Formula: Required kWh = Daily kWh × Days ÷ (DoD × System Efficiency)
LFP DoD tip: 80% DoD gives 4,000+ cycles. 100% DoD reduces cycle life significantly.
Autonomy days: 1–2 days for grid backup, 3–5 days for off-grid, 7+ for critical loads
About This Calculator
What this tool does: Estimates how many days a battery bank can support loads before reaching minimum allowable SOC.
Core idea: Backup duration comes from usable energy divided by average daily consumption.
Mini Example
With 12 kWh usable storage and 4 kWh/day critical load, backup duration is about 3 days.
Quick Literacy Notes
- Usable capacity is not nameplate capacity; account for DoD and conversion losses.
- Critical-load-only operation extends autonomy significantly during outages.
- Temperature and battery aging reduce real-world backup days over time.
Common Mistakes
- Using nameplate capacity instead of usable capacity.
- Not separating critical and non-critical loads.
Key Takeaways
- Backup duration comes from usable energy divided by average daily consumption.
- Usable capacity is not nameplate capacity; account for DoD and conversion losses.
- Avoid this mistake: Using nameplate capacity instead of usable capacity.
Practical Checklist
- Separate critical loads from optional loads before autonomy sizing.
- Use usable capacity after DoD, reserve SOC, and conversion losses.
- Stress-test outage scenarios with low-temperature and aged-capacity assumptions.
FAQ
Q1: Which load definition should I lock first for backup-day planning?
Quick Answer: Validate this first: Usable capacity is not nameplate capacity; account for DoD and conversion losses.
Engineer Note: If this assumption drifts from real conditions, downstream outputs can remain numerically neat but operationally wrong. Confirm with measured or site-specific inputs before locking decisions.
Q2: What mistake most often overstates outage autonomy?
Quick Answer: Avoid this first: Using nameplate capacity instead of usable capacity.
Engineer Note: In practice, the next failure mode usually follows: Not separating critical and non-critical loads. Address both together; correcting one while keeping the other often leaves the design bias unchanged.
Q3: When should I run outage scenarios with seasonal derating assumptions?
Quick Answer: Use this calculator for fast screening and scenario comparison.
Engineer Note: For procurement, warranty, compliance, or commissioning decisions, move to detailed verification with datasheets, measured conditions, and project constraints. Core rule: Backup duration comes from usable energy divided by average daily consumption.
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