Size your lithium iron phosphate battery bank for off-grid solar. Enter your energy needs and system voltage to get the exact configuration.
LiFePO4: 3.2V/cell nominal, 3.65V full, 2.5V cutoff, 85% usable DoD, 3,000–5,000 cycle life.
| Feature | LiFePO4 | Li-Ion (18650) | AGM Lead-Acid |
|---|---|---|---|
| Usable DoD | 85–100% | 80% | 50% |
| Cycle Life | 3,000–5,000 | 500–1,000 | 300–500 |
| Thermal Stability | Excellent | Moderate | Good |
| Weight (per kWh) | ~13 kg | ~7 kg | ~33 kg |
| Cost per cycle | Lowest | Medium | Highest |
| Fire Risk | Very Low | Higher | Low (but acid) |
Multiply your daily watt-hour usage by the number of days of autonomy you want, then divide by 0.8 (80% usable depth of discharge for LiFePO4). For example, 5,000 Wh/day × 3 days ÷ 0.8 = 18,750 Wh of battery storage. Convert to amp-hours by dividing by your system voltage (12V, 24V, or 48V). The calculator above does all this and recommends a series/parallel cell configuration.
4 cells in series make a nominal 12.8V LiFePO4 battery (4 × 3.2V). To increase capacity, add cells in parallel: 4s2p doubles capacity, 4s4p quadruples it. A 12V 200 Ah bank uses 4 × 200 Ah cells in series, or 8 × 100 Ah cells in 4s2p. Always use a 4s BMS rated for your maximum continuous discharge current.
LiFePO4 batteries can be discharged 80–90% without significantly affecting cycle life — far better than lead-acid (50%) or AGM (60%). Most off-grid system designs use 80% as the planning number to leave headroom for cold weather and aging. Quality LiFePO4 cells last 3,000–6,000 cycles at 80% DoD, which is 8–15+ years of daily cycling.
A quality LiFePO4 cell rated at 6,000 cycles will last 16+ years if cycled once per day. Even cheaper grade-B cells deliver 3,000+ cycles (8 years). Calendar aging adds slow degradation regardless of use, but properly stored LiFePO4 retains 80%+ capacity after 10 years. This is 4–5× longer than lead-acid for similar usable capacity.
LiFePO4 wins on every metric except upfront cost. You get 80% usable capacity vs 50% for lead-acid, 4–5× longer life, higher charge/discharge rates, no maintenance, and no off-gassing. Initial $/Wh is roughly 2× lead-acid, but $/cycle is 3–5× cheaper. Only choose lead-acid if upfront budget is tight and you don’t mind replacing batteries every 3–5 years.
LiFePO4 cells must not be charged below 32°F (0°C) — doing so causes lithium plating and permanent damage. Discharging is safe down to about -4°F (-20°C) but reduces capacity. For cold climates, choose batteries with built-in self-heating (most quality drop-in batteries have this) or install in a heated enclosure. Many cabins solve this with a small thermostatic battery heater pad.