mAh to Wh Converter (Battery Capacity)

What Is mAh? Milliamp-hours (mAh) is a measure of electric charge, meaning how much current a battery can deliver over time. 1 mAh = 1 milliamp flowing for 1 hour = 0.001 amp-hour (Ah). mAh tells you the current capacity but not the energy stored, because it ignores voltage. A 2,000 mAh battery at 3.7V holds twice the energy of a 2,000 mAh battery at 1.5V, even though both say “2,000 mAh.”

What Is Wh? Watt-hours (Wh) measure energy, which is the complete picture of a battery’s storage capacity. Wh = mAh × Voltage / 1,000 A 3,000 mAh Li-ion battery at 3.7V: Wh = 3,000 × 3.7 / 1,000 = 11.1 Wh. Wh is used for comparing batteries across different chemistries and voltages. It is also the unit used in airline lithium battery regulations (100 Wh limit for carry-on without permission).

Common Battery Voltages Lithium-ion (phone/laptop): 3.7V nominal (3.0V discharged, 4.2V fully charged). LiPo (Li-polymer): 3.7V nominal, same chemistry as Li-ion in a different form factor. LiFePO₄ (lithium iron phosphate): 3.2V nominal, more stable, used in EVs and solar storage. NiMH (AA/AAA rechargeable): 1.2V nominal. Alkaline (AA/AAA): 1.5V nominal (non-rechargeable). Lead-acid (car battery): 2.0V per cell → 12V for 6 cells. USB power bank output: 5V (boosted from 3.7V internal cells).

Calculating Device Runtime Runtime (hours) = Battery capacity (Wh) / Device power draw (W) A 10,000 mAh power bank at 3.7V = 37 Wh internally. At 5V USB output (with ~85% converter efficiency): usable energy = 37 × 0.85 ≈ 31.5 Wh. Charging a phone that needs 10 Wh: 31.5 / 10 = ~3 charges. Charging a 65W laptop for 30 minutes: 65 × 0.5 = 32.5 Wh needed → one partial charge.

Airline Lithium Battery Regulations (IATA) Under 100 Wh: allowed in carry-on without restrictions. 100 to 160 Wh: allowed with airline approval (1 spare battery). Over 160 Wh: generally not permitted in passenger aircraft. A 26,800 mAh power bank at 3.7V = 99.16 Wh, just under the 100 Wh limit. Many power banks are designed to fall just under 100 Wh for this reason.

The trap in that calculation: work the Wh out at 5V instead of 3.7V and the same 26,800 mAh pack looks like 134 Wh, which would put it in the approval-required band. The rating that counts is the one printed on the pack, based on the internal cell voltage, and a good manufacturer prints the Wh figure directly so nobody has to guess. If a pack quotes only mAh, assume 3.7V.

Energy Density Modern Li-ion cells: 150 to 265 Wh/kg gravimetric energy density. Tesla’s 4680 cell, introduced in 2022, lands around 240 to 250 Wh/kg. Its advantage is more about manufacturing cost and pack structure than a leap in density. Gasoline: roughly 12,000 Wh/kg, still vastly more energy-dense than any battery. The comparison is less lopsided than it sounds, because a petrol engine throws away 60 to 75% of that as heat while an electric drivetrain uses about 90% of what it stores. Production EV packs land at 150 to 200 Wh/kg once you count the casing, cooling and structure, well below the cell figure. Anything quoted above 300 Wh/kg at cell level is a lab result or a solid-state prototype, not something you can buy.

Power Bank Efficiency Power banks convert internal 3.7V cells to 5V USB output using a boost converter. Typical boost converter efficiency: 85 to 92%. A “20,000 mAh” power bank at 3.7V nominally holds 74 Wh. At 90% efficiency: 66.6 Wh usable at 5V = 13,320 mAh equivalent at 5V output. This is why a “20,000 mAh” power bank doesn’t actually give 20,000 mAh to your phone.