LiFePO4 Battery Sizing Guide
A useful battery estimate begins with energy use, not a popular amp-hour size. Build a load list, calculate watt-hours, include losses and reserve, then verify power and charging separately.
Build an energy budget
For each load, multiply average watts by hours of use. Add the results to obtain daily watt-hours. For cycling loads such as refrigerators, pumps, and compressors, measured average energy is more useful than multiplying nameplate watts by 24 hours.
Separate essential and optional loads. This shows which behavior change can reduce battery size during an outage or off-grid period.
| Load | Average watts | Hours | Energy |
|---|---|---|---|
| Refrigerator | 150W | 8 equivalent | 1,200Wh |
| Lighting | 60W | 6 | 360Wh |
| Internet | 25W | 24 | 600Wh |
| Laptop | 65W | 5 | 325Wh |
| Total | — | — | 2,485Wh |
Adjust for efficiency and reserve
Divide load energy by inverter efficiency and the usable battery fraction. For 2,485Wh, 90% inverter efficiency, and a 20% reserve, nominal capacity is 2,485 divided by 0.90 divided by 0.80, or about 3,451Wh.
Reserve is not a universal safety number. It can cover forecast error, battery aging, temperature, or the desire to avoid routine operation near disconnect limits.
| Nominal voltage | Approximate Ah for 3,451Wh |
|---|---|
| 12.8V | 270Ah |
| 25.6V | 135Ah |
| 51.2V | 68Ah |
Energy and power are different checks
A bank can store enough energy yet fail to start a pump or support the inverter’s continuous demand. Add simultaneous running watts, identify startup surge, and convert inverter power to DC current at the lowest expected battery voltage.
Confirm battery BMS continuous and peak current, inverter surge duration, conductor requirements, and overcurrent protection. Do not infer power capability from amp-hours alone.
- Daily energy in Wh
- Required days or hours of autonomy
- Inverter efficiency
- Usable-capacity reserve
- Simultaneous watts and surge
- Battery and BMS current
- Available recharge energy
Include how the bank will recover
A battery sized for two days without considering solar, alternator, generator, or grid recharge may be unnecessarily large—or too small if expected charging does not materialize. Estimate charging energy under realistic conditions and respect maximum charge current.
Recalculate with a low, expected, and high load case. A range is more honest than a single precise answer when usage is uncertain.
Editorial sources
Equipment-specific requirements can change. Check the current manual for the exact battery, motor, inverter, and charger you own.