How to Calculate Cordless Tool Battery Energy from Voltage and Amp-Hours

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To estimate how much energy a cordless tool battery stores, multiply its nominal voltage by its capacity in amp-hours:

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

A 20V, 5Ah battery is therefore rated at about 100Wh. That figure is useful for comparing batteries, estimating runtime, and understanding why two packs with different voltage and amp-hour ratings may hold similar amounts of energy. It is an estimate, not a promise of how long a tool will run.

The basic calculation

Voltage describes electrical potential; amp-hours describe how much charge a battery can deliver over time. Multiplying them gives watt-hours, a measure of energy. For example, a 12V, 2Ah pack works out to 24Wh, while an 18V, 4Ah pack works out to 72Wh.

Use the voltage printed on the battery label, not a charger’s output voltage or a tool’s marketing description. A nominal 18V battery may be advertised as “20V max” in some systems. Those labels can describe the same general battery class using different conventions. For a fair comparison, use the pack’s stated nominal voltage if available and compare packs within the same labeling convention.

Worked examples

Here are three simple calculations. The watt-hour totals are approximate because the voltage printed on a pack is a nominal value; actual voltage changes as the battery charges and discharges.

This is a practical way to compare packs for energy storage. A 36V, 4Ah pack has more rated energy than an 18V, 5Ah pack, despite having fewer amp-hours. Amp-hours alone are only a meaningful comparison when voltage is the same.

Compare energy, then check compatibility

Watt-hours help answer “which pack stores more energy?” They do not tell you whether a battery fits a tool, whether the tool can draw power safely from it, or whether it will deliver that power without overheating. Check the manufacturer’s compatibility information before buying. Batteries from different brands—or even different tool families within one brand—may not interchange.

If you are adding a pack to an existing system, a compatible replacement battery is usually a safer choice than a bargain pack that only matches the voltage printed on the label. Confirm the exact tool family and battery type before ordering.

Estimate runtime without overpromising

A rough runtime estimate is:

Hours = battery watt-hours ÷ average tool power in watts

For example, a 90Wh pack powering a tool that averages 300W would theoretically run for 0.3 hours, or 18 minutes. Real runtime will often be lower. The tool does not draw a constant amount of power, and the battery’s usable energy is less than its rated energy under many conditions.

For a simpler comparison within one tool platform, a 5Ah pack has roughly 25% more rated capacity than a 4Ah pack, so it may run about 25% longer under the same conditions. In practice, the difference can be smaller or larger depending on heat, battery age, tool efficiency, and how hard the tool is working. A drill driving small screws may run for a long time; a circular saw repeatedly cutting dense lumber can drain the same pack quickly.

Cold weather can reduce available capacity, and repeated high-current work can cause a pack or tool to heat up and cut power temporarily. Worn batteries may also deliver less energy than their label suggests. If runtime matters on a demanding job, carrying a charged spare is more dependable than relying on a calculation alone. A higher-capacity battery pack can reduce swaps, but it usually adds weight and cost.

Common amp-hour traps

Do not compare amp-hours across different voltages without converting to watt-hours. A 4Ah pack at 36V is rated at 144Wh; a 5Ah pack at 18V is rated at 90Wh. The first stores more energy by this calculation even though its amp-hour number is lower.

Also, do not assume that twice the amp-hours means twice the runtime in every tool. A larger pack may weigh more, change the balance of a tool, or be awkward in a compact drill. For overhead work or frequent ladder use, a smaller, lighter battery may be the better choice even if it needs changing sooner. A larger pack makes more sense for sustained cutting, grinding, or other demanding work where fewer interruptions matter.

Finally, watt-hours do not measure power output. A pack can have substantial energy capacity but still be unsuitable for a tool that demands high current. Battery design, cell condition, temperature, and the tool’s electronics all affect whether power can be delivered without excessive voltage drop or a thermal shutdown.

Use the number to make the buying decision

For comparing energy, multiply the battery’s nominal volts by its amp-hours and compare the resulting watt-hours. For buying into a cordless platform, compatibility, battery availability, charger speed, pack weight, and the tools you plan to use matter just as much. A basic battery and charger kit can be a reasonable buy if its packs suit your tools and workload; there is little benefit in paying for the largest pack if a lighter one already lasts through the job.

Before ordering, check the battery label, calculate Wh, and verify that the pack is listed for your tool. If two compatible batteries have similar watt-hours, choose based on weight, price, and how often you are willing to stop and swap packs—not amp-hours alone.