Nominal Energy
Voltage × amp-hours before reserve or conversion losses.
LiFePO4 Battery Calculator
The LiFePO4 Battery Calculator estimates nominal battery energy, usable watt-hours, delivered energy, battery-side current, runtime, and the amp-hour capacity needed for a daily energy target. It is designed for lithium iron phosphate systems used in solar storage, RVs, boats, backup power, cabins, and other deep-cycle applications.
Enter your actual battery-bank voltage and amp-hour rating instead of relying on a generic “12V” label. Common LiFePO4 examples such as 12.8V, 25.6V, and 51.2V are included as quick presets.
LiFePO4 Battery Sizing and Runtime Tool
Calculate battery Wh, usable energy, delivered energy, runtime, approximate battery current, and required Ah using your own voltage, capacity, load, discharge allowance, and efficiency assumptions.
Enter Your Battery and Load
Default values model a 12.8V 100Ah LiFePO4 battery supplying a 200W load with 90% planned depth of discharge and 90% delivery efficiency.
Calculation Results
A 12.8V 100Ah battery contains 1,280Wh nominally. With 90% planned depth of discharge and 90% delivery efficiency, about 1,036.8Wh reaches the modeled load.
Usable Energy
Nominal energy after the selected depth-of-discharge allowance.
Delivered Energy
Modeled load-side energy after applying delivery efficiency.
Estimated Runtime
Delivered watt-hours divided by average running watts.
Battery-Side Current
Approximate steady current for the entered load and efficiency.
Required Daily Capacity
Nominal Ah required for the optional daily Wh target before extra design margin.
How the LiFePO4 Battery Calculator Works
The calculation separates nominal capacity, planned discharge, conversion losses, runtime, and daily sizing so you can see what each assumption changes.
A battery's amp-hour rating is only part of the energy story. A 100Ah battery at 12.8V contains half the nominal energy of a 100Ah battery at 25.6V. That is why this LiFePO4 Battery Calculator begins by converting voltage and amp-hours into watt-hours. Watt-hours provide a common energy unit that can be compared directly with appliance and inverter loads.
If you only need to convert battery ratings, the Battery Capacity Calculator is a useful companion. For a broader lithium calculation that is not limited to LiFePO4 assumptions, compare the Lithium Battery Calculator.
The calculator then applies the planned depth of discharge. This value represents the share of nominal battery energy you intend to use. It is deliberately adjustable because there is no single discharge percentage that is correct for every battery, warranty, reserve strategy, temperature, or application.
Delivery efficiency is treated separately. If an inverter or other conversion stage loses energy, usable battery watt-hours and load-side watt-hours are not identical. Keeping discharge allowance and efficiency as separate inputs makes the estimate easier to understand and avoids hiding two different effects inside one generic derating percentage.
Start with Real Battery Ratings
Use the complete battery-bank voltage and Ah rating shown by the manufacturer. Common LiFePO4 systems use nominal values such as 12.8V, 25.6V, and 51.2V, but the battery documentation should always take priority.
Compare battery voltagesSeparate Nominal and Usable Energy
Nameplate Wh is not automatically the amount you plan to remove. The depth-of-discharge input gives you a transparent reserve rather than assuming the entire battery should always be used.
Check battery runtimeModel Delivery Losses
AC inverters and other power-conversion stages are not perfectly efficient. Enter a realistic efficiency figure so runtime is based on modeled energy reaching the load rather than only battery-side energy.
Plan charging timeLiFePO4 Energy, Runtime and Capacity Formulas
The formulas behind the calculator are intentionally simple enough to verify by hand. That makes it easier to identify an unrealistic input before relying on the output.
1. Convert amp-hours to watt-hours
Amp-hours describe electrical charge. Watt-hours describe energy. Multiply the complete battery-bank Ah rating by its nominal voltage to calculate nominal energy.
Nominal battery energy (Wh) = Battery capacity (Ah) × Nominal voltage (V)
A 12.8V 100Ah LiFePO4 battery therefore contains 1,280Wh, or 1.28kWh, nominally. The same 100Ah rating at 25.6V represents 2,560Wh. This is why comparing batteries by Ah alone can be misleading.
