Motors and compressors draw a short burst of extra power at startup. If your generator or inverter cannot handle that burst, equipment may hum, stall, or trip protection circuits.
For a complete sizing workflow, use the WattSizing Calculator.
Quick Answer
Use this estimate:
Surge Watts = Running Watts x Startup Multiplier
Typical startup multipliers:
- Small fan motors:
1.5x to 2.5x - Pumps/compressors:
2x to 4x - Hard-start compressors:
4x to 7x(equipment-specific)
Step-by-Step Method
- Determine running watts from nameplate or measured input.
- Select startup multiplier based on motor type.
- Estimate surge watts.
- Add other simultaneous loads during startup event.
- Add design margin (commonly 15% to 25%).
Formula for planning peak event:
Required Capacity = (Largest Surge Event + Other Running Loads) x Margin
Surge Estimation Table
| Load Type | Running Watts (Example) | Startup Multiplier | Estimated Surge Watts |
|---|---|---|---|
| Box fan motor | 120 W | 2x | 240 W |
| Refrigerator compressor | 180 W | 3x | 540 W |
| 3/4 HP well pump | 1,300 W | 2.5x | 3,250 W |
| Air compressor | 1,500 W | 4x | 6,000 W |
Worked Example
Scenario:
- Well pump running:
1,300 W - Pump startup multiplier:
2.5x - Refrigerator running:
180 W - Lighting and network:
220 W
Calculations:
- Pump surge:
1,300 x 2.5 = 3,250 W - Peak event:
3,250 + 180 + 220 = 3,650 W - Add 20% margin:
3,650 x 1.20 = 4,380 W
Practical selection: generator/inverter with at least 4,500 W to 5,000 W surge-capable capacity, depending on duty and environment.
Related guides:
Real-World Factors That Change Surge
- Supply voltage dip during startup.
- Cable length and wire gauge.
- Motor age, bearing condition, and load pressure.
- Ambient temperature and frequent restart intervals.
When possible, verify with a true power meter or clamp meter that captures inrush behavior.
FAQs
Is startup surge the same as locked-rotor current?
Not exactly. Locked-rotor is a worst-case condition; actual startup surge can be lower but is still substantial.
Can soft starters reduce surge watts?
Yes, for compatible motors they can reduce startup spikes and improve generator/inverter compatibility.
Should I add all surges together?
Usually size for the largest likely simultaneous event, not impossible worst-case stacking of every load.
Why does my generator trip only when the compressor starts?
Running load is fine, but startup surge briefly exceeds available surge capability.
CTA
Want a practical surge-aware sizing result instead of guesswork? Use the WattSizing Calculator to model running watts, startup events, and safety margin.
