Path-Q Solar
How should battery power be sized differently from battery energy capacity?
Answer
Size battery power in kilowatts for the loads that must operate at the same moment, and size energy capacity in kilowatt-hours for how long those loads must run. A battery can have enough stored energy but insufficient power for a high-demand load, or enough power but too little energy for the desired duration.
Key points
- Power determines whether the inverter can start and run the selected loads simultaneously.
- Energy determines runtime after accounting for usable capacity, reserve limits, efficiency, and battery conditions.
- Starting surges and intermittent loads must be included in power sizing.
- Longer backup duration may require more energy capacity even when peak power is unchanged.
What it depends on
- Critical-load list and simultaneous operating power
- Motor-starting and surge requirements
- Desired backup duration and load duty cycle
- Usable battery capacity, reserve settings, and efficiency
Risks and trade-offs
- Sizing only by daily kilowatt-hours can leave the system unable to start key appliances.
- Sizing only by peak kilowatts can provide inadequate runtime.
- Adding capacity for rare peaks may be less economical than managing or separating loads.
Information you may need
- Critical-load inventory and nameplate ratings
- Expected simultaneous and time-varying operation
- Desired outage duration
- Battery and inverter usable ratings
Your next step
What to do next
Create a critical-load schedule showing simultaneous kilowatts and hourly kilowatt-hours, then size inverter power and battery energy as separate requirements.
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