Battery Capacity Calculator
Convert mAh and voltage into watt-hours for power banks, e-bike packs, drone batteries and airline carry-on checks (100 Wh limit is the common cabin threshold).
Last updated: May 2026
Enter battery capacity and voltage to estimate watt-hours.
Why people need this
Battery listings often push mAh because it looks bigger, while transport rules and many technical comparisons use watt-hours. That mismatch causes confusion all the time.
This calculator fixes that immediately. It is especially useful for power banks, DIY electronics, travel checks and comparing cells with different voltages.
Typical use cases
- power banks
- battery packs
- travel allowance checks
- electronics projects
Frequently Asked Questions
Why do power banks list mAh instead of watt-hours?
Because mAh looks bigger and more impressive to consumers. A 10,000 mAh power bank might actually be only ~37 Wh, but "10,000" sounds more substantial than "37." This is intentional marketing—manufacturers know that most shoppers don't understand the difference. Airlines and shipping regulations use watt-hours as the safety limit, so you need to convert to know if your battery is allowed in carry-on luggage.
What does TSA allow for lithium batteries in carry-on luggage?
TSA allows a maximum of 100 Wh (watt-hours) per battery in carry-on baggage. Batteries between 100–160 Wh require airline approval before you board. Above 160 Wh is prohibited. A 20,000 mAh power bank at 3.7V nominal voltage converts to about 74 Wh, which is under the limit. But if it's 5V or higher nominal, or if it's a larger cell, you need this calculator to verify before packing—one miscalculation lands your power bank in confiscated baggage.
Why does voltage matter if I know the mAh?
Because the same mAh at different voltages stores different amounts of energy. A 5000 mAh cell at 3.7V nominal (lithium-ion) stores 18.5 Wh. The same 5000 mAh at 6V nominal (two cells in series) stores 30 Wh, which is 65% more energy in the same physical capacity. Device manufacturers exploit this by stacking cells to get higher nominal voltages, which is why you can't just glance at mAh and know the real capacity. Watt-hours unify the comparison across different battery architectures.
Can I compare power banks using mAh alone?
No—you need voltage too. A 10,000 mAh battery at 3.7V provides 37 Wh. A 10,000 mAh at 7.4V nominal (two cells stacked) provides 74 Wh—twice the usable energy despite the same mAh rating. Without converting to watt-hours, you might buy a power bank thinking it matches another brand's capacity, but actually get half the charge. This is especially misleading in online shopping where you can't see the cell architecture.
Why does charging efficiency matter for real-world capacity?
Power banks lose energy during the charge/discharge cycle—typically 10–15% is lost as heat and in the circuitry. A 37 Wh battery might only deliver 31–33 Wh to your device in practice. Watt-hours measure the cell's rated capacity, not what actually reaches your phone. Cheap power banks with poor charge controllers lose more. High-end units with better regulators lose less. The spec sheet shows watt-hours, but real-world performance depends on the circuitry quality—always budget for a 10% efficiency loss.