Lithium Battery Calculator

A general lithium battery calculator helps users answer the most common battery questions in one place. How long will this battery last, how much usable energy does it really have, what current will the load draw, and what battery size is needed for a target runtime are all part of the same planning job.

This version separates nameplate battery energy from usable battery energy. That matters because lithium batteries are usually planned using both a depth-of-discharge limit and a system efficiency factor, not just the full label capacity.

Short answer: Convert Ah to Wh using voltage, apply usable DoD and system efficiency to estimate realistic usable energy, then divide by load watts to estimate runtime. For sizing, reverse the formula to calculate the required Wh and Ah.

Free Lithium Battery Calculator

Calculate runtime, usable energy, discharge current, required battery size, and optional charging time

Chemistry presets update default usable DoD and charge efficiency.
Examples: 12.8V LiFePO4, 24V, 36V, 48V.
Enter rated battery capacity in amp-hours.
Power used by the device or system.
Example planning values: LiFePO4 often 80 to 90, other lithium often around 85.
Use this for inverter, wiring, and real-world conversion losses.
Used to estimate the battery size needed for your load.
Optional charging estimate starts from this charge level.
Set 80 if you want an estimate only to 80% charge.
Used only for the optional charging-time estimate.
Lithium charging is often very efficient, commonly around the mid to high 90% range.

What this calculator does

Estimate runtime

Calculates runtime from usable watt-hours and load watts so the answer is more realistic than using label capacity alone.

Show usable energy

Separates total battery energy from usable energy using your selected depth of discharge and system efficiency.

Size a battery bank

Calculates the required watt-hours and amp-hours needed to support a target runtime for a given load.

Estimate charge time

Optionally estimates charging time from charger power, charge efficiency, and selected start and target state of charge.

Core formulas

FormulaMeaningUse
Wh = Ah × VConverts battery capacity into stored energyFirst step for most battery calculations
Usable Wh = Ah × V × DoD × EfficiencyEstimates practical usable energyBest for runtime planning
Runtime = Usable Wh ÷ LoadEstimates runtime in hoursUseful for appliances, inverters, and DC loads
Required Ah = Required Wh ÷ VConverts required energy into battery capacityBest for sizing a new system
Current = W ÷ VEstimates discharge current in ampsUseful for wiring and current checks

Why DoD matters

Usable battery energy is not always the same as full nameplate battery energy. LiFePO4 is commonly planned at deeper discharge levels than lead-acid, and many lithium users work in the roughly 80% to 90% usable range rather than assuming the full battery label is available every cycle.

This matters because runtime planning should use the energy you expect to use in real operation, not just the marketing label on the battery. A calculator that separates total Wh from usable Wh gives more realistic answers for solar, RV, backup, marine, and off-grid systems.

Practical warning: If your system includes an inverter, surge loads, or cold-weather operation, real runtime can be lower than the simple result. Add margin instead of sizing a battery right to the minimum number.

Example results

12.8V 100Ah LiFePO4

A 12.8V 100Ah battery stores 1280Wh before any real-world adjustments. If you plan around 90% usable DoD and 95% system efficiency, the usable energy is about 1094Wh.

100W load example

If usable energy is about 1094Wh and the load is 100W, estimated runtime is about 10.9 hours. That is much more useful than thinking only in amp-hours.

Battery sizing example

If a 200W load must run for 8 hours, the load needs 1600Wh before losses. After adjusting for usable DoD and efficiency, the required battery bank will be larger than 1600Wh.

Current example

A 500W load on a 24V battery draws about 20.8A. This current figure is helpful when checking cable size, fuse selection, and battery discharge limits.

Frequently Asked Questions

How do I calculate lithium battery runtime?▼
First convert battery capacity into watt-hours using Ah × V. Then apply usable DoD and system efficiency if you want a more realistic estimate, and divide the usable watt-hours by the load in watts.
What is the formula for battery runtime?▼
A practical planning formula is Runtime = Usable Wh ÷ Load. Usable Wh is often estimated as Ah × V × usable DoD × system efficiency.
Why convert Ah to Wh?▼
Amp-hours alone do not show total stored energy unless voltage is included. Watt-hours make it easier to compare batteries at different voltages and plan runtime from actual energy.
Why use DoD instead of only efficiency?▼
DoD and efficiency describe different things. DoD limits how much of the battery you plan to use, while efficiency accounts for system or conversion losses.
What usable DoD should I use for LiFePO4?▼
A common planning range is around 80% to 90%, depending on how conservative you want to be. Many LiFePO4 systems are designed to use a deeper portion of capacity than lead-acid systems.
What system efficiency should I use?▼
For simple planning, many users choose something around the mid-90% range for efficient lithium-based systems. If you have an inverter and real conversion losses, a slightly lower value may be more realistic.
How do I calculate required battery size?▼
Multiply the load in watts by the required runtime in hours to get required load energy in Wh. Then divide by your usable DoD and system efficiency to estimate the battery energy needed, and finally divide by voltage to convert that into Ah.
How do I calculate discharge current?▼
For a DC estimate, divide watts by volts. A 240W load on a 12V battery draws about 20A.
Does this work for Li-ion, LiFePO4, and LiPo?▼
Yes. The same basic energy and power relationships apply across lithium chemistries, although real discharge limits, efficiency, voltage behavior, and battery management settings can differ.
How accurate is charge time from charger power?▼
It is a planning estimate. Real charging slows near the top of the charge curve, so the final part often takes longer than a simple average-power formula suggests.
Why use start and target SoC for charging?▼
Because you rarely charge from completely empty to completely full every time. Charging from 20% to 80% requires much less energy than charging from 0% to 100%.
Are calculator results exact?▼
No. They are estimates based on standard formulas and practical planning assumptions. Real results change with temperature, load variation, battery age, cell balancing, inverter losses, and manufacturer limits.

Practical rule: For lithium battery planning, think in usable watt-hours, not only amp-hours. That makes runtime, sizing, and charging estimates much easier to understand.

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Disclaimer: All calculator results are estimates based on standard formulas. Always verify with manufacturer specifications for critical applications.