Path-Q Solar
How should battery capacity be adjusted for reserve energy that cannot be used routinely?
Answer
Size the battery from the energy required by the loads, then select enough nominal capacity so the required routine usable energy remains available after accounting once for the protected reserve, minimum state of charge, applicable discharge-path losses, temperature derating, and other operating limits. Do not treat nameplate capacity as fully available routine-use energy.
Key points
- A reserve setting reduces the portion of nominal capacity available for ordinary cycling.
- Usable AC energy is lower than available battery energy because of applicable discharge-path losses.
- The required reserve depends on the purpose of the reserve, such as outage protection or battery protection.
- Reserve settings may be configurable, but changing them can affect backup duration and battery longevity.
What it depends on
- Required load energy and outage duration
- Nominal and usable battery capacity
- Reserve percentage or absolute reserve
- Applicable discharge-path efficiency
- Temperature and state-of-charge derating
Risks and trade-offs
- A larger reserve improves outage readiness but reduces routine bill-savings or self-consumption use.
- Using reserve energy routinely may leave insufficient backup during an outage.
- Overly conservative assumptions increase cost; optimistic assumptions reduce resilience.
Information you may need
- Critical-load energy profile
- Battery usable-capacity specification
- Configured reserve policy
- Expected temperatures and outage-duration target
Your next step
What to do next
Calculate the required energy delivered to the loads, adjust it once for the applicable discharge-path efficiency and usable-capacity factors, and account for the protected reserve once.
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