Amp-hours (Ah)
Ah measures how much electric charge a battery can deliver over time under its rated conditions. It is useful for comparing batteries at the same nominal voltage, but Ah alone does not tell you total stored energy.
Battery Capacity Calculator
Use this Battery Capacity Calculator to convert amp-hours, milliamp-hours, watt-hours, and kilowatt-hours, estimate battery runtime, or work backward from a load and target runtime to a required battery size. Enter the nominal voltage from your battery label or datasheet so the result reflects the energy stored by your actual system.
The calculator separates nominal capacity from usable energy by letting you model depth of discharge and delivery efficiency. That makes it useful for lithium batteries, solar storage, RV house banks, marine systems, portable power, backup power, and electronics.
Calculate Battery Capacity, Energy and Runtime
Choose the relationship you need, enter positive values, and calculate. Decimal inputs are supported, zero values are rejected, percentages are limited to 1–100%, and large inputs are checked before results are displayed.
All-in-One Battery Capacity Tool
Convert common battery ratings or estimate runtime and required Ah without mixing charge capacity and energy units.
Battery Capacity Basics: Ah, mAh, Wh and kWh
Battery ratings become much easier to compare once charge capacity and stored energy are kept separate. These six ideas explain what the calculator is doing and why voltage matters.
Battery capacity can be described in more than one unit. Amp-hours (Ah) and milliamp-hours (mAh) describe charge capacity, while watt-hours (Wh) and kilowatt-hours (kWh) describe energy. That distinction matters because a 100Ah battery is not a fixed amount of energy until its voltage is known. A 100Ah battery at 12.8V contains about 1.28kWh nominal, while 100Ah at 25.6V contains about 2.56kWh.
If you are only converting energy units, the Watt Hour Calculator provides a narrower workflow. If your battery label gives Ah and you want a direct conversion, the Battery Ah to Wh Calculator is useful as a cross-check.
Milliamp-hours (mAh)
mAh is simply a smaller charge unit: 1Ah equals 1,000mAh. It is common for phones, power banks, drones, and small electronics. For energy comparison, convert mAh to Wh using the voltage associated with the rating.
Convert mAh to WhWatt-hours (Wh)
Wh measures energy and is usually the clearest basis for comparing batteries across different voltages. It connects a battery’s stored energy with a load measured in watts, which is why Wh is central to runtime estimates.
Kilowatt-hours (kWh)
One kWh equals 1,000Wh. Larger solar and home-storage systems are often described in kWh because the numbers are easier to read. A 5,120Wh battery is 5.12kWh nominal.
Nominal voltage
Nominal voltage is the rated voltage used for energy arithmetic, not necessarily the live terminal voltage you will measure at every state of charge. Use the manufacturer’s stated nominal pack voltage for conversions.
Review lithium voltage rangesRuntime
Runtime connects stored energy to average load. The simplest ideal equation is hours = Wh ÷ W, but real systems also need allowance for reserve, conversion losses, variable loads, temperature, and battery protection limits.
Open the runtime calculatorBattery Capacity Calculator Formulas and Worked Examples
The formulas are simple enough to verify by hand. The important part is choosing inputs that describe the same battery bank, voltage basis, average load, and operating assumptions.
Convert Ah to Wh and kWh
Battery energy (Wh) = battery capacity (Ah) × nominal voltage (V)
For a 100Ah, 12.8V battery, nominal energy is 100 × 12.8 = 1,280Wh, or 1.28kWh. At 25.6V, the same 100Ah rating becomes 2,560Wh. This is why comparing batteries by Ah without voltage can be misleading. If your source data is in mAh, divide by 1,000 first, then multiply by voltage.
Convert Wh back to Ah
Battery capacity (Ah) = battery energy (Wh) ÷ nominal voltage (V)
If a battery is rated 2,560Wh at 25.6V, its equivalent charge capacity is 100Ah. The reverse conversion is useful when a power station or storage battery lists only Wh or kWh but you need an Ah figure for a same-voltage comparison. The Wh to Ah Calculator provides a dedicated version of this calculation.
