Battery capacity gets printed in two different units, and they do not measure the same thing. Amp-hours (Ah) measure electric charge. Watt-hours (Wh) measure energy. You can convert between them, but only if you know the battery's voltage — and that missing voltage is exactly where most capacity comparisons go wrong. Two batteries can share the same amp-hour rating and still store noticeably different amounts of energy.

This article gives you a durable way to read both numbers, one conversion you can check two ways, and a clear sense of what a capacity figure still does not tell you. It assumes you have already met the difference between power and energy; if not, that is the better place to start.

The short answer

  • Amp-hours (Ah) measure charge — how much electric charge the battery can pass. The formal SI unit of charge is the coulomb (C); one amp-hour is just one ampere flowing for one hour.
  • Watt-hours (Wh) measure energy — how much work that charge can do. One watt-hour is one watt sustained for one hour.
  • The two are linked by voltage:
Energy (Wh) = Charge (Ah) × Voltage (V)

So an amp-hour figure on its own is only half of an energy statement. Give it a voltage and it becomes watt-hours; leave the voltage unstated and you cannot compare two batteries fairly.

Charge and energy are different quantities

An amp-hour counts charge moved over time. A battery rated 100 Ah can, under its rated conditions, deliver 100 amperes for one hour, or 10 amperes for ten hours — the same total charge either way. This is useful, because much of a battery's own behaviour (its charge and discharge currents, its state-of-charge accounting) is naturally tracked in charge. The formal unit of charge is the coulomb, defined as one ampere-second, so one amp-hour is 3 600 coulombs — a count of charge, not of energy.

A watt-hour counts energy. Because the watt is one joule per second and an hour is 3 600 seconds, one watt-hour is 3 600 joules. Energy is what actually runs your loads: a light, a fridge, or a pump consumes watt-hours, and your storage has to supply them. When you want to know how long something will run, or whether a day's loads fit inside a battery, you are asking an energy question, and energy is measured in watt-hours.

These unit definitions are not Viherion's convention; they come from the SI, the international system of units maintained through the BIPM and documented by national metrology institutes such as NIST. The coulomb is the SI unit of charge, the joule is the SI unit of energy, and the watt is one joule per second.

The bridge between them is voltage

Charge becomes energy through voltage. The volt is defined as energy per unit charge — one volt means one joule is carried by each coulomb of charge (V = J/C). Multiply a quantity of charge by the voltage it moves through and you get energy:

Energy (J) = Charge (C) × Voltage (V)

Rewriting that in the practical battery units — amp-hours instead of coulombs, watt-hours instead of joules — the two "per hour" and "3 600" factors cancel, and you are left with the clean relationship:

Energy (Wh) = Charge (Ah) × Voltage (V)

One caution is built into that equation: it is exact only when the voltage is constant. A real battery's terminal voltage drifts as it charges and discharges. So in practice we multiply amp-hours by the battery's nominal voltage — a single representative figure the manufacturer assigns to the whole discharge. That makes Ah × V a very good estimate rather than a guaranteed exact energy, a point we return to at the end.

A map between the three quantities

Because charge, voltage, and energy are locked together, knowing any two gives the third:

                 Energy (Wh)
                /          \
     ÷ Voltage (V)          ÷ Charge (Ah)
              /              \
     Charge (Ah) ← × Voltage → Voltage (V)


  Energy  = Charge × Voltage    →  Wh = Ah × V
  Charge  = Energy ÷ Voltage    →  Ah = Wh ÷ V
  Voltage = Energy ÷ Charge     →  V  = Wh ÷ Ah

Keep the units beside the numbers and the arithmetic checks itself: amp-hours times volts gives watt-hours. If a calculation hands you plain amp-hours where you expected an energy, a voltage went missing somewhere.

Worked example: converting a real rating

Question: How much energy does a 12.8 V, 100 Ah battery store?

