SizingKitPower & current
Watt hour calculator
A watt-hour is an amp-hour multiplied by a volt, so a 100 Ah pack at 12.8 V holds 1,280 Wh and the same 100 Ah at 51.2 V holds 5,120 Wh. This page solves that identity in all three directions — capacity and voltage into energy, load and run time into energy, energy and voltage back into capacity — and prints the result in watt-hours, kilowatt-hours, BTU and kilojoules together. It also says which side of the 100 Wh airline threshold the answer lands on, because that limit is written in watt-hours and the label on most packs is not.
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- Wh, kWh, BTU, kJ
- Ah and mAh in
- 100 Wh air-travel check
Which two figures do you have?
The identity is one line — watt-hours are amp-hours multiplied by volts — so pick the direction you are missing and the other three follow.
A phone bank prints mAh, a deep-cycle block prints Ah, and 20,000 mAh is 20 Ah.
Nominal, not the resting or the charging figure. The chart below shows what the label on a pack is made of.
Energy
WATT-HOURS
1,280 Wh
KILOWATT-HOURS
1.28 kWh
- Thermal equivalent
- 4,368 BTU
- SI
- 4,608 kJ
- Charge at 12.8 V
- 100 Ah
Air travel. 1,280 Wh is over the 160 Wh ceiling, which puts it outside passenger baggage entirely — it moves as cargo under the packing instruction for its cell type. Source: 49 CFR 175.10(a)(18), the US enactment of the ICAO Technical Instructions passenger provisions. The rule counts watt-hours, never milliamp-hours, which is why a bank prints both.
100 Ah, at ten nameplate voltages
Every row holds the same count of charge and a different amount of energy. This is the whole reason a capacity in amp-hours cannot be compared with another one until both carry their voltage.
| Pack | Cells | Nominal | Energy |
|---|---|---|---|
| LiFePO4 cell | 1S x 3.2 V | 3.2 V | 320 Wh |
| Li-ion cell (18650, pouch) | 1S x 3.6 V | 3.6 V | 360 Wh |
| Lead-acid block, 3 cells | 3S x 2 V | 6 V | 600 Wh |
| Li-ion 3S laptop pack | 3S x 3.6 V | 10.8 V | 1,080 Wh |
| Lead-acid block, 6 cells | 6S x 2 V | 12 V | 1,200 Wh |
| LiFePO4 4S drop-in | 4S x 3.2 V | 12.8 V | 1,280 Wh |
| Lead-acid string, 12 cells | 12S x 2 V | 24 V | 2,400 Wh |
| LiFePO4 8S bank | 8S x 3.2 V | 25.6 V | 2,560 Wh |
| Lead-acid string, 24 cells | 24S x 2 V | 48 V | 4,800 Wh |
| LiFePO4 16S bank | 16S x 3.2 V | 51.2 V | 5,120 Wh |
- Nominal column generated as cells multiplied by cell voltage, never transcribed as a list of pack sizes.
- 3.2 V/cell — lithium iron phosphate cell datasheets — 3.2 V nominal, on a discharge plateau that barely moves between 90% and 20% charge. A few LiFePO4 datasheets publish 3.3 V for the same cell, which shifts a 16S pack from 51.2 V to 52.8 V.
- 3.6 V/cell — cobalt-oxide and NMC lithium-ion cell datasheets — 3.6 V nominal. 3.7 V is the other published nominal for the same cell chemistry, and it is the whole reason one laptop pack is labeled 10.8 V and the next 11.1 V.
- 2 V/cell — lead-acid cell nominal — six of them are what makes a block a 12 V block. A healthy flooded cell rests near 2.12 V fully charged and 1.95 V at half, so nominal is a naming convention rather than a reading.
- Where the case prints its own watt-hour figure, that figure wins over this table.
How to turn a battery label into watt-hours
Three fields, one identity. The only figure people leave out is the one that makes the answer possible.
Pick the direction you are missing
Capacity and voltage into energy is the usual one — a 7.2 Ah sealed block at 12 V. Choose load and run time instead when you know the draw and how long it has to hold, or energy at a voltage when a datasheet gave you 1,280 Wh and you need the amp-hours a charge controller is set in.
Type the capacity in the unit the case prints
A phone bank is marked in milliamp-hours and a deep-cycle block in amp-hours, so the field takes both and 20,000 mAh becomes 20 Ah. Units written into the box beat the selector beside it, so 7.2 Ah pasted into a field set to mAh still reads as amp-hours.
Put the nominal voltage beside it
Use the figure the case is named after — 12.8 V for a four-cell lithium iron phosphate drop-in, 12.0 V for a six-cell lead-acid block, 3.6 V for a bare lithium-ion cell — not the 13.6 V your meter reads on a charger. The chart under the answer shows what each of those nameplates is made of, and prints on one sheet.
