What a Battery Label Can Tell You: Volts, Amp-Hours and Watt-Hours Explained

A battery label might show 18 V, 5 Ah and 90 Wh. All three can describe the same battery, but each answers a different question. Understanding the difference makes workplace inventories more useful—and helps prevent assumptions about runtime, charging and compatibility.

In our previous article, we looked at the batteries that can go unnoticed inside ordinary workplace equipment. Once you have found them, the next challenge is recording what they are.

Writing “five large batteries” leaves plenty of room for interpretation. Recording the model, chemistry, voltage, capacity and energy rating gives the next person something more useful to work with.

You do not need to become an electrical engineer to read those details. Start by separating three measurements that are often treated as interchangeable.

Three units, three different meanings

• Volts (V) describe electrical voltage.
• Amp-hours (Ah) describe charge capacity.
• Watt-hours (Wh) describe an energy rating.

For a simple example, a battery rated at 18 V nominal and 5 Ah has a calculated energy rating of approximately 90 Wh. The numbers belong together; choosing only the largest number tells you very little.

Volts: check which voltage the label means

Voltage is the electrical potential difference between two points. On a battery label, the nominal voltage is a reference value used to describe the battery. Its actual voltage changes during use and charging.

“Nominal” matters because the largest voltage printed on a product may describe something else. For example, DEWALT explains that its 20V MAX designation refers to maximum initial voltage without a workload, while nominal voltage is 18 V. This is an example of how a manufacturer presents its ratings, not a recommendation for a particular product.

When estimating watt-hours, use the manufacturer's documented nominal voltage and the corresponding capacity for that battery. Prefer the manufacturer's stated Wh rating where it is available. If the figures appear inconsistent, check the model documentation instead of choosing whichever number produces the result you expect.

Some packs support more than one voltage configuration. Do not combine a voltage from one configuration with an Ah figure from another.

Amp-hours: capacity is not a current limit

Amp-hours describe an amount of electrical charge. They combine current, measured in amps, with time.

A 5 Ah rating does not mean “this battery supplies exactly 5 amps”, and it does not tell you the maximum current the battery can safely deliver. That requires separate manufacturer information.

At the same nominal voltage, a higher Ah rating represents more rated energy. It can be useful when comparing compatible batteries within a tool system. It is not, by itself, a promise that a tool will run for a particular number of hours.

The job being performed, the equipment's power demand and the battery's condition all affect the practical result. A workshop drill used intermittently and a machine running continuously are different workloads.

Watt-hours: a more useful way to compare energy

Watt-hours bring voltage and charge capacity together. The basic calculation is:

Nominal voltage (V) × capacity (Ah) ≈ energy rating (Wh)

The FAA's battery guidance explains the volts-times-amp-hours calculation. Here, we are using it to understand label information, not to establish transport permission or a workplace storage limit.

Consider two illustrative batteries:

• Battery A: 18 V × 5 Ah = 90 Wh.
• Battery B: 36 V × 5 Ah = 180 Wh.

Both say 5 Ah, but Battery B has twice the calculated energy rating. This is why comparing Ah alone across different voltages can be misleading. It does not mean the two batteries are interchangeable.

A Wh rating is also different from the amount of energy available right now. A partially charged or aged battery does not necessarily contain or deliver its full original rated energy. The label describes the rating; it is not a live measurement of charge or condition.

If a label already states Wh, record that value and its source. A calculation can help check your understanding, but it should not silently replace the manufacturer's specification.

Milliamp-hours: convert the units before comparing

Smaller batteries and power banks often use milliamp-hours, written as mAh. There are 1,000 mAh in 1 Ah, so divide by 1,000 before multiplying by voltage.

For example, 2,500 mAh is 2.5 Ah. If the corresponding nominal battery voltage is 7.2 V, the calculation is 7.2 × 2.5 = 18 Wh.