2. Apply the planned discharge allowance
Usable battery energy (Wh) = Nominal Wh × (Depth of discharge ÷ 100)
At a 90% planned depth of discharge, a 1,280Wh battery provides 1,152Wh of modeled usable battery-side energy. A different battery, warranty, reserve target, operating temperature, or application may justify a different value, so this percentage should be based on the specific system rather than copied blindly.
3. Apply delivery efficiency
Delivered load energy (Wh) = Usable Wh × (Efficiency ÷ 100)
If the modeled usable energy is 1,152Wh and the chosen delivery efficiency is 90%, about 1,036.8Wh reaches the load in the simplified model. A direct DC load may have a different loss profile from an AC appliance powered through an inverter.
4. Calculate runtime
Runtime (hours) = Delivered energy (Wh) ÷ Average load (W)
At 1,036.8Wh delivered and a constant 200W load, estimated runtime is 5.184 hours, or about 5 hours and 11 minutes. Real loads may cycle on and off, change power level, or have startup surges, so average watts should represent expected operation rather than only a nameplate maximum.
5. Estimate battery-side current
Battery current (A) ≈ Load W ÷ [Nominal V × (Efficiency ÷ 100)]
For the default example, 200W divided by 12.8V × 0.90 is approximately 17.36A. This current estimate helps show scale, but it does not replace cable, fuse, connector, inverter, or BMS calculations.
6. Size capacity from a daily Wh target
Required Ah = Daily load Wh ÷ [Voltage × DoD fraction × Efficiency fraction]
A 1,200Wh daily target at 12.8V, 90% planned depth of discharge, and 90% efficiency requires about 115.7Ah before any extra autonomy or engineering margin. For charging after capacity is selected, review the lithium battery charging guide.
LiFePO4 Voltage Basics
A commonly used nominal LiFePO4 cell voltage is about 3.2V. Four cells in series are commonly represented as a 12.8V pack, eight as 25.6V, and sixteen as 51.2V. These are convenient planning references, not substitutes for the exact battery's specification.
Terminal voltage changes with state of charge, current, temperature, and battery design. The voltage entered here is a nominal energy-calculation value rather than a prediction of what a meter will display throughout discharge.
Series and Parallel Banks
Compatible batteries connected in series increase bank voltage while Ah remains the same. Compatible parallel batteries increase Ah while the nominal bank voltage stays the same. Manufacturer rules can restrict series or parallel operation.
After establishing a supported configuration, enter the resulting full-bank voltage and Ah rather than the rating of one individual battery. For voltage-specific planning, compare the 12V, 24V, and 48V lithium battery calculators.
BMS, Current and Protection Limits
A battery can contain enough energy for a load and still be unable to deliver the required current. The BMS may enforce continuous-current, surge-current, low-voltage, high-voltage, or temperature limits.
Always compare the calculated current with the battery's specifications and review the site's Lithium Battery BMS guide before treating an energy result as an installation decision.
Charging and Temperature
The calculator does not choose charging voltage, current, absorption settings, balancing behavior, or temperature limits. Those settings are product-specific and must come from the battery and charger documentation.
For one manufacturer example, see Victron Energy's LiFePO4 battery documentation and its operating guidance. Apply only specifications that match the battery you actually own.
| Example battery | Nominal energy | Delivered energy* | Example load | Estimated runtime |
|---|---|---|---|---|
| 12.8V 100Ah | 1,280Wh | 1,036.8Wh | 100W | 10.37 hours |
| 12.8V 200Ah | 2,560Wh | 2,073.6Wh | 500W | 4.15 hours |
| 25.6V 100Ah | 2,560Wh | 2,073.6Wh | 300W | 6.91 hours |
| 51.2V 100Ah | 5,120Wh | 4,147.2Wh | 800W | 5.18 hours |
*Example delivered energy assumes 90% planned depth of discharge and 90% delivery efficiency. These values are examples for transparent arithmetic, not universal operating recommendations.
Accuracy and safety note: actual capacity and runtime can vary with state of charge, temperature, battery age, cell balance, discharge current, inverter standby consumption, changing conversion efficiency, wiring losses, BMS behavior, and variable loads. Medical, emergency, navigation, fire-safety, and other critical systems should not be designed around a single calculator result.
