Free tool
mAh to Wh Calculator
Watt-hours are milliamp-hours times voltage, divided by 1,000. The voltage is the part a mAh rating leaves out, and it is why mAh figures cannot be compared across batteries.
Wh = mAh × V ÷ 1,000
Wh = mAh × volts ÷ 1,000. A 10,000 mAh power bank at its 3.7 V cell voltage holds 37 Wh; a 20,000 mAh bank holds 74 Wh. Going the other way, mAh = Wh × 1,000 ÷ V. Use the voltage the mAh is rated at — for a power bank that is the internal cell, not the 5 V USB port.
For packs rated in amp-hours, the amp-hours to watt-hours calculator does the same step without the factor of 1,000.
The battery
Power banks rate mAh at the internal lithium-ion cell — about 3.6–3.7 V — not at the 5 V USB output. Check the label: many print the Wh figure, or the cell voltage, beside the mAh.
The answer
Energy
37 Wh
Wh = mAh × V ÷ 1,000 → 10,000 × 3.7 ÷ 1,000
Same battery, other units
10 Ah at 3.7 V · 0.037 kWh. At 5 V the same energy would be stated as 7,400 mAh — the reason a power bank's mAh and its USB output do not match.
Flying with it
37 Wh is at or under the FAA's 100 Wh line for a lithium-ion battery: spare batteries and power banks go in carry-on only, never checked bags. Airlines can be stricter; check before you fly.
Informational use only, please verify before you rely on it
This reproduces the relationship Wh = mAh × V ÷ 1,000 and its inverse, with an optional charge estimate at an efficiency you choose. It is a preparation aid, not a determination. It does not measure a battery's actual capacity, confirm what an airline will accept, or size a home battery system.
This tool is provided for general informational and educational purposes only. Its output is an illustrative estimate generated from the values you enter and from general assumptions that will not match every deal, market, lender, or homeowner. It is not tax, legal, accounting, financial, or professional advice, and it is not a quote, an offer, a credit decision, or a guarantee of pricing, approval, timing, savings, or eligibility.
You are solely responsible for independently confirming all information presented here including any figures, rates, fees, margins, timelines, tax treatment, and federal, state, local, or utility incentives, with the applicable lender, authority having jurisdiction, and your own qualified tax, legal, and financial advisors before acting on it, relying on it, or presenting it to a homeowner or any third party. Incentive programs, lender terms, and permitting requirements change frequently and vary by jurisdiction.
Seamless Home is not a tax advisor, law firm, lender, or licensed installing contractor, and makes no representation or warranty as to the accuracy, completeness, or currency of the information produced by this tool. To the fullest extent permitted by law, Seamless Home accepts no liability for any decision made or action taken in reliance on it.
mAh to Wh at 3.7 V
| Capacity (mAh) | Energy at 3.7 V (Wh) |
|---|---|
| 1,000 mAh | 3.7 Wh |
| 2,000 mAh | 7.4 Wh |
| 3,000 mAh | 11.1 Wh |
| 5,000 mAh | 18.5 Wh |
| 10,000 mAh | 37 Wh |
| 20,000 mAh | 74 Wh |
| 26,800 mAh | 99.16 Wh |
| 30,000 mAh | 111 Wh |
26,800 mAh at 3.7 V is 99.2 Wh — just under the FAA's 100 Wh line, which is why that capacity turns up so often on large power banks. At 30,000 mAh the same cells are over it.
Why mAh misleads and Wh does not
mAh is charge; Wh is energy. A milliamp-hour says how much current a battery can deliver for how long. It only becomes energy once it is multiplied by the voltage that current flows at. Two batteries both labelled 5,000 mAh hold different amounts of energy if one is a 3.7 V cell and the other a 7.4 V two-cell pack — the second holds twice as much.
The same energy, three mAh figures. A 37 Wh power bank is 10,000 mAh at its 3.7 V cell, about 7,400 mAh at 5 V, and about 1,850 mAh at 20 V USB-C. None of those is wrong; they describe one battery at three voltages. Only the Wh figure is the same in all three, which is why it is the one the FAA regulates and the one worth comparing.
Charges are energy delivered, not mAh divided. Boosting a cell to USB voltage and charging a phone both lose energy as heat. The calculator's charge estimate starts from an efficiency you choose rather than pretending there is no loss. For a laptop, the laptop watts post works through battery size and runtime the same way.
Bigger batteries move to Ah and kWh. Portable power stations and home batteries are rated in Ah at 12, 24 or 48 V, or directly in kWh, because the numbers in mAh would be unwieldy. A 10 kWh home battery is 10,000 Wh; how long it carries a house is a question of the loads, which how many watts a house uses covers.
Frequently Asked Questions
How do you convert mAh to Wh?+
Multiply milliamp-hours by the voltage and divide by 1,000: Wh = mAh × V ÷ 1,000. A 10,000 mAh power bank rated at 3.7 V holds 10,000 × 3.7 ÷ 1,000 = 37 Wh. Without the voltage there is no conversion — mAh is charge, and energy is charge times voltage.
How do you convert Wh to mAh?+
Multiply watt-hours by 1,000 and divide by the voltage: mAh = Wh × 1,000 ÷ V. 100 Wh at 3.7 V is about 27,027 mAh. The same 100 Wh stated at 5 V would be 20,000 mAh, which is why a mAh figure means nothing until you know the voltage it was rated at.
What voltage should I use for a power bank?+
The internal cell voltage, usually 3.6 or 3.7 V for lithium-ion — not the 5 V USB output. Power banks almost always state mAh at the cell. Many labels also print the Wh figure or the cell voltage next to the mAh; if the label gives Wh, use that directly.
How many watt-hours is a 20,000 mAh power bank?+
About 74 Wh at 3.7 V, or 72 Wh at 3.6 V. That is under the FAA's 100 Wh limit for a spare lithium-ion battery without airline approval.
Why doesn't a 10,000 mAh power bank charge a 5,000 mAh phone twice?+
Two reasons. Energy is lost boosting the cell voltage up to the USB output and again in the phone's own charging circuit, and the phone's cell is often at a slightly higher voltage than the bank's. Compare watt-hours and allow for losses: 37 Wh at an assumed 85 percent delivers about 31 Wh, and a 5,000 mAh phone battery at 3.85 V holds about 19 Wh — roughly 1.6 charges, not 2.
How many mAh can I take on a plane?+
The FAA limit is in watt-hours, not mAh: a spare lithium-ion battery or power bank may be up to 100 Wh without airline approval, and up to two spares over 100 Wh up to 160 Wh with airline approval. Spares go in carry-on only. At 3.7 V, 100 Wh is about 27,000 mAh. Airlines can set stricter limits.
Is this a determination?+
No. It reproduces the relationship Wh = mAh × V ÷ 1,000 as a preparation aid, and the charge count uses an efficiency you choose. It does not test a battery's real capacity, confirm what an airline will accept, or size a home battery system.
Related resources
Amp Hours to Watt Hours Calculator
Wh from an Ah rating, with usable energy after depth of discharge.
Learn more →Milliamps to Amps Calculator
mA to A, and why mAh is a capacity rather than a current.
Learn more →How Many Watts Does a Laptop Use?
Charger ratings, real draw, battery Wh and outage runtime.
Learn more →Home Battery Backup
When the battery you need is measured in kWh, not mAh.
Learn more →From power banks to a battery for the house
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