Primary Result
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Main calculated battery value for the selected mode.
Battery Capacity Calculator
Calculate battery capacity in Ah, Wh, and mAh, convert between common battery units, and estimate runtime from voltage, load, and energy use. This tool is useful for lithium batteries, solar storage, RV systems, power stations, and electronics.
Capacity and Runtime Calculator
Switch between conversion modes to calculate battery energy, charge capacity, and estimated runtime using standard battery formulas.
All-in-One Battery Capacity Tool
Use the tabs below to convert Ah to Wh, Wh to Ah, mAh to Wh, or estimate battery runtime from a known load.
Calculation Results
Use these results to compare batteries, estimate energy storage, or size a battery for your expected load and runtime.
Educational Estimates Only: Real battery performance depends on chemistry, discharge rate, inverter losses, depth of discharge, temperature, and battery age. For critical systems, always verify with the battery datasheet.
How Battery Capacity Works
Battery capacity can be expressed in several units, and understanding how they connect makes it much easier to compare batteries correctly.
Battery capacity is commonly listed in amp-hours, milliamp-hours, or watt-hours. Amp-hours measure electrical charge, while watt-hours measure actual stored energy. This difference matters because two batteries can have the same Ah rating but very different total energy if they operate at different voltages. For that reason, watt-hours are often the best unit when comparing batteries across different systems.
Wh = Ah × V
This formula converts battery charge into stored energy. For example, a 12V 100Ah battery stores about 1200Wh, while a 48V 100Ah battery stores about 4800Wh. The Ah rating stayed the same in both cases, but the energy is very different because the voltage changed.
Ah = Wh ÷ V
This reverse formula helps when you know how much energy you need and want to choose a battery size. If you need 2400Wh in a 24V system, you divide 2400 by 24 to get 100Ah. This method is common in solar, RV, marine, and backup battery sizing.
Runtime (hours) = Wh ÷ W
Battery runtime is estimated by dividing the battery's stored energy in watt-hours by the load power in watts. For example, a 1000Wh battery running a 100W device may last about 10 hours in ideal conditions. Real runtime is usually lower because of inverter efficiency losses, discharge limits, and temperature effects.
Common conversions
- 1Ah = 1000mAh.
- Wh = Ah × Voltage.
- Ah = Wh ÷ Voltage.
- Runtime = Wh ÷ Load Watts.
Example capacities
- 12V 100Ah = 1200Wh.
- 24V 50Ah = 1200Wh.
- 3.7V 5000mAh = 18.5Wh.
- 1000Wh battery at 100W ≈ 10 hours.
Why Wh is better for comparison
If you compare only Ah, you may think two batteries are equal when they are not. Watt-hours account for both voltage and capacity, so they represent the real energy stored in the battery.
What changes real runtime
High loads, heat, cold weather, inverter losses, battery aging, and safe discharge limits all reduce real-world battery runtime. A calculator gives a good estimate, but not a guaranteed field result.
Frequently Asked Questions
These are the most common questions people ask when converting battery capacity and estimating runtime.
What is battery capacity?
Battery capacity is the amount of electrical charge or energy a battery can store. It is commonly expressed as amp-hours, milliamp-hours, or watt-hours depending on the battery type and application.
What is the difference between Ah and Wh?
Ah measures charge, while Wh measures energy. Two batteries with the same Ah can have different Wh if their voltages are different. That is why Wh is usually better for comparing batteries across different systems.
How do I convert Ah to Wh?
Multiply amp-hours by voltage. For example, 100Ah at 12V equals 1200Wh because 100 × 12 = 1200.
How do I convert Wh to Ah?
Divide watt-hours by voltage. For example, 2400Wh at 24V equals 100Ah because 2400 ÷ 24 = 100.
How do I convert mAh to Wh?
First divide mAh by 1000 to get Ah, then multiply by voltage. For example, 5000mAh at 3.7V equals 18.5Wh.
How do I calculate battery runtime?
Divide battery watt-hours by the load in watts. A 1000Wh battery running a 100W device lasts about 10 hours in ideal conditions.
Is mAh the same as Ah?
No. 1000mAh equals 1Ah. mAh is simply a smaller unit used often for phones, tablets, and smaller electronics.
Why do batteries with the same Ah have different energy?
Because energy depends on both Ah and voltage. A higher-voltage battery stores more energy at the same Ah rating.
Is the calculator exact?
No. It gives planning estimates using standard battery math. Actual results depend on battery chemistry, discharge rate, inverter efficiency, temperature, and the condition of the battery.
Which unit should I use to compare batteries?
Use watt-hours whenever possible because it represents real stored energy and allows fair comparison across different voltages.
Can I use this for lithium batteries?
Yes. The formulas work for lithium, LiFePO4, lead-acid, AGM, and other battery types. Real usable capacity still varies by battery chemistry and safe depth of discharge.
What reduces real battery runtime?
High loads, inverter losses, cold weather, battery age, voltage sag, and avoiding full discharge all reduce actual runtime compared with the theoretical result.
Understand Battery Energy Faster
Use this calculator to convert capacity, estimate runtime, and compare batteries properly before you choose a battery for your home, RV, solar, or electronics project.
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