Path-Q Solar

How can I estimate backup duration when appliances cycle on and off?

Answer

Estimate duration from usable battery energy divided by the average load over time, then account for inverter losses, reserve capacity, startup surges, and changing duty cycles. A time-based load profile is more useful than adding every appliance's nameplate wattage as if it ran continuously.

Key points

  • Separate continuous loads from cycling loads such as refrigeration, pumps, and heating equipment.
  • Estimate each cycling appliance's average operating fraction over the outage.
  • Check peak simultaneous power and starting surges independently from energy use.
  • Include a reserve so the battery is not planned to reach its absolute minimum state of charge.

What it depends on

  • Usable battery capacity and discharge limits
  • Average and peak load profile
  • Appliance duty cycles and starting requirements
  • Inverter efficiency and reserve settings

Risks and trade-offs

  • Duty cycles can increase during extreme weather or unusual household use.
  • Averages can hide short peak loads that trip the inverter.
  • Actual runtime will vary with battery age, temperature, and operating settings.

Information you may need

  • Appliance power measurements or reliable specifications
  • Expected outage duration and weather conditions
  • Battery usable capacity and efficiency assumptions
  • Loads that can be excluded or automatically shed

Your next step

What to do next

Create an hourly or shorter-interval load schedule for the outage scenario and test it against both battery energy and inverter power limits.

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