Lithium Battery Calculator
The Lithium Battery Calculator estimates battery energy, usable watt-hours, runtime, battery-side current, and the amp-hour capacity needed for a daily energy target. Enter the real ratings from your lithium battery and load to compare 12V, 24V, 48V, or custom-voltage systems without treating amp-hours as a complete energy measurement.
Use it for solar storage, RV house power, marine systems, portable power, or backup planning, then confirm the result against the exact battery, BMS, inverter, charger, cable, and protection-device specifications.
Calculate Lithium Battery Runtime and Capacity
Use the interactive tool to convert Ah to Wh, apply a planned depth of discharge and delivery efficiency, estimate runtime under a steady load, and calculate the nominal Ah needed for a daily energy target.
Runtime, Energy and Ah Sizing Estimator
Defaults show a 12.8V 100Ah LiFePO4 example. Change any value, including decimals, and calculate again.
Estimated Results
Continuous runtime at the entered average load
1,280 Wh × 90% DoD × 90% efficiency ÷ 100 W = 10.368 hours.
For a dedicated load-duration comparison, cross-check the result with the Battery Runtime Calculator.
How the Lithium Battery Calculator Works
The formulas are intentionally transparent so you can reproduce the result by hand, understand what each percentage changes, and avoid common mistakes when comparing batteries at different voltages.
The Lithium Battery Calculator begins by converting amp-hours into watt-hours. Amp-hours measure charge, not complete stored energy, so the battery's nominal voltage is required. A 100Ah battery at 12.8V contains 1,280Wh nominally, while a 100Ah battery at 25.6V contains 2,560Wh. The Ah numbers match, but the energy does not. If you only need that conversion, use the site's Watt Hour Calculator or the broader Battery Capacity Calculator.
Next, the tool applies the planned depth of discharge. This is a user-selected operating assumption, not a universal recommendation. At 90% DoD, a 1,280Wh battery contributes 1,152Wh of usable battery-side energy to the model. Delivery efficiency is then applied separately. At 90% efficiency, the modeled load receives 1,036.8Wh. Keeping reserve and conversion losses separate makes the result easier to interpret.
The Lithium Battery Calculator also keeps runtime and daily sizing separate because they answer different planning questions. Runtime asks how long an existing battery may support a steady average load. Daily sizing asks how much nominal Ah is required to deliver a known amount of load-side energy in one day. A 100W device running continuously for 12 hours consumes 1,200Wh, but a 1,200Wh daily total could also come from several appliances that cycle on and off. Treating those scenarios as identical without understanding the load pattern can hide inverter standby draw, surge requirements and periods of unusually high demand.
1. Convert Ah to Wh
Use nominal pack voltage rather than a momentary terminal reading. Live battery voltage changes with state of charge, temperature and load. The Lithium Battery Voltage Chart explains why nominal, charging and resting voltages should not be treated as the same measurement.
2. Apply Usable DoD and Efficiency
Enter DoD and efficiency as percentages. For example, 90% becomes 0.90 in the math. A direct DC load may have different losses from an AC appliance on an inverter, and an inverter can also consume standby power that is not represented by one efficiency percentage.
3. Estimate Runtime
Runtime uses average running watts. Motors, refrigerators, pumps and compressors can have changing duty cycles and startup surges, so a single steady-load estimate should be checked against measured use when accuracy matters.
4. Size Daily Ah
This result answers a different question from runtime: how much nominal battery capacity is needed to deliver a daily load-energy target under the selected assumptions. Once capacity is chosen, the Battery Charge Time Calculator can help estimate charging duration.
| Battery example | Nominal energy | Delivered energy* | Average load | Estimated runtime |
|---|---|---|---|---|
| 12.8V 100Ah | 1,280Wh | 1,036.8Wh | 100W | 10.37 h |
| 25.6V 100Ah | 2,560Wh | 2,073.6Wh | 300W | 6.91 h |
| 51.2V 100Ah | 5,120Wh | 4,147.2Wh | 800W | 5.18 h |
| 12.8V 200Ah | 2,560Wh | 2,073.6Wh | 500W | 4.15 h |
*Examples use 90% planned DoD and 90% delivery efficiency. They are mathematical planning examples, not guaranteed field runtime.
