Path-Q Solar
How should battery sizing change if I want to preserve reserve capacity during outages?
Answer
Size the battery for the energy you want available to critical loads after applying the reserve percentage, rather than sizing only for normal daily cycling. A higher reserve reduces usable routine capacity and may require a larger battery to meet both daily-use and outage objectives.
Key points
- Define the minimum energy to reserve and the outage duration or operating objective.
- Estimate critical-load energy use and starting or surge power separately.
- Usable capacity is lower than the battery's nameplate capacity because of reserve limits and system losses.
- A larger battery may be needed if the reserve must remain available while the battery also serves daily loads.
What it depends on
- Reserved energy percentage
- Critical-load energy consumption
- Desired outage duration
- Battery usable capacity and round-trip efficiency
- Whether solar can recharge the battery during an outage
Risks and trade-offs
- A larger battery increases cost and space requirements.
- A high reserve can leave capacity unused during normal operation.
- If starting loads exceed inverter capability, additional battery energy alone will not solve the problem.
Information you may need
- Which circuits must operate during an outage
- Their expected wattage, operating hours, and motor-starting loads
- Desired reserve percentage and outage duration
- Battery and inverter usable-capacity specifications
Your next step
What to do next
Compare at least two designs using the same critical-load list: one optimized for routine cycling and one with the requested reserve, including usable energy, surge power, and expected outage duration.
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