorb that spike, it shuts down. Not because the battery is empty. Not because steady load is too high. Because startup demand exceeded surge capability at the exact wrong moment.

Most labels show running amps or running watts. LRA is often buried in a compressor nameplate or technical datasheet. Even when you find it, converting LRA into practical inverter requirements still depends on voltage, phase, and power-factor assumptions.

This calculator handles both directions:

  • LRA -> Surge: Convert nameplate LRA into surge apparent power (VA), PF-adjusted reference watts, and a buffered peak target.
  • Running -> Surge: Estimate startup demand from running watts with load-class multipliers when LRA is unknown.

Both modes use the same engineering assumptions behind GeneratorChecker compatibility workflows.

How to Read Your Results

Surge Apparent Power (VA) is the raw startup demand:

  • Single-phase: LRA x Voltage
  • Three-phase: LRA x Voltage x 1.732

For inverter sizing, this is the critical number. Portable inverters must handle startup apparent power.

Low PF and Typical PF watts are reference views only. They help you understand real-power range, but inverter sizing should still be based on VA and the buffered peak.

Recommended Peak is the value to compare against the unit’s published surge or peak rating. It adds your selected safety buffer to startup demand.

Default safety buffer is 15%. For critical loads (for example flood-season sump pumps), 20% to 25% is usually safer.

Status badge thresholds in this tool:

  • Green: <= 2200W (fits many small stations)
  • Amber: 2201W to 4200W (mid/high tier needed)
  • Red: > 4200W (large inverter class)

Common Inverter Tiers compare your recommended peak against real-world classes in the market:

  • Small: about 2,000W peak
  • Mid: about 3,600W peak
  • Large: about 7,200W peak

Mode Descriptions

LRA -> Surge

Use this when you have Locked Rotor Amps from a nameplate, compressor sticker, or OEM datasheet. Typical for well pumps, HVAC compressors, and air compressors.

Running -> Surge

Use this when you only know running watts. Select load type and the calculator applies a startup multiplier based on that class.

Load Type Descriptions (Running Mode)

  • Resistive / heating element (1.0x): Space heaters, kettles, toasters, hair dryers. No meaningful startup spike.
  • Electronic / PSU loads (1.2x): Laptops, routers, TVs, monitors. Small inrush at power-on.
  • Motor baseline (3.0x): Box fans, some power tools, garage door systems.
  • Compressor baseline (3.0x): Refrigerators, many window and portable AC units.
  • High-inrush compressor (5.15x): Pancake compressor class and similar hard-start motor loads.

Quick Preset Notes

  • Liberty 287 (1/2 HP): Typical residential sump pump scenario.
  • Central AC (3-ton): Common high-start-demand residential HVAC case.
  • Heater 1500W (resistive): No startup surge, simple resistive reference.
  • Pancake compressor (5.15x): One of the hardest portable-start classes.

Default buffer is 15%, but some presets intentionally change buffer to reflect common field assumptions for that load type.

How to Use This Result

  1. Use Typical PF as your planning baseline.
  2. Size from Recommended Peak, not from continuous watts.
  3. If your load is 240V, confirm split-phase or native 240V support before comparing watts.

Related guides:

Advisory Messages Explained

  • 240V selected: Verify split-phase or native 240V output first.
  • Recommended peak above 3000W: Soft-start hardware can reduce inrush on some compressor loads.
  • High-inrush mode selected: If you have a real LRA value, switch to LRA mode for a tighter estimate.

FAQ

What is LRA in simple terms?

LRA means Locked Rotor Amps: startup current before a motor reaches normal speed. It is usually the highest electrical stress point and is often the reason an inverter trips at startup.

Why can a device trip a power station even if running watts look fine?

Running watts describe steady operation, not startup. Motors and compressors can need a short surge that is multiple times higher than running load. If surge demand exceeds inverter peak support, the unit can trip instantly even with a full battery.

Is this calculator exact for every appliance?

No. It is an engineering estimate. Real startup behavior depends on voltage conditions, temperature, cable quality and length, soft-start hardware, and equipment condition. Use a safety buffer and validate critical combinations in real conditions.

Where This Data Comes From

  • LRA conversion: Standard electrical formulas (LRA x V single-phase, LRA x V x 1.732 three-phase).
  • Running-watt multipliers: Appliance-class assumptions from tested startup behavior and conservative engineering practice.
  • Power-factor brackets: Planning ranges for residential motor loads.
  • Inverter tiers: Clustered from published surge ratings in the GeneratorChecker catalog.
  • Safety buffer: User-adjustable 0% to 50%, default 15%.

Same conversion logic is used across GeneratorChecker compatibility workflows.

Methodology: Read the full methodology.