Path-Q Solar
How should a backup battery's power rating be selected to handle the startup surge of refrigerators or pumps?
Answer
Select a battery and inverter based on both continuous power and the highest starting surge of the loads that may start together. The system must also sustain the surge for the required duration and remain within its voltage, current, motor-starting, and backup-output limits.
Key points
- Motor loads can briefly draw substantially more power during startup than during normal operation.
- The relevant calculation includes simultaneous starts, compressor or pump type, starting method, and whether loads can be sequenced.
- Battery energy capacity determines runtime, while inverter and battery power capability determine whether loads can start and run.
What it depends on
- Measured or manufacturer-listed running and starting power
- Number of loads that may start at the same time
- Inverter surge duration and output capability
- Battery maximum continuous and peak discharge limits
- Load-management or staggered-start controls
Risks and trade-offs
- A system with adequate energy capacity may still trip if its instantaneous power rating is insufficient.
- Oversizing for rare simultaneous starts increases cost and may reduce efficiency.
- Starting loads sequentially can reduce required peak capacity but adds controls and operational constraints.
Information you may need
- Nameplate and, preferably, measured startup characteristics
- List of backed-up loads and possible simultaneous operation
- Battery and inverter continuous, peak, and surge specifications
- Required backup duration and load-management options
Your next step
What to do next
Create a backed-up-load and startup-surge schedule, then have the designer verify it against the exact battery and inverter specifications.
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