About the Battery Capacity (Wh)
This calculator converts a battery's milliamp-hour (mAh) rating into watt-hours (Wh), the unit that actually reflects how much energy a battery stores. It's useful when comparing batteries that run at different voltages, such as a phone battery against a laptop or power station battery, where the mAh number alone doesn't tell the full story.
How It Works
Enter the battery's capacity in mAh and its voltage, and the calculator divides the mAh figure by 1000 to convert it to amp-hours, then multiplies by the voltage to produce the energy stored in watt-hours.
Formula & Methodology
Milliamp-hours measure electric charge, not energy, so a mAh rating by itself only tells you how long a battery can supply a given current, not how much total energy it holds. Converting to watt-hours requires bringing voltage into the equation, since energy equals charge multiplied by voltage. Dividing the mAh figure by 1000 converts it into amp-hours (Ah), and multiplying that by the battery's voltage gives watt-hours, the standard unit for comparing stored energy across devices regardless of their operating voltage.
Examples
Typical phone battery
A 5000 mAh battery at 3.7V (a common lithium-ion cell voltage) stores (5000 / 1000) x 3.7 = 18.5 Wh.
Portable power station battery
The same 5000 mAh capacity at 12V stores (5000 / 1000) x 12 = 60 Wh, more than triple the energy of the phone battery despite the identical mAh rating.
Advantages
- Converts mAh into the watt-hour figure that airlines, manufacturers, and energy comparisons actually rely on.
- Makes it possible to fairly compare batteries running at different voltages, which raw mAh numbers can't do.
- Requires only two commonly listed specs (mAh and voltage), both of which are printed on nearly every battery label.
Common Mistakes
- Comparing two batteries by mAh rating alone without checking whether they run at the same voltage, leading to a false impression of which one stores more energy.
- Using the wrong voltage, such as a device's input voltage instead of the battery cell's actual nominal voltage, which produces a skewed Wh figure.
- Treating the calculated Wh as exact usable energy, when real-world factors like discharge inefficiency and voltage sag mean actual usable energy is typically somewhat lower.
Edge Cases to Watch For
- The formula assumes the battery delivers its full rated mAh at a constant voltage; real batteries' voltage sags somewhat as they discharge, so actual usable energy can differ slightly from this calculation.
- Comparing two batteries by mAh alone is only valid when they run at the same voltage; otherwise the Wh figure is the more accurate basis for comparison.
- A zero or unusually low voltage entry produces a proportionally low or zero watt-hour result, since the formula scales directly with voltage.
Common Use Cases
- Travelers checking whether a power bank's watt-hour rating falls under airline carry-on battery limits, which are set in Wh rather than mAh.
- Shoppers comparing battery capacity across devices with different voltages, like phones, laptops, and portable power stations.
- Electronics hobbyists and DIY builders calculating how much energy a battery pack will actually deliver in a project.