Path-Q Solar

How much battery capacity is needed to keep essential loads running overnight?

Answer

Estimate the energy used by the essential circuits during the intended backup period, then add allowance for inverter losses, battery reserve limits, temperature, aging, and uncertain usage. A simple starting calculation is: required nominal battery capacity equals essential energy use divided by the usable fraction and system efficiency. The battery must also meet the loads' instantaneous power and starting demands.

Key points

  • List each essential load, its running watts, hours of use, and any motor-starting surge.
  • Separate energy capacity, measured in kilowatt-hours, from inverter power capacity, measured in kilowatts.
  • Usable capacity is less than the nameplate rating because systems commonly reserve energy and experience conversion losses.
  • Heating, pumps, refrigerators, medical equipment, communications, and lighting may have different duty cycles.

What it depends on

  • Essential-load wattage and operating hours
  • Desired backup duration and usage behavior
  • Battery usable-capacity limits and inverter efficiency
  • Starting or surge requirements and ambient temperature

Risks and trade-offs

  • A battery sized only by average energy can fail when multiple loads start together.
  • Oversizing raises cost and may leave capacity unused, while undersizing requires load shedding.
  • Actual overnight demand can differ substantially from estimates during extreme weather or emergencies.

Information you may need

  • Circuit-level load list or measured consumption
  • Desired overnight hours and minimum state of charge
  • Battery usable capacity, inverter rating, and surge specification
  • Loads that must start automatically or run continuously

Your next step

What to do next

Create an essential-load schedule with watts, operating hours, and starting requirements, then have a designer size both the usable battery capacity and inverter power.

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