Practical LiFePO4 Battery Examples
These worked examples show how voltage, capacity, load, reserve, and efficiency interact. They are arithmetic demonstrations rather than product recommendations.
1
12.8V 100Ah for a 200W Load
Nominal energy is 12.8 × 100 = 1,280Wh. Using 90% gives 1,152Wh of battery-side usable energy. At 90% delivery efficiency, the load receives about 1,036.8Wh.
Divide 1,036.8Wh by 200W and the estimate becomes 5.184 hours. In practice, an inverter's idle consumption and a device's changing power draw can shorten or lengthen elapsed runtime.
2
51.2V Bank for a 4kWh Daily Target
Suppose the load requires 4,000Wh per day. At 51.2V, 90% DoD, and 90% delivery efficiency, the battery requirement is about 96.5Ah before extra reserve or backup-day capacity.
That number addresses storage only. A solar system must also replenish the energy, so array size, weather, controller limits, and charging time remain separate checks.
3
RV or Marine Load Planning
A refrigerator, lights, pumps, fans, electronics, and inverter loads rarely run at one constant wattage all day. Estimate each device's watt-hours first, then combine them into a realistic daily energy budget.
Use the dedicated RV lithium sizing tool or marine battery calculator when application-specific load planning is more useful than one steady-load runtime estimate.
LiFePO4 Battery Calculator FAQs
Answers to common questions about battery watt-hours, runtime, discharge allowance, voltage, efficiency, current, and real-world accuracy.
Multiply nominal battery voltage by amp-hour capacity. A 12.8V 100Ah battery has 1,280Wh of nominal energy. A 25.6V 100Ah battery has 2,560Wh. This is why both voltage and Ah are needed when comparing stored energy.
It converts Ah and voltage into nominal Wh, applies the selected depth of discharge, applies delivery efficiency, then divides the remaining load-side Wh by average load watts. The calculation assumes a steady representative load rather than automatically modeling duty cycles or startup surges.
Use the planned usable percentage appropriate to your specific battery, reserve needs, warranty, operating conditions, and manufacturer guidance. The 90% default is an example, not a universal recommendation. Using 100% mathematically consumes all modeled nominal energy and leaves no planning reserve.
Energy can be lost between the battery and the load, especially when DC power is converted through an inverter. The efficiency input keeps this loss separate from depth of discharge. A 90% efficiency setting means the model assumes 90% of usable battery-side energy reaches the load.
Nominal energy is 12.8V × 100Ah = 1,280Wh, or 1.28kWh. Usable and delivered energy will be lower if you reserve part of the battery's capacity or account for conversion losses.
Yes. At 51.2V, 100Ah equals 5,120Wh. At 12.8V, 100Ah equals 1,280Wh. The Ah ratings are the same, but the higher-voltage bank contains four times the nominal energy in this comparison.
For the same power and similar efficiency, increasing voltage reduces current because power is related to volts multiplied by amps. That can be useful in larger systems, but voltage alone does not determine correct wiring, protection, inverter compatibility, or battery configuration.
No. The current output is only a steady-state planning estimate. Cable and protection sizing also depend on conductor length, insulation, ambient temperature, allowable voltage drop, fault current, connector limits, applicable standards, equipment requirements, and installation method.
Real loads can consume more power than expected, an inverter can draw standby energy, temperature can affect usable capacity, battery state of charge may be lower than assumed, and the BMS may disconnect before all modeled energy is used. Battery age, wiring losses, and load variation can also change measured runtime.
Yes, the underlying energy arithmetic applies to those applications when the inputs represent the actual battery and loads. Application-specific planning may still require additional checks for solar charging, alternator charging, shore power, environmental conditions, redundancy, inverter surge, and critical loads.
Size Your LiFePO4 System with Better Inputs
Use the LiFePO4 Battery Calculator with the ratings from your real battery, then verify the result against BMS current limits, inverter requirements, charger settings, wiring, fusing, temperature limits, and the manufacturer's documentation. A transparent estimate is most useful when every input reflects the system you actually plan to build.
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