Power banks introduce another possible mix-up: the advertised capacity may refer to the internal cells, while a USB output has a different voltage. Anker's explanation of power-bank capacity uses a 10,000 mAh, 3.7 V example: 10 Ah × 3.7 V = 37 Wh.

Multiplying that same 10 Ah by a 5 V USB output would incorrectly produce 50 Wh by mixing ratings from different sides of the device. Conversion losses also reduce the energy delivered at the output. Use the manufacturer's stated Wh figure or the voltage explicitly associated with the capacity; do not assume every power bank uses the same internal voltage.

Watts and watt-hours are different measurements

Watts (W) measure power: the rate at which energy is transferred. Watt-hours (Wh) measure an amount of energy.

A charger marked 65 W is describing a power capability. A battery marked 65 Wh is describing an energy rating. The similar numbers do not make them equivalent, and neither figure alone establishes charging time.

As a simplified arithmetic example, 90 Wh divided by a constant 30 W load gives three hours. That assumes all 90 Wh is usable at the load, with no losses. Real equipment has changing demand, conversion losses and operating limits, so this is an explanation of the units—not a runtime guarantee.

The same distinction matters when recording a charging area: the batteries' combined Wh rating does not tell you the electrical load drawn by their chargers. Record charger information separately.

A matching voltage does not prove compatibility

Two batteries can share a voltage rating while belonging to different equipment systems. A connector that appears to fit is not enough to establish that a charger is suitable.

Identify the exact battery and charger models and follow the manufacturer's compatibility instructions. Michigan EGLE's charging guidance advises using charging equipment supplied or recommended by the device manufacturer.

If compatibility is uncertain, ask the manufacturer or an authorised supplier before connecting anything. Reading a label should support that check, not become a reason to try an unverified combination.

Likewise, Wh is not a safety score. It does not tell you whether a battery has been damaged, is subject to a recall or is suitable for a particular charging or storage arrangement. A smaller rating does not cancel a concern about condition. For those situations, see our article on battery warning signs and what to do next.

Turn the label into a useful inventory entry

Choose one type of battery-powered equipment in your workplace. From an accessible label and the relevant documentation, record:

• Manufacturer and exact battery or equipment model.
• Battery chemistry, where confirmed.
• Nominal voltage, with the source of that information.
• Capacity in Ah, converting from mAh if necessary.
• Manufacturer-stated Wh, or a clearly marked calculated estimate.
• A label photograph or document reference, where it is safe to obtain one.

Keep that information alongside the quantity, location, status and responsible person identified in your walkthrough. For ten batteries each rated at 90 Wh, the combined nameplate rating is 900 Wh, or 0.9 kWh. Keep the individual ratings and quantities as well; the total alone leaves out how the batteries are distributed and used.

If a label is inaccessible or unclear, use the equipment model to obtain documentation. Record “unconfirmed” where necessary. Do not open a battery pack, disturb damaged equipment or test-discharge a battery just to complete an inventory.

Our Battery Inventory & Site Risk Assessment guide provides a broader structure for the record. The useful first step is modest: replace vague descriptions with traceable information, and make the remaining questions visible.

Sources and further reading

• Samsung SDI: battery power, charge capacity and energy.
• FAA: determining a battery's watt-hour rating.
• DEWALT: maximum and nominal voltage terminology.
• Anker: internal capacity, output voltage and conversion losses.
• Michigan EGLE: manufacturer-recommended charging equipment.

Add those label details to your workplace battery inventory. Next in the series: Why One Battery Safety Rule Won’t Fit Every Workplace, which connects the equipment information with local working conditions.

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Editorial update: October 25, 2026. Prepared by Battery Safe Systems with AI assistance as general education for U.S. workplace readers. Follow manufacturer instructions, site procedures and applicable local requirements; this is not a site-specific assessment or independent certification. Numerical examples and diagrams are illustrative; use the actual equipment specifications. Workplace photographs are AI-generated and do not demonstrate approved installations.