Worked example: 12.8V 100Ah at a 100W load
The Lithium Battery Calculator first multiplies 100Ah by 12.8V to get 1,280Wh of nominal energy. At a planned 90% depth of discharge, usable battery-side energy is 1,152Wh. After applying 90% delivery efficiency, the load-side energy estimate becomes 1,036.8Wh. Dividing by 100W gives 10.368 hours, or about 10 hours 22 minutes. The current estimate is about 8.68A because the 100W load is divided by 12.8V and the 0.90 efficiency factor.
Worked example: 25.6V 100Ah at a 300W load
At 25.6V, the same 100Ah rating represents 2,560Wh nominally. With 90% DoD and 90% efficiency, the model delivers 2,073.6Wh to the load. Dividing by 300W gives 6.912 hours, or roughly 6 hours 55 minutes. This example shows why a higher-voltage battery with the same Ah can contain more energy. It also shows why comparing battery products only by amp-hour labels can be misleading when their nominal voltages differ.
Worked example: daily sizing for 4,000Wh
Suppose a 51.2V system must deliver 4,000Wh per day. At 90% planned DoD and 90% delivery efficiency, the Lithium Battery Calculator divides 4,000 by 51.2 × 0.90 × 0.90, producing about 96.5Ah of required nominal capacity before any additional engineering margin. That does not automatically mean a 100Ah battery is sufficient for every real installation. Weather, charging opportunity, temperature, reserve needs, inverter idle draw and future load growth can justify more capacity.
Practical Lithium Battery Sizing Guide
Turn the calculator output into a more realistic system plan by checking voltage, load behavior, reserve, wiring, charging and application-specific constraints.
12V, 24V and 48V Systems
Higher system voltage can deliver the same power at lower current. For a 1,200W load, the ideal current is roughly 100A at 12V, 50A at 24V and 25A at 48V before losses. That can influence cable size, inverter selection and system architecture, but voltage should never be changed casually because every connected component must be compatible.
For focused comparisons, use the published 12V lithium calculator, 24V lithium calculator, or 48V lithium calculator.
Average Load vs. Surge Load
The Lithium Battery Calculator uses average watts because energy over time determines runtime. Equipment compatibility also depends on short-duration current. A refrigerator compressor, pump or motor can draw much more at startup than after it reaches normal operation. The battery, BMS and inverter must support those peaks even if the average energy budget looks comfortable.
When possible, measure real power over a representative period. For variable loads, daily watt-hours can be more useful for battery sizing than one nameplate wattage.
Series, Parallel and Bank Ratings
In a compatible series arrangement, voltage adds while Ah remains the same. In a compatible parallel arrangement, Ah adds while voltage remains the same. Energy in Wh therefore changes with either a higher bank voltage or more parallel capacity. Enter the completed bank's nominal voltage and rated Ah, not the rating of one battery, unless you are calculating that single battery.
Battery manufacturers may restrict series or parallel operation, so use the series-parallel tool for arithmetic and the exact product documentation for permission and limits.
Wiring, Fusing and Charging
Energy math does not size conductors or overcurrent protection. Cable length, current, insulation rating, ambient temperature, allowable voltage drop, connector ratings and fault-current behavior all matter. Review the site's battery wiring guides and battery safety guidance before installation.
Charging is another separate design step. Charger voltage, current, temperature limits and BMS behavior must match the battery. Manufacturer instructions outrank generic chemistry assumptions.
Accuracy and safety note: Real usable capacity and runtime can change with temperature, battery age, cell balance, state of charge, load profile, wiring loss, inverter idle draw, BMS protection and voltage cutoff. For critical backup, medical, navigation, emergency or commercial systems, include appropriate reserve and professional engineering review.
Chemistry and nominal voltage matter
The quick presets on this page use common nominal LiFePO4 examples: 12.8V, 25.6V and 51.2V. Other lithium-ion chemistries can use different nominal cell voltages and charge limits. Enter the nominal pack voltage stated by the battery manufacturer rather than forcing a LiFePO4 preset onto a different chemistry. Victron's official Lithium Battery Smart introduction is one manufacturer example of product-specific battery documentation.
For installation and operating limits, consult the exact manual for the battery you own. As another manufacturer reference, Victron publishes model-specific guidance in its Lithium Battery Smart operation guide. Do not copy charge voltages, temperature limits or current limits from an unrelated battery.
The Lithium Battery Calculator becomes more useful when the inputs come from measured behavior rather than optimistic guesses. A plug-in energy meter can help estimate AC appliance watt-hours, while a shunt or battery monitor can reveal battery-side current and daily energy flow. Compare a few ordinary days instead of one unusually light day. If the system is seasonal, repeat the measurement during hotter, colder, busier or lower-sun conditions. Good input data usually improves sizing decisions more than adding extra decimal places to the formula.