Estimate runtime with usable energy
Runtime (h) = nominal Wh × DoD × efficiency ÷ average load W
Using a 1,280Wh battery, 90% planned depth of discharge, 90% delivery efficiency, and a constant 100W load gives 1,280 × 0.90 × 0.90 ÷ 100 = 10.368 hours. This is about 10 hours and 22 minutes. A refrigerator, pump, inverter, or computer may not draw constant power, so a measured average load is more useful than a peak rating for runtime.
Work backward to required Ah
Required Ah = (load W × runtime h) ÷ [voltage × DoD × efficiency]
A 300W average load for four hours needs 1,200Wh delivered to the load. At 12.8V, 90% planned DoD, and 90% delivery efficiency, required nominal capacity is about 115.7Ah. That number is a mathematical baseline before any additional design margin, aging allowance, seasonal reserve, or manufacturer-specific constraint.
How to use Battery Capacity Calculator results in practice
Start with the rating printed on the battery or in its datasheet, then use the mode that matches the question you are trying to answer. If you own a battery and want to know its stored energy, convert Ah to Wh. If you know the energy rating but need a same-voltage Ah comparison, convert Wh to Ah. For phones, power banks, and small packs, mAh to Wh puts the rating on an energy basis that is easier to compare across different voltages.
For a system design, treat runtime and required-Ah results as the beginning of the sizing process. RV loads may cycle differently from marine electronics or a portable power station, so application-specific estimates can improve the load model. Compare your baseline with the RV Lithium Battery Calculator, Marine Lithium Battery Calculator, or Portable Power Station Calculator when those use cases match your project. The most useful result is not the largest number; it is the number tied to realistic loads, operating time, reserve, and voltage.
| Example battery | Nominal energy | Delivered energy at 90% DoD / 90% efficiency | 100W load runtime |
|---|---|---|---|
| 12.8V 100Ah LiFePO4 example | 1,280Wh / 1.28kWh | 1,036.8Wh | 10.37 hours |
| 25.6V 100Ah LiFePO4 example | 2,560Wh / 2.56kWh | 2,073.6Wh | 20.74 hours |
| 51.2V 100Ah LiFePO4 example | 5,120Wh / 5.12kWh | 4,147.2Wh | 41.47 hours |
| 12.8V 200Ah LiFePO4 example | 2,560Wh / 2.56kWh | 2,073.6Wh | 20.74 hours |
Why voltage changes the answer
Ah is a charge rating, so the same Ah value at a higher voltage represents more energy. A 100Ah battery at 51.2V contains four times the nominal Wh of a 100Ah battery at 12.8V. That does not mean either battery is automatically better; it means the systems are operating at different voltage and energy levels.
For LiFePO4 examples, manufacturers may publish 12.8V or 25.6V nominal products. Victron, for example, lists those nominal voltages in its Lithium Battery Smart product documentation. Always use the rating for your exact battery rather than assuming every “12V” or “24V” product has the same nominal voltage.
Why Ah and Wh are not interchangeable
Wh already includes voltage, while Ah does not. A useful way to think about it is that Ah tells you the amount of charge available, and Wh tells you how much electrical energy that charge represents at the stated voltage. NIST lists the ampere-hour as a charge unit equivalent to 3,600 coulombs in its Guide to the SI conversion factors.
For practical battery planning, Wh is usually the cleaner bridge between storage and loads because appliance consumption is commonly expressed in watts or watt-hours.
Series and parallel banks
In a compatible series bank, voltage adds while Ah remains the same. In a compatible parallel bank, Ah adds while voltage remains the same. Calculate the complete bank rating first, then enter that bank voltage and bank Ah into this Battery Capacity Calculator.
Do not assume every lithium battery can be placed in series or parallel. Follow the product manual and BMS restrictions. Use the Battery Series Parallel Calculator to model the arithmetic before checking the hardware rules.
Capacity is only one part of sizing
A battery can have enough total energy and still be unsuitable for a load if the BMS current limit, inverter surge rating, cable size, connector rating, or fuse is inadequate. Capacity answers “how much energy?” while current and power limits answer “how fast can that energy be delivered?”
After estimating capacity, use the Wire Gauge Calculator as an initial voltage-drop check and review the Lithium Battery Safety Guide before installation.
Accuracy, Limitations and Safe Interpretation
A useful capacity calculation is transparent about what it includes and what it does not. Real batteries are electrochemical systems, so measured performance can differ from nominal arithmetic.
