Path-Q Solar

How should motor-starting surges affect the inverter size for backup power?

Answer

Size the inverter to handle both the continuous load and the highest relevant motor-starting surge, including situations where more than one motor may start close together.

Key points

  • Refrigerators, pumps, compressors, fans, and well equipment can briefly draw much more power when starting.
  • Use measured or manufacturer-provided starting data when available rather than relying only on running watts.
  • The inverter's surge rating must specify both its magnitude and duration.

What it depends on

  • Starting characteristics of each motor
  • Whether motors start simultaneously or sequentially
  • Inverter surge capacity and duration
  • Battery voltage and ability to sustain the surge

Risks and trade-offs

  • An undersized inverter may trip or fail to start a motor even when average demand is modest.
  • A larger inverter can increase equipment and installation cost and may have higher standby consumption.
  • Soft-start devices or controls may reduce surge demand but require compatibility assessment.

Information you may need

  • Running and locked-rotor or startup current for each motor load
  • The intended backup operating sequence
  • The inverter's continuous and surge specifications

Your next step

What to do next

Have the installer calculate the worst credible simultaneous startup condition and match it to the inverter's documented surge capability.

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