What Size Solar Panel for a 100Ah LiFePO4 Battery?
Estimate solar watts for a 100Ah lithium battery from voltage, state of charge, target recharge time and peak-sun-hours.
A 100Ah battery has different energy depending on its voltage. Convert the battery to watt-hours before deciding how much solar you need.
A nominal 12.8V 100Ah LiFePO4 battery stores about 1.28kWh. Recharging most of that battery in one strong day commonly points to roughly 300–500W of solar, depending on sun hours, charging losses and how deeply the battery is discharged.
What matters
- A nominal 12.8V 100Ah battery is about 1.28kWh.
- You rarely need to replace 100% of capacity every day.
- Charging efficiency and real-world panel yield reduce delivered energy.
- The charge controller must support the planned array.
Decision factors that change the answer
Battery energy
100Ah alone is incomplete; 100Ah at 12.8V is about 1.28kWh, while 100Ah at 24V is roughly double the energy.
Depth of discharge
Replacing 50% state of charge requires about half the energy of a near-empty refill.
Sun hours
More peak-sun-hours reduce the array size needed for same-day recovery.
Charge current
Controller and battery charge-current limits must support the proposed array power.
How to size it
- Multiply nominal volts by Ah to get battery Wh.
- Estimate the Wh you need to replace after a normal day.
- Divide by effective peak-sun-hours and system efficiency.
- Verify battery charge-current and controller limits.
Replacing about 1kWh in five effective sun-hours requires more than 200W of nameplate solar once real-world losses are included.
Worked example
Replacing 1kWh with 4 peak-sun-hours and 80% solar-to-battery efficiency requires about 313W of panels. A 400W array gives practical margin; 200W may take closer to two good solar days.
Common buying mistakes
- Using Ah without voltage.
- Sizing from perfect laboratory panel output.
- Ignoring the battery manufacturer’s maximum charge current.
For a 12V-class 100Ah LiFePO4 battery, a few hundred watts of solar is often a rational starting range; calculate from the energy you actually need to replace.
Check the underlying numbers
Use these site resources to replace generic assumptions with your own load or the source-linked data behind this guide.
Use the numbers before choosing a model
Start with your load list. The battery needs enough watt-hours for the desired runtime and enough continuous/surge watts to operate the appliances at the same time. Then check whether your recharge method can replace the energy you expect to consume.
For outage planning, we recommend keeping reserve capacity instead of designing a system that reaches 0% at the end of the forecasted outage.
Next step
Estimate battery runtime → or Build a critical-load energy budget →.