How to Size a Home LiFePO4 Battery Bank: A Step-by-Step Sizing Guide
Undersize and you run out at night; oversize and you overpay. Here's the engineer's formula for sizing an off-grid or backup LiFePO4 battery bank — with a worked example.
The core sizing formula
1. Add up your daily energy (kWh/day)
2. Choose days of autonomy
| Use case | Days of autonomy |
|---|---|
| Grid-tied backup (essentials only) | 1 day |
| Off-grid, sunny climate | 2 days |
| Off-grid, cloudy/winter climate | 3–5 days |
3. Apply usable depth of discharge (DoD)
4. Add efficiency and a safety margin
Worked example
| Step | Value | Running total |
|---|---|---|
| Daily energy | 5 kWh/day | 5 kWh |
| × 2 days autonomy | × 2 | 10 kWh |
| ÷ 0.85 usable DoD | ÷ 0.85 | ≈ 11.8 kWh |
| ÷ 0.88 inverter eff. | ÷ 0.88 | ≈ 13.4 kWh |
| × 1.15 safety margin | × 1.15 | ≈ 15.4 kWh |
A note on system voltage (12 / 24 / 48 V)
Watch our aging & test line on YouTubeFrequently asked questions
What size battery bank do I need for a 5 kWh/day home?+
For 2 days of autonomy with LiFePO4, roughly 15 kWh after accounting for ~85% usable depth of discharge, inverter efficiency and a 10–15% margin. Multiply daily kWh by days of autonomy, then divide by DoD and efficiency.
What depth of discharge should I use for LiFePO4 sizing?+
Use 80–90% usable DoD for LiFePO4. That means a 10 kWh nominal bank delivers about 8–9 kWh before the BMS protects the cells — far more usable energy than lead-acid's ~50%.
How many days of autonomy should I plan for?+
1 day for grid-tied backup of essentials, 2 days for off-grid in sunny climates, and 3–5 days for off-grid systems in cloudy or winter conditions.
Should I choose a 12V, 24V or 48V system?+
Higher voltage means lower current, thinner cables and less loss. 12V suits small RV/marine systems, 24V mid-size, and 48V/51.2V is standard for whole-home storage. Match the bank to your inverter's input voltage.
