Compare battery capacity in watt-hours (Wh), especially when the batteries have different voltages. Milliamp-hours (mAh) measure electric charge; Wh measure energy. To convert a battery’s rated capacity, use Wh = (mAh ÷ 1,000) × nominal voltage (V).
What mAh and Wh tell you
Milliamp-hours describe how much electric charge a battery is rated to hold. Watt-hours describe energy. Because energy depends on both charge and voltage, an mAh figure alone cannot show how much energy a battery stores unless you also know its voltage.
If two batteries have the same nominal voltage, their mAh ratings can be compared directly as a measure of charge capacity. If their voltages differ, compare their Wh ratings instead. A larger Wh rating indicates more rated stored energy, but does not by itself guarantee longer runtime or more device charges.
How to convert mAh to Wh
First convert milliamp-hours to amp-hours by dividing by 1,000. Then multiply amp-hours by the battery’s nominal voltage:
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Ah = mAh ÷ 1,000
Wh = Ah × V
The Federal Register states the relationship as “Watt-hour (Wh) = ampere-hour (Ah) × volts (V).” For battery ratings, use the rated capacity and nominal voltage. Do not multiply by a different battery’s voltage or by a voltage selected from a device’s marketing specifications.
Worked example
An FAA guide illustrates a battery rated at 1,030 mAh and 7.4 V. Convert 1,030 mAh to 1.030 Ah, then calculate 1.030 Ah × 7.4 V = approximately 7.7 Wh. The FAA guide’s example identifies the battery as 7.7 Wh; it is a worked calculation, not a runtime test.
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How to compare two batteries
- Find each battery’s rated capacity and nominal voltage. If the manufacturer prints a Wh rating, use it for an initial energy comparison.
- Convert any mAh-only rating. Divide its mAh value by 1,000 to get Ah, then multiply by that same battery’s nominal voltage.
- Compare the resulting Wh figures. This puts batteries with different nominal voltages on a common rated-energy basis.
- Check practical requirements separately. Confirm compatibility, power delivery, physical fit, and any travel restrictions; a higher Wh figure does not answer those questions.
Power banks and portable chargers
A power bank’s mAh number is not enough to compare its stored energy with another battery’s if the nominal voltages differ. Look for its Wh rating or calculate it from its rated capacity and nominal voltage. A more energy-dense rating does not establish how quickly the power bank can deliver power or how much energy a connected device will receive after conversion losses.
What a Wh comparison does—and does not—establish
Wh is useful for comparing rated stored energy, but it is not a complete measure of real-world performance. The U.S. Department of Energy distinguishes energy from power: energy is an amount, while power describes how quickly energy is transferred or used. A battery with more Wh may store more rated energy, but Wh alone does not tell you whether it can supply the power a device requires.
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- Runtime: Actual operating time depends on the device’s energy demand and system conversion losses. A Wh rating is not a guaranteed number of hours.
- Number of charges: A battery’s Wh value does not translate directly into a guaranteed number of phone or laptop charges; energy is lost in conversion and device use varies.
- Compatibility: The battery must meet the device’s electrical and physical requirements. More Wh does not make an incompatible battery suitable.
- Condition and usable energy: The rating is based on rated capacity and nominal voltage; it does not model every discharge condition or establish a battery’s current health.
Using Wh to check U.S. air-travel limits
The FAA’s passenger guidance says spare lithium-ion batteries, including portable chargers, are generally limited to 100 Wh per battery. It allows up to two spare batteries in the 101–160 Wh range with airline approval. Airlines may impose stricter limits, so check the current FAA PackSafe guidance and the operating airline’s rules before travel. These are U.S. passenger-travel rules, not a general safety or quality rating.
For travel, follow the FAA’s handling guidance as well as its capacity limits. It includes protecting battery terminals against short circuits and avoiding travel with certain damaged or recalled batteries. A battery’s Wh rating alone does not establish that it is safe to carry in every condition.
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