Battery Watt-Hours Calculator

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.

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. 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.

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.

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. The battery life calculator turns capacity and load into runtime, and 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.

The formula

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

Worked example with the defaults: 100 Ah × 12 V = 1,200 Wh, which is 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.

Two limits are worth keeping in view. Watt-hours describe the energy a healthy pack stores at its nominal voltage, not the energy you should draw out of it: depth of discharge decides that, and it varies sharply by chemistry. And capacity ratings are quoted at a specified discharge rate, conventionally the twenty-hour rate for deep-cycle lead-acid, so a battery emptied far faster than that delivers noticeably less than its nameplate suggests. Neither effect changes the arithmetic on this page; both change what the resulting number means once the battery is in service.

Frequently asked questions