Amp Hours vs Watt Hours: Compare Drill Batteries Correctly

Amp hours (Ah) measure electrical charge capacity. Watt hours (Wh) measure nominal energy capacity after voltage is included.

For cordless-tool batteries, use amp hours to compare packs on the same nominal-voltage platform. Use watt hours when the voltages differ.

The conversion is:

Watt hours = nominal volts × amp hours Wh = V × Ah

IATA defines a lithium battery’s watt-hour rating as its amp-hour capacity multiplied by its nominal voltage (IATA).

The practical difference

Rating	What it describes	Best use when buying
Amp hours (Ah)	Charge capacity	Comparing batteries within one voltage platform
Watt hours (Wh)	Nominal energy capacity	Comparing battery capacity across different voltages
Watts (W)	Rate of power use or delivery	Assessing load or output—not stored energy

An 18 V, 5 Ah pack has a nominal energy rating of:

18 V × 5 Ah = 90 Wh

A 36 V, 2.5 Ah pack also has:

36 V × 2.5 Ah = 90 Wh

The second battery has half the amp-hour rating but the same calculated watt-hour capacity. That is why Ah alone can mislead when comparing different voltage platforms.

Use nominal voltage, not the larger “max” number

Battery voltage changes with state of charge and load. Tool brands may emphasize either nominal voltage or the maximum initial voltage of a freshly charged pack, so first establish which figure the label uses.

For example, DeWalt sells its DCB205 as a “20V MAX” 5 Ah battery but states that 20 V is the maximum initial voltage measured without a workload and that nominal voltage is 18 V (DeWalt). Its nominal energy is therefore:

18 V nominal × 5 Ah = 90 Wh

Using 20 V would produce 100 Wh, but that combines a no-load maximum with an amp-hour capacity rating rather than following the nominal-voltage calculation.

This distinction also matters with small packs. Compatibility still comes before arithmetic when choosing a 3.6 V lithium battery.

When amp hours are enough

Within one battery platform at the same nominal voltage, a 5 Ah battery has 2.5 times the rated charge capacity of a 2 Ah battery:

5 Ah ÷ 2 Ah = 2.5

That makes Ah a useful capacity comparison between compatible packs. In the same drill under the same conditions, the larger pack has more potential runtime. It does not guarantee exactly 2.5 times as many holes.

Bit diameter, material, hole depth, feed pressure, gearing, motor efficiency, temperature, battery condition and protective cutoffs can all change the work completed per charge. A larger pack may also be heavier, more expensive or capable of supplying current differently from a compact pack.

Do not treat matching voltage and Ah as proof of compatibility. The pack must also have the correct physical interface, charger support and battery-management communication for the tool system.

Watt hours do not tell you battery power

A watt-hour rating describes nominal stored energy, not how quickly a pack can deliver that energy. Two 90 Wh packs can differ in:

• maximum current delivery;
• voltage sag under heavy load;
• heat management;
• protective electronics;
• cell format and internal resistance;
• performance as the pack warms or discharges.

DeWalt’s standard DCB205 and DCBP520 POWERSTACK illustrate the distinction. Both are rated at a nominal 18 V and 5 Ah, but DeWalt claims that the DCBP520 supplies 50% more power than the DCB205. Its footnote says that comparison is “not in application,” so it should not be read as 50% more drilling performance in every tool or task (DeWalt).

The useful lesson is narrower: identical Ah and nominal Wh do not establish identical high-load output.

For ordinary drilling and screwdriving, capacity and pack weight may matter more than maximum output. Large hole saws, self-feed bits and sustained masonry drilling place heavier demands on a pack, making current delivery and thermal management more relevant.

Can watt hours predict drill runtime?

Only as a rough theoretical calculation. If a device drew a constant 90 W and all 90 Wh were usable:

90 Wh ÷ 90 W = 1 hour

A drill is not a constant load. Demand changes during startup, breakthrough and binding, and the tool draws little or nothing during pauses. Electrical losses and battery protection also separate a label calculation from usable work.

Watt hours can therefore compare nominal battery energy, but they cannot responsibly be converted into a hole count without specifying the drill, battery, bit, material, hole diameter and depth, operating method and test conditions.

Which number should decide the purchase?

• Choosing between 2 Ah and 5 Ah packs for the same platform: compare Ah, then consider weight, price, compatibility and required output.
• Comparing 18 V, 36 V or other platforms: calculate Wh using each pack’s nominal voltage, while remembering that the tools themselves may differ in efficiency and capability.
• Running demanding bits or sustained loads: Wh is not enough; check the manufacturer’s tool-and-battery compatibility and output guidance.
• Estimating holes or work per charge: prefer a matched runtime test with stated conditions over either label number.

The short rule is: Ah compares capacity within one voltage family; Wh is the fairer nominal-energy comparison across voltage families. Neither rating alone guarantees drill power or real runtime.