How to size a LiFePO4 battery bank for backup: loads, runtime, usable kWh, and inverter power

List every circuit and device the battery bank will support during an outage. Include resistive loads (heating, lights), inductive loads (motors, pumps), and electronics (routers, PLCs). For each item record continuous power (W), surge power (W), and duty cycle or expected hours per day.

Calculate average daily energy (Wh) and peak instantaneous power. For B2B projects, keep an allowance for future expansion, startup currents, and inefficiencies from inverters and loads. Factory testing and sample approval should validate that reported surge and continuous ratings meet real application needs.

LiFePO4 cells are often rated in nominal kWh, but usable kWh depends on recommended depth-of-discharge and battery management system settings. For backup, many integrators choose 80–90% DoD to maximize runtime while preserving cycle life—but verify the exact DoD and recommended operating limits in the product documentation.

Calculate usable energy: Usable kWh = Nominal kWh × Usable DoD. Then convert to Wh and divide by the average daily energy from your load audit to estimate runtime. Remember system losses: inverter efficiency, wiring losses, and temperature derating reduce available energy—use measured or factory-tested efficiency figures for accuracy.

Select an inverter with continuous and surge capacities above your highest expected loads. For critical backup, size continuous power to at least the summed continuous demand, and ensure the inverter’s surge rating covers motor starts and compressor inrush.

Match inverter DC input voltage to the battery bank configuration (series/parallel arrangement for LiFePO4 modules). Confirm model-specific documentation and factory test reports for continuous duty, thermal limits, and recommended ambient conditions. Also verify transport documents and installation instructions, as weight and mechanical dimensions affect mounting and shipment planning.

Modern LiFePO4 battery packs use communication interfaces such as CAN bus and RS485 to report state of charge, cell voltages, temperatures, and alarms. For tight inverter integration, verify CAN bus battery inverter communication and CAN BMS battery compatibility in the product specification. Ask for model-specific CAN message maps and wiring diagrams during sample approval.

Ask "what is CAN communication in a solar battery" and require the vendor to provide the exact CAN protocol (e.g., J1939-like frames or proprietary mapping). Also verify the physical CAN port battery inverter compatibility: connector type, termination requirements, and voltage levels.

RS485 is still common for legacy or industrial inverters. Confirm RS485 battery inverter communication and ask "what is RS485 on a LiFePO4 battery" in procurement queries. Request Modbus register maps or protocol documentation and test cases to validate RS485 LiFePO4 battery inverter compatibility before bulk shipment.

Verify shipping documents, UN transport classification, and any country-specific battery transport requirements for the United States and European Union. Confirm that the product documentation lists operating temperature ranges, IP ratings, and mechanical mounting details so installation teams can prepare racks, ventilation, or enclosures.

Check certificates and compliance statements relevant to your market and application. State explicitly in procurement that compatibility, compliance, and certificates must be verified for the exact product and market—do not assume generic claims. Include factory testing and sample approval as conditions of purchase.