Battery Watt-Hours Calculator
Amp-hours × volts to watt-hours — the only fair way to compare batteries of different voltages.
Updated
You need
1,200 Whof energy
100 Ah × 12 V
- As kilowatt-hours
- 1.20 kWh
Over the 100 Wh airline carry-on limit — 100–160 Wh needs airline approval, and over 160 Wh is barred from passenger flights.
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In short
How many watt-hours is a 100 Ah 12 V battery?
Watt-hours = amp-hours × volts, so 100 Ah at 12 V is 1,200 Wh, or 1.2 kWh. Change only the voltage and the energy follows: the same 100 Ah label holds 2,400 Wh at 24 V and 4,800 Wh at 48 V. That is why amp-hours alone cannot compare two batteries, and why airlines regulate lithium packs in watt-hours.
A nameplate rating is stored energy, not usable energy, because how much of it you should actually draw depends on the chemistry and its depth-of-discharge limit.
How to use the battery watt-hours calculator
Enter the amp-hour capacity and the nominal voltage and the tool returns the energy in watt-hours and kilowatt-hours. That single number is the only fair way to compare two batteries, because an amp-hour rating on its own describes a quantity of charge and says nothing about the energy that charge carries.
A 100 Ah label on a 12 V leisure battery means 1,200 Wh. The identical 100 Ah label on a 48 V rack battery means 4,800 Wh — four times the energy behind the same headline figure. Convert both to watt-hours and the comparison becomes honest, whatever the chemistry and whatever the voltage.
Watt-hours is also the unit that connects a battery to a load, because appliances, daily household usage, and your electricity bill are already measured in watt-hours and kilowatt-hours.
1,200 Wh
100 Ah at 12 V
the same figure as 1.2 kWh
2,400 Wh
Same label at 24 V
double the energy, identical Ah
4,800 Wh
Same label at 48 V
four times the 12 V figure
The place this bites hardest is the power bank in your bag. A pack labelled 10,000 mAh is 10 Ah at the 3.7 V nominal voltage of its lithium cells, which is 37 Wh of stored energy. Your phone charges over USB at 5 V, and what crosses the cable is energy rather than charge, so the same 37 Wh becomes 37 divided by 5, or 7.4 Ah, once it is counted at the higher voltage.
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That is 7,400 mAh before the boost converter has taken its cut, and converters commonly run somewhere in the 85 to 90 percent efficiency band, which lands the delivered figure nearer 6,300 to 6,700 mAh. Nothing is broken and nobody is cheating you: the label counts charge at the cell voltage while your phone counts charge at the USB voltage, and only the watt-hours stay constant between the two.
To size a battery against a load, work in watt-hours from both ends. Add up what the load consumes in a day in watt-hours, compare that against the battery watt-hours, then derate for the fraction of the pack you can safely use.
A load drawing 1,000 Wh a day does not need a 1,000 Wh battery. On a lead-acid bank designed around 50 percent depth of discharge it needs roughly 2,000 Wh of nameplate capacity, and on LiFePO4 designed around 80 percent it needs about 1,250 Wh.
Need hours rather than watt-hours?
The battery life calculator turns capacity and load into runtime, once you have derated the nameplate for the depth of discharge your chemistry supports.
Open the battery life calculator →The solar battery bank calculator does the same job for a whole off-grid bank. Treat all of it as planning arithmetic rather than a design: anything that will be wired into a building or a vehicle should be sized and signed off by a licensed electrician or installer.
Do
- Enter the nominal voltage the pack is rated at, not its charging voltage.
- Divide a milliamp-hour label by 1,000 before entering it as amp-hours.
- Rank every candidate battery in watt-hours rather than amp-hours.
- Check the figure against the 100 Wh and 160 Wh aviation thresholds before flying.
Don't
- Compare two batteries on amp-hours, a label that describes charge and not energy.
- Read the nameplate watt-hours as the energy you should actually draw out.
- Expect a 10,000 mAh power bank to hand a phone 10,000 mAh at the USB voltage.
- Pack a spare lithium battery or a power bank in checked baggage.