The Battery Capacity Calculator is designed for planning and conversion, not certification. Ah-to-Wh and Wh-to-Ah conversions are direct arithmetic when nominal voltage is known, but real delivered energy and runtime can be affected by temperature, battery age, discharge rate, BMS cutoffs, state of charge, inverter efficiency, cable loss, idle consumption, and load variation.
For lithium systems, the most reliable inputs come from the exact battery datasheet and measured load data. For lead-acid batteries, usable capacity can also vary substantially with discharge rate, so a simple Wh ÷ W runtime estimate may be less accurate at high current. For critical backup, navigation, medical, or emergency loads, keep reserve capacity and verify actual runtime under expected conditions.
Temperature
Cold or hot conditions can change available capacity, charge acceptance, and BMS protection behavior. The calculator does not apply a universal temperature correction because the effect depends on chemistry and battery design.
Variable loads
Motors, refrigerators, pumps, and inverters may cycle or surge. Runtime mode uses an average load and does not test whether the battery or inverter can support startup current.
BMS and cutoff limits
A BMS may disconnect for voltage, current, or temperature protection before the simple energy model reaches zero. Always check the battery’s current limits and protection settings.
Conversion losses
AC loads usually involve inverter loss, and some inverters consume standby power even with a small load. Enter a realistic efficiency and include idle consumption in the average load when it matters.
Aging and reserve
Capacity can decline with age and usage. A design that only works when every rated watt-hour is available has little resilience, so practical systems often include reserve beyond the mathematical minimum.
Installation checks
This page does not size fuses, conductors, busbars, chargers, breakers, or enclosures. Those choices require current, cable length, fault protection, environment, and the applicable manufacturer or electrical guidance.
Practical rule: use the calculator to establish a clean energy baseline, then verify battery current limits, charging requirements, wiring, protection, and application-specific reserve. If charging time is part of the plan, compare the result with the Lithium Battery Charge Time Calculator.
Battery Capacity Calculator FAQs
Answers to common questions about converting battery ratings, estimating runtime, choosing voltage inputs, and interpreting usable capacity.
Multiply amp-hours by nominal voltage: Wh = Ah × V. A 100Ah battery at 12.8V is 1,280Wh nominal. If the rating is in mAh, divide by 1,000 first to convert mAh to Ah, then multiply by voltage.
It depends on voltage. At 12V, 100Ah is 1,200Wh. At 12.8V it is 1,280Wh. At 24V it is 2,400Wh, and at 51.2V it is 5,120Wh. Ah alone is not enough to determine energy.
Ah measures charge capacity; Wh measures energy. Wh includes voltage, so it is usually better for comparing batteries across different system voltages and for estimating runtime from a load measured in watts.
Use the nominal voltage stated by the manufacturer for the exact battery. Many 12V-class LiFePO4 products are rated 12.8V nominal, but the battery label or datasheet is the correct source for your input.
Yes. Runtime mode starts with nominal Wh, applies the planned depth of discharge and delivery efficiency, then divides the modeled delivered energy by average load watts. It is a planning estimate because real loads and battery behavior vary.
Use a planned value consistent with your battery documentation, warranty, reserve strategy, and application. The calculator accepts 1–100% because it is an input model, not a universal recommendation. Leaving reserve can be especially important for critical loads.
Possible causes include a higher real average load, inverter standby power, lower starting state of charge, temperature, aging, voltage drop, protection cutoffs, or lower conversion efficiency. Measured battery-side power is usually a better runtime input than a guessed load.
Not reliably if the ratings use different voltage bases. Convert mAh to Wh using the associated nominal voltage. Wh gives a more meaningful energy comparison when voltage differs.
The unit conversions do. Runtime is less straightforward because usable lead-acid capacity can change with discharge rate and operating conditions. Use manufacturer discharge curves and a conservative reserve for final planning.
No. Capacity and runtime are energy calculations. Wiring, fuses, BMS current limits, and inverter selection require current, surge power, cable length, voltage drop, fault protection, and manufacturer specifications.
Turn Battery Capacity Into a Better System Plan
Use the Battery Capacity Calculator to create a clear Ah, Wh, kWh, or runtime baseline, then compare that result with your battery datasheet, real loads, charging plan, BMS limits, wiring, and reserve requirements.
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