Amp-Hours to Watt-Hours: Convert Ah to Wh Correctly

To convert amp-hours to watt-hours, multiply the battery’s amp-hour rating by its nominal voltage:

Watt-hours (Wh) = amp-hours (Ah) x volts (V)

The Federal Aviation Administration uses the same formula in its current battery guidance. A battery rated at 100Ah and 12.8V stores 1280Wh of nameplate energy:

100Ah x 12.8V = 1280Wh

That does not mean an AC appliance will receive all 1280Wh. The conversion answers one question: how much energy the battery label represents. Battery limits, reserve, conversion loss, temperature, age, and load behavior come afterward.

Use the amp-hours to watt-hours calculator for the conversion, including a selectable reserve and a runtime example.

Why Ah alone cannot compare batteries

Amp-hours describe charge capacity, not energy by themselves. Voltage supplies the missing part.

Battery label	Calculation	Nameplate energy
50Ah at 12V	50 x 12	600Wh
100Ah at 12.8V	100 x 12.8	1280Wh
50Ah at 24V	50 x 24	1200Wh
25Ah at 48V	25 x 48	1200Wh

The 50Ah, 24V battery and 25Ah, 48V battery in this table contain the same nominal energy even though their Ah ratings differ by two to one. Comparing Ah without voltage would reach the wrong conclusion.

Use nominal voltage, not a live meter reading

A battery’s terminal voltage changes with state of charge, load, charging, temperature, and chemistry. Capacity specifications are based on a stated nominal voltage and test method. For label-to-label comparison, use the nominal voltage in the data sheet.

That is why a product sold in the broad “12V” category may be specified as 12V, 12.8V, or another nominal value. Do not replace the manufacturer’s number with 12 just because the system is called 12V.

If the label shows milliamp-hours, convert first:

Ah = mAh / 1000
Wh = (mAh / 1000) x V

For example, 10,000mAh at 3.7V is 37Wh, not 120Wh and not 10,000Wh.

Stored energy, planned battery energy, and delivered AC energy

Keep these three values separate:

1. Stored nameplate energy: Ah x nominal V
2. Energy available after your reserve: stored Wh x usable fraction
3. Estimated energy delivered through an inverter: available Wh x inverter efficiency

Here is an illustrative calculation for a 100Ah, 12.8V battery. Assume a 20 percent reserve and 90 percent inverter efficiency:

Stored energy = 100Ah x 12.8V = 1280Wh
After 20% reserve = 1280Wh x 0.80 = 1024Wh
Estimated AC energy = 1024Wh x 0.90 = 922Wh
Runtime for a constant 100W AC load = 922Wh / 100W = 9.2 hours

The 20 percent reserve and 90 percent efficiency are user-selected planning assumptions, not universal values for lithium, lead-acid, or every inverter. Use the battery manual’s discharge limits and the inverter’s efficiency information when available. Also account for inverter idle power during long, light-load operation.

For an all-in-one power station, start with the manufacturer’s stated Wh rather than reconstructing cell voltage and Ah. Enter that Wh directly in the power station runtime calculator.

Series and parallel change different parts of the label

Only combine batteries in configurations the manufacturer permits. Batteries should be compatible in model, chemistry, capacity, age, state, protection system, and wiring arrangement as required by their manuals.

For two identical 12V, 100Ah batteries:

• Series: voltage adds, Ah stays the same. 24V x 100Ah = 2400Wh.
• Parallel: Ah adds, voltage stays the same. 12V x 200Ah = 2400Wh.

The total nameplate Wh is 2400Wh in both examples. Current, cable, fuse, balancing, and equipment-voltage requirements are different, so equal energy does not make the two layouts interchangeable.

Common conversion mistakes

Treating Ah as runtime

A 100Ah battery does not automatically run a 10A load for exactly ten hours. The rating depends on the manufacturer’s discharge test, cutoff voltage, temperature, battery condition, and chemistry. Convert to Wh for energy planning, then use the product’s documented limits.

Applying a blanket depth-of-discharge rule

Rules such as “use only half of every lead-acid battery” or “all lithium is 100 percent usable” are too broad for a purchase decision. Different products specify different cutoff behavior, cycle-life conditions, and recommended operating ranges. State the reserve you are using and verify it against the exact manual.

Ignoring the battery management system

Watt-hours do not tell you the maximum charge or discharge current. A battery may contain enough energy but have a battery-management-system limit below what an inverter or motor load demands. Check continuous and peak current, permitted connection arrangements, and low-temperature charge behavior.

Sizing wire or a fuse from Wh

Energy capacity does not determine conductor and overcurrent protection by itself. Current, voltage, cable length, installation method, equipment terminals, and available fault current matter. Victron’s current battery guidance, for example, requires battery cables sized for expected current and a fuse appropriate to the lowest applicable cable, battery, or system current rating. Follow the exact equipment manuals and local requirements.

A clean battery comparison process

1. Record the exact model’s nominal voltage and Ah.
2. Calculate nameplate Wh.
3. Check the manual’s operating, discharge, charging, temperature, and connection limits.
4. Apply an explicit reserve for your use case.
5. Apply conversion efficiency only when a converter or inverter is in the power path.
6. Compare delivered energy, maximum current, cycle conditions, warranty, dimensions, and weight separately.

Next action: convert the exact battery label with the Ah-to-Wh calculator, then put the resulting Wh into the power station size calculator or runtime calculator for the actual load.