The 12V Lithium Battery Calculator estimates runtime, watt-hours, usable energy, battery-side current and daily amp-hour requirements for a 12V lithium battery system. It is designed around common 12.8V LiFePO4 house batteries while keeping nominal voltage editable for the exact battery you own.
Use it for RV house power, marine electronics, solar storage, portable DC systems and backup loads, then verify the result against your battery, BMS, inverter, charger, cables and protection devices.
Enter the ratings from your battery and the average power of the load. The 12V Lithium Battery Calculator keeps depth of discharge and delivery efficiency separate so you can see how reserve and conversion losses affect the result instead of hiding them inside one assumption.
Defaults use a common 12.8V, 100Ah LiFePO4 example. Replace them with the values from your battery label or datasheet.
Estimated continuous runtime
A 100Ah, 12.8V battery at 90% planned DoD and 90% delivery efficiency is estimated to support a constant 100W load for about 10.37 hours.
1,280 Wh × 90% DoD × 90% efficiency ÷ 100 W = 10.368 hours.
The calculator follows the same energy relationships used when converting battery amp-hours into watt-hours and then comparing that stored energy with a load measured in watts.
A battery label often emphasizes amp-hours, while appliances and inverters are usually discussed in watts. Those units describe different things. Amp-hours measure electrical charge capacity. Watt-hours describe energy. To estimate runtime, the first step is therefore to convert battery Ah into Wh using nominal battery voltage. If you need only that conversion, the Battery Ah to Wh Calculator provides a focused version of the same arithmetic.
A 100Ah battery at 12.8V has 1,280Wh of nominal energy. A 200Ah battery at the same voltage has 2,560Wh. This is why an Ah number should not be interpreted without voltage when you are comparing stored energy. The broader Battery Capacity Calculator is useful when you want to compare Ah, Wh and kWh across several system voltages.
The next step applies planned depth of discharge. This is the percentage of nominal energy you intend to remove before recharging or leaving a reserve. At 90% planned DoD, a 1,280Wh battery contributes 1,152Wh of usable battery energy in the model.
Depth of discharge is not the same thing as conversion efficiency. The calculator keeps them separate. If a load is powered through an inverter, energy is lost during conversion. At 90% delivery efficiency, 1,152Wh becomes 1,036.8Wh available to the modeled load.
Finally, runtime is delivered watt-hours divided by the average load in watts. A steady 100W load supplied with 1,036.8Wh gives 10.368 hours. For a dedicated cross-check, use the Battery Runtime Calculator, which focuses specifically on runtime rather than the wider 12V planning workflow.
The current estimate works in the other direction. For a simplified battery-side current calculation, load watts are divided by nominal voltage and delivery efficiency. At 100W, 12.8V and 90% efficiency, the model returns about 8.68A. That number helps show the scale of the load, but it does not replace a proper conductor or protection calculation. Use the site's Wire Gauge Calculator as a first voltage-drop check and then confirm the final circuit against the equipment instructions and applicable electrical requirements.
A “12V lithium battery” is a system class, not a promise that terminal voltage remains exactly 12.0V at every state of charge.
Many 12V-class LiFePO4 batteries are built from four cells in series and are commonly specified around 12.8V nominal. Their actual terminal voltage changes during charging and discharging. The calculator therefore uses 12.8V as a practical default but lets you edit the value so the arithmetic matches your battery datasheet.
If you are unsure how nominal voltage differs from resting or charging voltage, the Lithium Battery Voltage Chart explains the difference and provides a better context for voltage readings.
Two batteries can both be rated 100Ah and still store different amounts of energy if their nominal voltages differ. A 12.8V 100Ah battery is 1.28kWh nominal, while a 25.6V 100Ah battery is 2.56kWh nominal. That is why watt-hours are useful when comparing systems.
If your project is not limited to 12V, compare the same load with the 24V Lithium Battery Calculator or 48V Lithium Battery Calculator. Higher system voltage can reduce current for the same power level.
For batteries that the manufacturer allows to be paralleled, voltage stays approximately the same while Ah adds. Two compatible 12.8V 100Ah batteries can be treated arithmetically as a 12.8V 200Ah bank. The bank then has 2,560Wh nominal energy.
Compatibility still matters. Battery model, age, state of charge, cable arrangement and manufacturer limits can affect whether parallel operation is appropriate. The Series / Parallel Battery Calculator can help with the electrical arithmetic after you verify the permitted configuration.
Runtime is based on a particular average load. Daily sizing is based on total watt-hours consumed over a day. A refrigerator may draw a higher power while its compressor runs but cycle on and off, while lights and electronics may run for many hours at lower power.
For a daily target, the calculator divides load-side Wh by voltage, DoD and efficiency to estimate nominal Ah. Use the Battery Sizing Calculator when you need a broader appliance-by-appliance sizing workflow.
These 12V Lithium Battery Calculator examples use 12.8V nominal LiFePO4 batteries and fixed assumptions so the calculations are easy to reproduce and compare by hand.
| Battery | Efficiency | Delivered Energy | Average Load | Estimated Runtime |
|---|---|---|---|---|
| 12.8V 100Ah | 100% | 1,152Wh | 100W | 11.52 hours |
| 12.8V 100Ah | 90% | 1,036.8Wh | 100W | 10.37 hours |
| 12.8V 100Ah | 90% | 1,036.8Wh | 300W | 3.46 hours |
| 12.8V 200Ah | 90% | 2,073.6Wh | 500W | 4.15 hours |
| 12.8V 280Ah | 92% | 2,801.7Wh at 85% DoD | 800W | 3.50 hours |
Start with 100Ah × 12.8V = 1,280Wh nominal. At 90% DoD, usable battery energy is 1,152Wh. At 90% delivery efficiency, 1,036.8Wh reaches the modeled load. Divide by 100W and the result is 10.368 hours, or about 10 hours and 22 minutes.