Technical specifications
| Identity | Wh = Ah × V, solved in all three directions. Power × time goes through the same joule base, so 500 W for 3 h and 125 Ah at 12 V return the identical 1,500 Wh. |
|---|---|
| Energy units reported | Wh, kWh, BTU and kJ from a single entry. The energy input accepts all ten the site carries, including the therm at exactly 100,000 BTU and the International Table calorie. |
| Capacity units accepted | Ah and mAh, with 1 Ah = 1,000 mAh. The number may be written with or without its unit, and thousands separators are read: 20,000 mAh and 20000mah are the same entry. |
| Voltage range read | 0.5 V to 1,500 V. Below half a volt is not a battery terminal, and above 1,500 V is a DC bus rather than a pack this page is written for. |
| Nameplate chart | 10 rows, from a single 3.2 V LiFePO4 cell to a 16-cell 51.2 V bank. Every nominal voltage in it is generated as cell count × cell voltage rather than transcribed. |
| Air-travel thresholds | 100 Wh with no approval; 101–160 Wh with the operator's approval and two spare batteries maximum; above 160 Wh not in passenger baggage at all. 49 CFR 175.10(a)(18). |
| Cell figures used | 3.2 V lithium iron phosphate, 3.6 V lithium-ion, 2.0 V lead-acid — all three shown as datasheet values rather than facts, with what moves them printed under the chart. |
| Runs with no signal | The arithmetic is JavaScript in the tab, so the page still answers at a boarding gate on airplane mode once it has loaded. |
Frequently asked questions
Why do amp-hours mean nothing until a voltage is written next to them?
Because an amp-hour counts charge, not energy: it is 3,600 coulombs, and coulombs do no work until they fall through a voltage. A 100 Ah pack at 12.8 V and a 100 Ah pack at 51.2 V hold the same charge and four times different energy, which is why a specification sheet that lists only amp-hours has told you a quarter of what you need.
Is my 20,000 mAh power bank under the 100 Wh airline limit?
Almost certainly yes, at about 72 Wh. The cells inside are lithium-ion at 3.6 V nominal, so 20 Ah × 3.6 V is 72 Wh; the mistake that puts people over is multiplying by the 5 V of the USB output instead, which returns 100 Wh and describes a battery that does not exist. Regulators write the limit in watt-hours for exactly this reason, and most reputable banks now print the watt-hour figure on the case so nobody has to do the multiplication at a security desk.
Is a 12 V battery actually 12 V?
Three different packs are all sold as 12 V and none of them is 12.0 V in service. Six lead-acid cells at 2.0 V nominal make 12.0 V, four lithium iron phosphate cells at 3.2 V make 12.8 V, and three lithium-ion cells at 3.6 V make 10.8 V — a spread of nearly 20% in the energy the same amp-hour rating represents. The 3.6-versus-3.7 V disagreement between lithium-ion datasheets is why one laptop pack is labeled 10.8 V and the next 11.1 V.
What is the difference between a watt and a watt-hour?
A watt is a rate and a watt-hour is an amount, the same way a gallon per minute is not a gallon. A 1,500 W heater draws 1,500 W whether it runs for a minute or a week; run it for two hours and it has consumed 3,000 Wh. Confusing the two is what produces a battery specified in watts, which is a statement with no time in it and therefore no meaning.
Nominal voltage, or the voltage my meter reads?
Nominal, every time, for an energy figure. Terminal voltage moves through the whole discharge — a lithium iron phosphate cell sits near 3.65 V full and 2.5 V empty — and using the top of that range inflates the energy by a fifth. Nominal is the manufacturer's own average over the discharge, which is precisely the number that makes amp-hours times volts come out right.
How does this relate to the kWh on my electricity bill?
It is the same unit, a thousand times larger: 1 kWh is 1,000 Wh, 3,600 kJ and 3,412 BTU. A 5 kWh home battery therefore holds about five hours of a typical evening's household draw, and the figures on this page convert straight into the ones your utility charges against.
Does this tell me how much of the battery I can actually use?
No — everything here is the number on the label, taken at face value. Depth of discharge, the Peukert effect at high currents and the cold-weather penalty are all separate reductions applied to that figure and they live on the amp hour calculator, where a lead-acid bank worked hard comes out well under half of what this page reports.
The watt-hour, and the voltage hidden inside every amp-hour
The watt-hour is a unit of energy defined as one watt sustained for one hour, which is 3,600 joules exactly. The amp-hour is not a unit of energy at all — it counts charge, one ampere flowing for one hour, or 3,600 coulombs — and the gap between those two ideas is where nearly every battery misunderstanding starts. Charge only becomes energy once it falls through a potential difference, so the amp-hour figure that dominates battery marketing is meaningless on its own and merely misleading when two packs at different voltages are compared with it. Multiply by the nominal voltage and the comparison becomes honest, which is the entire job of this page.
Air-transport regulators reached the same conclusion decades ago and wrote their limits in watt-hours rather than in the milliamp-hours printed on consumer goods. Under 49 CFR 175.10(a)(18) — the US enactment of the ICAO Technical Instructions that most of the world flies under — a lithium-ion battery of 100 Wh or less travels in the cabin without anybody's permission, one between 101 and 160 Wh needs the operator's approval and is capped at two spares, and anything above 160 Wh leaves passenger baggage entirely. The arithmetic that trips people is the power bank: its cells are 3.6 V, not the 5 V it hands to a phone, and using the output voltage inflates a compliant 72 Wh bank into an apparently non-compliant 100 Wh one.
Everything on this page treats the label as true, and the label is not the usable capacity. What you may actually take out is the label reduced by depth of discharge, by the rate you draw it at and by temperature, which is the subject of the amp hour calculator; how big a bank has to be to carry a load list for a stated number of days belongs to the battery capacity calculator. If what you are converting is a load rather than a store — volts, amps and watts in the same breath — the volts amps watts calculator solves that triangle, and watts to amps handles the power-factor case a motor nameplate needs. Energy stored in a capacitor rather than a cell obeys a different law again: it drains on an exponential governed by the RC time constant, not on an amp-hour rating.
What happens to the numbers you type here
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The capacity, voltage and load you enter never leave the tab, so a pack you are checking against an airline limit — and the trip it belongs to — is not information this site holds.