Path-Q Solar

How should battery backup be sized for the startup surges and ongoing energy needs of refrigerators or pumps?

Answer

Size the system for both the motor's brief starting demand and its ongoing running energy. Battery energy capacity handles duration, while the inverter and battery power capability handle startup and simultaneous loads.

Key points

  • Identify the appliance's running watts, starting watts or locked-rotor demand, and expected operating hours.
  • Allow for other loads that may start at the same time.
  • Check the inverter's surge rating, duration of that rating, and motor-starting capability.
  • Include usable battery capacity rather than only the battery's nominal capacity.
  • Account for conversion losses, reserve capacity, temperature effects, and battery operating limits.

What it depends on

  • Measured or documented starting and running demand
  • Required backup duration and duty cycle
  • Other backed-up loads and their coincidence
  • Inverter surge capability and battery discharge limits

Risks and trade-offs

  • A battery with enough energy for several hours may still fail if the inverter cannot start the motor.
  • Oversizing for rare starts increases cost, while undersizing can cause nuisance shutdowns or equipment failure.
  • Repeated motor starts can reduce available backup duration and increase battery stress.

Information you may need

  • Appliance nameplate data or measured startup current
  • Expected operating schedule and backup duration
  • Complete list of simultaneous backup loads
  • Candidate inverter and battery power and energy ratings

Your next step

What to do next

Create a load schedule showing running watts, starting watts, start timing, and required hours, then have the candidate system checked against both power and usable-energy limits.

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SQ02683