A widely sold lithium iron phosphate (LiFePO₄) battery is rated at a nominal 12.8 V and 100 Ah. Applying the relationship:

  • Formula: Energy = Charge × Voltage
  • Inputs: Charge = 100 Ah, Voltage = 12.8 V (nominal)
Energy = 100 Ah × 12.8 V = 1 280 Wh = 1.28 kWh

That matches the 1 280 Wh printed on such a battery's own data sheet, which is a good sign the units line up. Check it a second way through the base SI units: 100 Ah is 360 000 coulombs, and each coulomb carries 12.8 joules at 12.8 V, so 360 000 C × 12.8 J/C = 4 608 000 J. Dividing by 3 600 joules per watt-hour gives 1 280 Wh — the same answer by an independent path. The result is stated to three significant figures because the inputs justify no more.

Why the same amp-hours can mean different energy

Here is the trap. Suppose two batteries are both rated 100 Ah, but one is a lead-acid battery at a nominal 12 V and the other is a LiFePO₄ battery at a nominal 12.8 V:

  • Lead-acid: 100 Ah × 12 V = 1 200 Wh
  • LiFePO₄: 100 Ah × 12.8 V = 1 280 Wh

Same charge rating, but about 80 Wh — roughly 7% — more energy in the second, purely because its nominal voltage is higher. The difference comes from chemistry: a lead-acid cell is nominally about 2 V, so a 12 V battery stacks six of them, whereas a LiFePO₄ cell is nominally about 3.2 V, so a "12 V" LiFePO₄ battery is really four cells at 12.8 V. Other lithium chemistries sit at different cell voltages again.

This is why "it's a 12-volt battery" is a loose statement, and why comparing two batteries by amp-hours alone compares the wrong quantity. To compare storage fairly:

  • Convert both to watt-hours at each battery's own nominal voltage.
  • Compare the watt-hour figures, not the amp-hour figures.
  • Only compare amp-hours directly when the two batteries are genuinely at the same nominal voltage.
Battery (example) Charge Nominal voltage Energy
Lead-acid "12 V" 100 Ah 12 V 1 200 Wh
LiFePO₄ "12 V" 100 Ah 12.8 V 1 280 Wh
LiFePO₄ "24 V" 100 Ah 25.6 V 2 560 Wh

The third row makes the same point more dramatically: the same 100 Ah at a 24 V nominal holds roughly twice the energy of the 12 V version, because the charge is moved through twice the voltage.

What the number still does not tell you

Converting amp-hours to watt-hours correctly gets you a trustworthy nameplate energy. It does not yet tell you how much energy you can actually use, and it says nothing about the battery's health. A few reasons the real figure is lower and more conditional than the label:

  • Not all of it is usable. Most batteries should not be fully discharged every cycle. How much you can safely draw — the usable fraction — depends on chemistry and on how hard you cycle it. The same example battery is rated for far more cycles when discharged to 60% than to 100%.
  • The rating assumes specific conditions. Amp-hour capacity is measured at a stated discharge current and temperature. Draw current faster, or operate cold, and the charge you actually get out can be less than the label.
  • Voltage sags under load. Because Ah × V is exact only at constant voltage, the energy a battery delivers in practice follows its real discharge voltage, which sags under heavy load — so treat the nominal-voltage product as a close estimate, not a guarantee.
  • Capacity fades with age. A battery's charge capacity declines over its life. A monitor's amp-hour count and the original nameplate are not direct measurements of present health.

Each of these deserves its own treatment. The single most important next step is separating the nameplate from the part you can plan around, which is exactly the subject of usable vs. nominal battery capacity.

Key takeaways

  • Amp-hours measure charge; watt-hours measure energy. They are different quantities, not two names for the same thing.
  • Voltage bridges them: Wh = Ah × V, exact at constant voltage and a close estimate at a battery's nominal voltage.
  • Two batteries at the same amp-hours but different nominal voltages hold different energy — always compare in watt-hours at each battery's own voltage.
  • A correct watt-hour figure is still only the nameplate: usable fraction, discharge rate, temperature, and ageing all reduce or condition what you can actually rely on.

Where to go next