What familiar batteries actually hold once amp-hours are multiplied by volts, and which side of the aviation watt-hour thresholds each one lands on. The point of the table is the ordering: a tool pack you would not think twice about can outrank a power bank you assume is the bigger battery, because the voltage does most of the work.
| Battery or device | Illustrative nameplate rating | Watt-hours (Ah × V) | Where that lands on the aviation thresholds |
|---|---|---|---|
| Phone battery, installed in the handset | 4.0 Ah at 3.7 V | 14.8 Wh | Under 100 Wh, no approval needed |
| Mirrorless or DSLR camera spare | 2.0 Ah at 7.2 V | 14.4 Wh | Under 100 Wh, carry-on only as a spare |
| Power bank labelled 10,000 mAh | 10 Ah at 3.7 V | 37 Wh | Under 100 Wh, carry-on only |
| Laptop battery | 5.0 Ah at 11.4 V | 57 Wh | Under 100 Wh, no approval needed |
| Power bank labelled 20,000 mAh | 20 Ah at 3.7 V | 74 Wh | Under 100 Wh, carry-on only |
| Drone flight pack | 5.0 Ah at 15.4 V | 77 Wh | Under 100 Wh, carry-on only |
| Cordless tool pack, 5 Ah | 5.0 Ah at 18 V | 90 Wh | Under 100 Wh, close to the line |
| Power bank labelled 26,800 mAh | 26.8 Ah at 3.7 V | 99.2 Wh | Under 100 Wh by less than a watt-hour |
| Power bank labelled 30,000 mAh | 30 Ah at 3.7 V | 111 Wh | In the 100 to 160 Wh band, operator approval required |
| Cordless tool pack, 9 Ah | 9.0 Ah at 18 V | 162 Wh | Above 160 Wh, not accepted on a passenger aircraft |
| Electric bike battery | 10 Ah at 36 V | 360 Wh | Far above 160 Wh, not accepted |
| Portable power station | 42 Ah at 12 V | 504 Wh | Far above 160 Wh, not accepted |
| Leisure or deep-cycle battery | 100 Ah at 12 V | 1,200 Wh | Nowhere near cabin legal |
| Home storage rack battery | 100 Ah at 48 V | 4,800 Wh | Nowhere near cabin legal |
Will it fly? The airline watt-hour bands
Watt-hours is also the exact unit aviation regulators work in, which makes this the conversion travellers need. FAA PackSafe guidance and IATA passenger guidance draw the same two lines. A lithium-ion battery rated at 100 Wh or less travels in carry-on without approval.
A battery rated above 100 Wh and up to 160 Wh needs the operator approval, and passengers are generally limited to two of those larger spares. Anything rated above 160 Wh is not accepted on a passenger aircraft at all. Spare batteries and power banks belong in the cabin and never in checked baggage.
Run your own packs through the tool before you pack: a 20,000 mAh power bank is 74 Wh and travels freely, while an 18 V 9 Ah cordless tool pack is 162 Wh and does not. Your airline is the final authority on the middle band.
100 Wh
Carry-on without approval
at or under this line
160 Wh
Ceiling with operator approval
generally two spares at most
74 Wh
20,000 mAh power bank
travels freely in the cabin
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The formula, worked line by line
Charge multiplied by potential difference is energy. An amp-hour is one amp flowing for one hour, which is 3,600 coulombs of charge moved; a volt is one joule of energy carried per coulomb. Multiply the two and the coulombs cancel, leaving joules. Expressed in the units actually printed on battery labels, that is amp-hours times volts, and the answer arrives in watt-hours.
