Ah, Wh, Runtime, and Charging: Battery Terms Explained

Ah measures rated electric charge. Wh measures rated energy. Runtime is the result of operating a particular tool under particular conditions. Charge time is the result of a specific battery-and-charger cycle. These terms are related, but none should be substituted for another.

At the same nominal voltage, a 5Ah pack has 2.5 times the rated charge capacity of a 2Ah pack. That does not guarantee exactly 2.5 times the runtime in every tool.

Term	What it describes	Useful relationship	Does not prove
Ah	Rated charge capacity	5Ah ÷ 2Ah = 2.5	Power or exact runtime
Wh	Rated energy	Wh = nominal V × Ah	Usable minutes in every tool
Runtime	Time operating under stated conditions	Depends on tool, load, mode, pack, and environment	A fixed property of Ah alone
Charge time	Time for a defined battery/charger cycle	Use the exact manufacturer table	Ah divided by charger amps in all conditions

Amp-hour (Ah)

An amp-hour is a unit of electric charge. NIST gives 1 Ah = 3,600 coulombs. On a tool battery, Ah is a rated capacity label—not “amps per hour.”

Within the same approved voltage platform, a higher-Ah pack has more rated charge capacity. It may support more work between charges, but pack design, output capability, temperature, condition, electronics, and the tool load still affect the result.

Ah is not torque, speed, or power. Two packs with the same Ah rating can differ in size, weight, cell layout, supported output, and approved applications.

Watt-hour (Wh)

A watt-hour is a unit of energy. NIST gives 1 Wh = 3,600 joules. For a battery rating, the FAA uses:

Wh = nominal V × Ah

Example:

• 18 V × 2 Ah = 36 Wh
• 18 V × 5 Ah = 90 Wh

This is a rated-energy comparison. It is not a promise that a tool will deliver all of that energy as useful work or run for a fixed number of minutes.

Wh is more useful than Ah for an initial energy comparison across different voltages. Platform compatibility still comes first: a larger Wh value does not allow a battery to be used in an unrelated system.

Nominal voltage

Nominal voltage is the rated value used to describe a battery system under defined conventions. Use the manufacturer-supplied nominal voltage or the Wh printed on the pack for energy calculations.

DEWALT explains that its 20V MAX label refers to maximum initial voltage without a workload and that nominal voltage is 18 volts. Using 20 instead of 18 in the calculation would overstate the rated Wh for that example.

Maximum initial voltage

Maximum initial voltage is a label tied to the pack's initial no-load condition. It should not be silently treated as nominal operating voltage.

This difference is also a reminder that the number on the platform badge does not establish cross-brand fit. Manufacturer-defined interfaces, controls, and approvals remain decisive.

Runtime

Runtime is how long a charged battery operates one tool under stated conditions. The load can change continuously: a drill starts and stops, a saw meets different material resistance, and a blower can run at several speeds.

An idealized planning estimate is:

hours ≈ rated Wh ÷ average watts

Real tools have changing power demand, electronics, thermal limits, voltage drop, conversion losses, and low-voltage cutoffs. Treat the equation as context, not a product promise.

Trust a runtime claim only when it identifies the exact tool, battery, mode, accessory or material, and test wording. “Up to” is a condition, not a normal-use guarantee.

For a deeper capacity comparison, read 2Ah vs. 5Ah Tool Batteries: Capacity, Weight, and Runtime.

Charge time

Charge time depends on the battery, charger, temperature, pack condition, control system, and stated starting/end condition. Dividing Ah by the charger's printed amps can miss current tapering, temperature delays, system limits, and efficiency.

Milwaukee states that charge time varies with battery-pack type, charger type, and surrounding environment. DEWALT describes model-specific hot/cold delay behavior and notes that a charger's maximum output rate applies only to packs designed to accept it.

Check:

• Exact battery catalog number
• Exact charger model
• Manufacturer compatibility table
• Published charge-time conditions
• Whether multiple ports charge simultaneously or sequentially
• Ventilation and temperature instructions

Safety boundary

Use only batteries and chargers identified for the device by the manufacturer. OSHA warns that improper charging, physical damage, and temperature exposure can contribute to lithium-battery failure. Remove a pack from use and follow official guidance if it is swollen, cracked, leaking, hissing, smoking, or unusually hot.

Do not defeat temperature or low-voltage protection to extend runtime. Check exact models in the CPSC recall database before merchandising or recommending an older pack.

Frequently asked questions

Does 5Ah run exactly 2.5 times longer than 2Ah?

Not necessarily. It has 2.5 times the rated charge capacity at the same voltage. Actual runtime depends on the tool, load, mode, pack design, condition, temperature, and protection controls.

Is a higher-Ah pack more powerful?

Ah is a capacity rating, not a universal power rating. Use model-specific manufacturer data for output claims.

Can I use maximum voltage to calculate Wh?

Use nominal voltage or the printed Wh value unless the manufacturer directs otherwise.

To compare a current setup, check DeWalt-compatible tools and batteries. If a product page does not identify the exact pack or charger, ask product support.