Application-specific planning
An RV can have intermittent inverter loads, parasitic draw and alternator charging. A marine bank can have navigation, refrigeration and bilge loads plus stricter redundancy needs. Solar storage can depend on weather and desired autonomy. After the broad calculation, the RV Lithium Battery Calculator and Marine Lithium Battery Calculator can help you work in a more specific context.
5 Checks Before You Choose a Battery Size
The Lithium Battery Calculator gives you a transparent mathematical estimate. These checks help you decide whether the inputs and resulting battery size are realistic for the system you actually plan to build.
A strong battery plan is not simply the smallest capacity that makes the runtime equation work. It also needs enough current capability, charging opportunity and reserve for the intended use. Review these checks after every major change in load, inverter size, system voltage or expected daily energy.
Verify the battery label
Confirm the exact nominal voltage and rated Ah of the battery or completed bank. A small input error multiplies through every Wh, runtime, current and required-capacity result.
Use realistic load energy
Do not size a system around best-case appliance labels. Include normal duty cycles, electronics that remain on, inverter standby consumption and seasonal changes that affect how long equipment operates.
Keep a useful reserve
A mathematical 100% DoD scenario leaves no planning reserve. Choose a discharge window that matches the battery documentation, warranty, operating conditions and your tolerance for reaching low-voltage protection.
Check current separately
Large loads on low-voltage systems can create high current. Confirm continuous and surge limits for the battery, BMS, inverter, disconnects, cables, lugs, busbars and fuses rather than assuming an energy result approves the circuit.
Plan how energy returns
A battery sized for one day of demand is only useful if the charging system can restore that energy in the available time. Compare solar, shore, alternator or generator charging power with the battery's permitted charge rate.
If the output suggests a much larger battery than expected, first check the daily Wh input, DoD and efficiency instead of immediately shopping for more capacity. If the result seems unusually small, make sure you have not entered only one appliance while ignoring the rest of the system. The strongest sizing process is iterative: calculate, compare with measured use, verify equipment limits, then calculate again.
Lithium Battery Calculator FAQs
Common questions about battery runtime, amp-hours, watt-hours, depth of discharge, efficiency and how to interpret the calculator safely.
Multiply battery Ah by nominal voltage to get Wh. Multiply by the planned depth-of-discharge fraction and delivery-efficiency fraction, then divide by the average load in watts. A 100Ah, 12.8V battery at 90% DoD and 90% efficiency provides 1,036.8Wh to the model, which gives 10.368 hours at a constant 100W load.
It depends on nominal voltage. At 12.8V, 100Ah equals 1,280Wh. At 25.6V, it equals 2,560Wh. At 51.2V, it equals 5,120Wh. This is why Ah alone is not enough to compare energy storage across different battery voltages.
Enter the planned usable percentage that fits the exact battery documentation and your reserve goal. The calculator accepts 1% to 100%, but it does not prescribe one universal value because chemistry, BMS settings, warranty terms, temperature and application needs vary.
Yes, through the delivery-efficiency input. If you enter 90%, the model reduces usable battery energy by 10% before calculating runtime. It does not separately model inverter idle watts or an efficiency curve that changes with load, so include significant standby consumption in the load estimate.
Yes for the energy arithmetic, as long as you enter the correct nominal pack voltage, rated Ah, planned DoD and delivery efficiency. Do not use the 12.8V, 25.6V or 51.2V LiFePO4 presets as specifications for a different lithium chemistry.
Possible causes include a higher real load, inverter standby draw, colder temperature, battery aging, less than a full starting state of charge, voltage drop, cell imbalance or a BMS cutoff. The calculator assumes the entered values remain constant, while real systems change during operation.
The tool divides load-side daily Wh by nominal voltage, planned DoD and delivery efficiency. For 1,200Wh per day at 12.8V, 90% DoD and 90% efficiency, the mathematical requirement is about 115.7Ah before any additional design margin.
No. It estimates battery energy, runtime, approximate battery-side current and required Ah. Solar array sizing depends on charging energy and available sun, while wires and fuses need current, conductor, run-length and protection calculations. Use the dedicated tools and installation guidance for those tasks.
Size Your Lithium System With Better Inputs
Run the Lithium Battery Calculator with the real label values from your battery and measured or carefully estimated load data. Then compare runtime, charging, wiring and application-specific tools before finalizing the system.
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