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Use this calculator to

  • Convert a battery's watt-hour rating into milliamp-hours at its cell voltage
  • Check a power bank or spare battery against the 100 Wh airline carry-on limit
  • Compare batteries quoted in mAh and Wh on a like-for-like basis
  • Size replacement batteries for laptops, tools and portable kit

Wh to mAh Calculator

Convert watt-hours (Wh) and voltage (V) to milliamp-hours (mAh).

Wh
V
Result
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Formulas

  • mAh = Wh × 1000 / V

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Battery energy storage systems (BESS) on commercial sites are specified in kWh at the system level but built from cells and modules rated in mAh at a specific voltage. Converting watt-hours to milliamp-hours at the cell voltage is the working calculation behind every BESS proposal, every IoT power budget and every standby battery bank specification. A 100kWh BESS built from 3.2V LFP cells needs around 31,250 Ah of cell capacity at the cell level, distributed across series and parallel strings to hit the system voltage.

For Industrial & Commercial operators using BESS to manage MIC headroom, capacity charges, triad-replacement charges or to shift load off peak periods, the modular sizing question drives the capital cost. A 500kWh BESS rated for a 1C discharge can offset 500kW of demand for an hour, which is enough to ride through the worst of a winter evening peak on a manufacturing site. Get the cell-level capacity right and the system delivers the bill saving the financial model promised. Get it wrong and the system underdelivers in the first cold snap.

Purely Energy works with clients designing BESS into their procurement strategy. We model the bill saving from MIC reduction, capacity-charge avoidance and time-of-use arbitrage alongside the supply contract itself, so the BESS investment case stands up against the alternative of buying capacity from the grid. For IoT and comms power budgets on remote industrial sites, the same calculation supports the design of standalone solar plus battery installations that keep monitoring kit running through the winter without a mains connection.

Common questions

How do I convert Wh to mAh?

Multiply the watt-hours by 1,000 and divide by the voltage: mAh = Wh x 1000 / V. An 11.1 Wh phone battery at the 3.7 V lithium-ion cell voltage is 11.1 x 1000 / 3.7, which is 3,000 mAh. Use the battery's nominal voltage, which is printed on the label, not the charger voltage or the device's USB output voltage, otherwise the capacity figure will be wrong.

Why do I need the voltage to convert Wh to mAh?

Because the two units measure different things. Watt-hours measure energy; milliamp-hours measure electric charge. The voltage is the bridge between them: energy equals charge times voltage. The same 57 Wh of energy is 5,000 mAh in an 11.4 V laptop pack but would be about 15,400 mAh at a single cell's 3.7 V. Comparing mAh figures across batteries with different voltages is meaningless; convert both to Wh first for a fair comparison.

Which voltage should I use for the conversion?

Use the nominal voltage of the battery pack. A single lithium-ion cell is 3.7 V, a two-cell pack is 7.4 V, a three-cell pack is 11.1 V (or 11.4 V for some chemistries), and a lead-acid block is 12 V. The nominal voltage and usually the Wh figure are printed on the battery label or in the datasheet. Power banks are conventionally rated in mAh at 3.7 V cell voltage, even though their output socket delivers 5 V or more.

Why does a 20,000 mAh power bank deliver less than the maths suggests?

The 20,000 mAh rating applies at the internal cell voltage of 3.7 V, which is 74 Wh of stored energy. To charge your device, the power bank boosts that to 5 V or more at the USB output, and the conversion plus the device's own charging circuitry lose energy as heat. Real-world usable output is commonly 20 to 30 percent below the headline figure, which is why a 20,000 mAh bank fills a 3,000 mAh phone roughly four to five times, not six.

What is the airline limit for lithium batteries in watt-hours?

Standard aviation rules allow lithium-ion batteries up to 100 Wh in carry-on baggage without airline approval; batteries between 100 Wh and 160 Wh need the airline's agreement, and larger ones are generally not allowed as passenger baggage. Spare batteries and power banks must travel in the cabin, not the hold. If your battery only states mAh, convert it: Wh = mAh x V / 1000, so 20,000 mAh at 3.7 V is 74 Wh, comfortably under the limit.

Related calculators

The conversions that procurement and engineering use alongside this one.

Wh to mAh Calculator | Purely Energy