LiFePO₄ Battery Runtime Calculator
This calculator helps you estimate how long a LiFePO₄ (Lithium Iron Phosphate) battery will power a device based on battery capacity (Ah), voltage, and load (watts).
List of the Best LiFePO₄ Battery:
How to Calculate LiFePO₄ Battery Runtime
Estimating how long a LiFePO₄ (Lithium Iron Phosphate) battery will power a device depends on three core factors:
- Battery capacity (Ah): How much charge the battery can hold
- Voltage (V): The system’s nominal operating voltage (often 12.8V, 25.6V, or 51.2V for LiFePO₄)
- Load power (W): How much energy your device consumes per hour
The basic formula is:
Runtime (hours) = (Ah × V × Efficiency) / Load (W)
Where efficiency accounts for inverter or controller losses (usually ~85–95%).
Example:
A 100Ah battery @ 12.8V powers a 150W fridge at 90% efficiency:
100 × 12.8 × 0.9 = 1152 Wh usable
Runtime = 1152 ÷ 150 = 7.68 hours (~7h 41min)
Why Use LiFePO₄ Batteries?
LiFePO₄ batteries are popular in solar, RV, marine, backup, and mobile power applications because of their unique advantages:
- Longer lifespan: 2000–5000+ cycles vs 300–800 for lead-acid
- Deeper discharge: Safe to use 90–100% of rated capacity
- Lighter weight: Up to 60% lighter than lead-acid
- Consistent voltage: Holds steady voltage until nearly depleted
- Built-in BMS: Many come with Battery Management Systems for protection
What Voltage Should I Use?
Most LiFePO₄ batteries are labeled at nominal voltages that correspond to 4-cell multiples:
- 12.8V = 4S (4 x 3.2V cells)
- 25.6V = 8S
- 51.2V = 16S
Make sure you use the battery’s rated nominal voltage, not peak or charging voltage (e.g., 14.6V for 12.8V systems).
Including Efficiency Losses
In real-world conditions, not all stored energy reaches your device. Losses occur in:
- DC-AC inverters (typically 85–95%)
- Voltage regulators or converters
- Wiring and connection points
Our calculator includes an optional efficiency input. A safe assumption is:
- Pure DC system (no inverter): ~95%
- With inverter: ~85–90%
Runtime Estimation Table
Assuming 12.8V LiFePO₄ battery @ 90% efficiency:
| Battery (Ah) | Load (W) | Estimated Runtime (Hours) |
|---|---|---|
| 50Ah | 100W | ~5.76 hrs |
| 100Ah | 200W | ~5.76 hrs |
| 200Ah | 300W | ~7.68 hrs |
| 100Ah | 75W | ~15.36 hrs |
| 50Ah | 25W | ~23 hrs |
Frequently Asked Questions (FAQ)
Can I fully drain a LiFePO₄ battery to 0%?
Technically yes — many LiFePO₄ batteries support 90–100% depth of discharge. However, discharging to 10–20% prolongs lifespan.
Is voltage constant during discharge?
LiFePO₄ maintains a flat discharge curve. Voltage stays ~12.8V for most of the cycle and drops rapidly near 5–10% capacity.
Should I size my system with a margin?
Yes — add 20–30% margin to runtime estimates to accommodate inverter startup surges, temperature effects, or aging batteries.
How accurate is this calculator?
It gives a good estimate under ideal conditions. Real-world factors like load fluctuation, inverter idle draw, and temperature may reduce runtime.
Can I use this for other battery types?
Yes — the math applies to any battery chemistry, but LiFePO₄ is especially suited due to its flat voltage curve and high discharge tolerance.
Real-World Use Cases
- RV boondocking: Estimate how long a 100Ah 12V LiFePO₄ battery can power lights, fridge, fan, and a small inverter.
- Solar storage: Plan battery bank size based on overnight power needs.
- Off-grid cabin: Understand how long LED lighting or a TV can run after sunset.
- Portable power station: Know runtime for CPAP, laptops, or tools before recharging.
Conclusion
LiFePO₄ batteries offer reliable, long-lasting energy storage. With their consistent voltage, deep discharge ability, and high cycle count, they’re ideal for modern off-grid and mobile power systems. This calculator helps you predict runtime with practical accuracy by factoring in real-world efficiency. Use it to design smarter systems, prevent surprises, and maximize your power budget.