The only judgement call is which voltage to use. Battery labels quote a nominal voltage, an agreed mid-discharge figure rather than a measured one: 3.7 V for a lithium-ion cell that in practice swings from about 4.2 V full down to 3.0 V empty, 12 V for a lead-acid battery that rests near 12.7 V and climbs above 14 V on charge. Nominal is the correct number here, and it is the number aviation regulators and manufacturers both quote.
watt-hours = amp-hours × volts
kilowatt-hours = watt-hours ÷ 1000
amp-hours = watt-hours ÷ volts (the reverse)
100 Ah × 12 V = 1,200 Wh = 1.2 kWh- Amp-hour rating
- 100 Ah
- Nominal voltage
- 12 V
- Multiply
- 100 × 12
- Stored energy
- 1,200 Wh, or 1.2 kWh
Hold the amp-hours still and change only the voltage and the energy tracks it exactly — 100 Ah at 24 V is 2,400 Wh, and at 48 V it is 4,800 Wh. Run the comparison the other way and a 50 Ah 24 V battery also comes to 1,200 Wh, identical in energy to the 12 V 100 Ah battery despite carrying half the amp-hour label. That pair is the clearest demonstration of why amp-hours cannot rank batteries on their own.
A second worked example, in the units that mislead people most. A 10,000 mAh power bank is 10 Ah at a 3.7 V nominal cell voltage, so 10 × 3.7 = 37 Wh. Divide that 37 Wh by the 5 V that USB delivers and you get 7.4 Ah, or 7,400 mAh, as the theoretical charge available at the port.
Apply a boost-converter efficiency in the 85 to 90 percent range and roughly 6,300 to 6,700 mAh actually reaches the phone. The watt-hours never changed. Only the voltage the charge is being counted at changed, and the converter took its share on the way through.
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Questions people ask
How many watt-hours is a 100 Ah battery?
It depends entirely on the voltage, which is the whole point of the conversion. At 12 V a 100 Ah battery holds 1,200 Wh, or 1.2 kWh. At 24 V the same amp-hour rating is 2,400 Wh, and at 48 V it is 4,800 Wh. The amp-hour figure is only half the story: on its own it describes charge, and until you multiply it by the voltage it tells you nothing about how much energy the battery actually stores or how long it will run anything.
Why convert amp-hours to watt-hours at all?
Because amp-hours cannot be compared across different voltages and watt-hours can. A 100 Ah 12 V battery and a 50 Ah 24 V battery look completely different on their labels yet hold the same 1,200 Wh. Converting to watt-hours puts every battery on one scale regardless of chemistry or system voltage, and it is also the unit your loads are already measured in, so it is the only figure that lets you match a battery to a daily energy demand without a second conversion.
What is the airline watt-hour limit for lithium batteries?
FAA PackSafe and IATA passenger guidance both set the same thresholds. Lithium-ion batteries rated at 100 Wh or less travel in carry-on baggage without approval. Batteries rated above 100 Wh and up to 160 Wh require the operator approval and passengers are generally limited to two such spares. Anything above 160 Wh is not accepted on a passenger aircraft. Spares and power banks must be carried in the cabin, not in checked baggage, and your airline is the final authority in every case.
Why does my 10,000 mAh power bank not give my phone 10,000 mAh?
Because the two numbers are counted at different voltages. The 10,000 mAh label is charge at the 3.7 V nominal cell voltage, which is 37 Wh of energy. USB delivers at 5 V, and the energy is what carries across, so 37 Wh at 5 V is only 7.4 Ah, or 7,400 mAh, before any losses. Boost converters typically run about 85 to 90 percent efficient, so roughly 6,300 to 6,700 mAh reaches the phone. The watt-hours are conserved even though the milliamp-hours are not.
How do I size a battery to my daily energy use?
Work in watt-hours at both ends, then derate. If a load uses 1,000 Wh a day and you want a full day of cover, you need 1,000 Wh of usable energy, which is not the same as 1,000 Wh of nameplate capacity. Divide by the depth of discharge your chemistry supports: about 0.50 for lead-acid gives roughly 2,000 Wh of nameplate, while 0.80 for LiFePO4 gives about 1,250 Wh. The battery life calculator converts that into runtime for a specific load.
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
The cabin thresholds this page reports: lithium batteries at 100 Wh or less travel without approval, 100 to 160 Wh needs operator approval and is generally limited to two spares, and spares must be in carry-on rather than checked baggage.
PackSafe — Lithium Batteries — US Federal Aviation Administration, 2026
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