The battery energy is unchanged, but a 300W load consumes it three times faster than a 100W load. Using the same 1,036.8Wh delivered-energy estimate gives about 3.46 hours. The estimated battery-side current also rises to roughly 26A at 90% efficiency.
A 200Ah battery at 12.8V stores 2,560Wh nominal. With 90% DoD and 90% efficiency, the model delivers 2,073.6Wh. Dividing by 500W gives about 4.15 hours. Actual inverter efficiency can vary with load, so use the manufacturer's data when available.
A 2,400Wh daily load at 12.8V, 90% DoD and 90% efficiency requires about 231.5Ah nominal before extra reserve. Round according to available battery sizes, future loads and charging conditions rather than treating the calculated value as an exact product recommendation.
The energy math is the same, but the way you estimate loads and charging opportunities changes with the application.
List refrigerator energy, lights, fans, water pump use, electronics and inverter loads. Daily Wh is often more useful than one appliance runtime because many RV loads cycle throughout the day. Use the RV Lithium Battery Calculator for a more application-specific estimate.
A battery must be large enough for the loads and practical to recharge with the available solar energy. Daily consumption, weather, controller limits and desired autonomy all matter. Continue with the Solar Battery Calculator or Solar Battery Bank Calculator when charging and backup days are part of the design.
Navigation electronics, lighting, pumps, refrigeration and communications can have very different duty cycles. A house-bank runtime estimate does not prove suitability for engine starting or propulsion. Use the Marine Lithium Battery Calculator for boat-specific planning.
12V current rises quickly with power: a 1,000W AC load at 12.8V and 90% efficiency represents about 86.8A at the battery in this simplified model. That is why BMS limits, cable size, connections and over-current protection become especially important as inverter loads increase.
A transparent calculator is useful because you can inspect its assumptions. It is also important to understand what the equations do not model.
The 12V Lithium Battery Calculator assumes a steady average load, a fixed nominal voltage and fixed percentages for planned DoD and delivery efficiency. Real batteries and electrical systems are dynamic. Available capacity can change with temperature, age, state of charge and operating conditions. An inverter can have idle consumption, and its efficiency may change with load. Compressors, pumps and motors can draw much more power for a short time during startup.
A battery management system can also disconnect a battery because of current, voltage or temperature protection before a simple energy calculation reaches zero. The Lithium Battery BMS Guide explains why those protection limits must be checked independently from stored-energy math.
Do not use 12.8V as a charger setting. Nominal battery voltage and charging voltage are different. Follow the exact battery and charger instructions. As manufacturer-specific references, see Victron's Lithium Battery Smart introduction and its operation guidance. Their values apply to the products they document and should not be generalized to every lithium battery.
Common 12V Lithium Battery Calculator questions about runtime, watt-hours, current, efficiency assumptions and capacity sizing.
Using 12.8V nominal voltage, 90% planned DoD and 90% delivery efficiency, the estimate is about 10.37 hours. With an idealized 100% delivery setting, the same battery and DoD would calculate to 11.52 hours. Real runtime can be shorter because of load variation, temperature, inverter idle consumption and other losses.
At 12.8V nominal, 100Ah × 12.8V = 1,280Wh, or 1.28kWh. If you use exactly 12.0V in the arithmetic, the result is 1,200Wh. Enter the nominal voltage specified for the battery you actually own rather than assuming every 12V-class lithium battery is identical.
Many 12V-class LiFePO4 batteries use four cells in series and are commonly described with a nominal pack voltage around 12.8V. The field remains editable because lithium chemistry and product design vary. Nominal voltage is also different from the live voltage you measure while charging or discharging.
Enter the percentage of nominal energy you intend to use before recharging or keeping a reserve. There is no single percentage that is correct for every battery. Follow the battery documentation, warranty conditions and your own reserve requirements.
Yes. The delivery-efficiency field reduces usable battery energy before runtime is calculated. For example, 90% efficiency removes 10% from the modeled energy reaching the load. The calculation does not separately add inverter standby consumption, so include meaningful idle power in the average load when necessary.
At 12.8V nominal and 90% efficiency, the simplified calculation is about 86.8A. Actual current varies with battery voltage and inverter behavior, and startup surges can be higher. Confirm the inverter, battery, BMS, cables and protection devices for the actual operating current.
The basic Ah, voltage, Wh and runtime equations still apply if you enter the correct nominal voltage and realistic operating assumptions for that battery. Do not treat the 12.8V LiFePO4 default or any charge guidance as a specification for another lithium chemistry.
Enter the total load-side watt-hours you expect to use in a day. The calculator divides that value by nominal voltage, planned DoD and delivery efficiency. The result is a nominal Ah estimate before adding any extra reserve for future loads, weak charging conditions, aging or additional autonomy.
No. It calculates battery energy, runtime, current and a daily Ah target. Solar and charger sizing also depend on charging power, available sun, controller limits, battery charge acceptance and the time available to recharge. Use a solar or charge-time calculator for those decisions.
No. It is a planning estimate based on the numbers you enter. Real performance depends on the battery, temperature, state of charge, aging, load variation, wiring, BMS behavior and conversion losses. Measure actual system power where possible and retain reserve for important loads.
Run the calculator with your actual battery label and average load, compare a few capacity options, then verify the electrical system before installation. For broader voltage and chemistry comparisons, continue with the Lithium Battery Calculator.
Recalculate Your Battery