Cordless power tool batteries are sold on two headline numbers: voltage and amp-hours. Voltage is easy to grasp as the power class of the system, while amp-hours reads like the size of the fuel tank. When every major brand sold 18V packs built from similar cells, a 4Ah battery was a 4Ah battery no matter which logo sat on the case. That is no longer true. Brands now sell handheld tools from 12V all the way past 60V, and packs built with several different cell generations sit on the same shelf. Amp-hours alone cannot tell you which pack holds more energy, so watt-hours deserve a closer look. Before you spend money on extra batteries, run the math on the label, especially when evaluating cordless tool combo kits where the battery platform is part of the deal.
What an Amp-Hour Actually Measures
An amp-hour is a unit of charge capacity. If a battery can push a continuous current of 1 amp for one hour, it holds 1 amp-hour. Deliver 2 amps and the same pack runs for half an hour. That simple definition worked well when every brand sold two battery sizes for one voltage. A cordless pack is not a single cell, though. Inside the plastic shell sit multiple lithium-ion cells, a wiring harness, and a battery management system that balances cells and protects the pack from over-discharge, over-current, and heat.
The catch is that a battery’s amp-hour rating shifts with discharge current. A pack rated 4Ah at a light 1-amp draw delivers less total energy when a circular saw pulls 30 amps. Manufacturers rate packs under controlled conditions, so the number on the label is a reference point, not a guarantee. Old charging habits linger here too; some crews still believe a pack needs a full drain before recharging, a habit carried over from older chemistries. Modern lithium packs do not work that way, and cordless power tool battery care has more to do with heat and storage voltage than with draining cycles.
Why the discharge rate changes the rating
Internal resistance in the cells and the pack wiring turns some energy into heat as current climbs. Draw more current and you recover less of the stored charge, especially near the end of the discharge. A 4Ah pack on a drill driver behaves differently from the same 4Ah pack on a grinder, which is why runtime claims rarely line up with a stopwatch.
Pack voltage versus cell voltage
An 18V pack uses five cells in series, each with a nominal 3.6V, for a nominal 18V. The same pack charges to about 20V, which is why one brand prints 20V Max on the case while another prints 18V. The tools and packs are interchangeable within the same platform, and the voltage label is a naming choice, not a compatibility break.
| Rating | Typical cells | Rough runtime on a drill driver | Weight class |
|---|---|---|---|
| 2.0Ah | 5 x 18650 | Quick jobs and light duty | Compact |
| 4.0Ah | 10 x 18650 or 5 x 21700 | Standard all-day work | Mid |
| 5.0Ah | 5 x 21700 | Extended work | Mid-heavy |
| 6.0Ah | 6 x 21700 | High-draw tools | Heavy |
Watt-Hours: The Energy Number That Levels the Field
Watt-hours combine voltage and amp-hours into one number that represents stored energy. The formula is simple: volts times amp-hours equals watt-hours. An 18V 4Ah pack holds 72 watt-hours. A 12V 4Ah pack holds 48 watt-hours. A 54V 4Ah pack holds 216 watt-hours. The amp-hour rating is identical in all three cases, but the energy is completely different, which is why comparing batteries by amp-hours alone across platforms falls apart.
Watt-hours also explain why a compact 12V driver feels weak next to a full-size 18V model even when both carry 4Ah packs. The 12V pack has less energy and the motor is usually smaller, so the tool cannot sustain the same output. When brands introduced power stations that run corded-style tools from battery packs, watt-hours became the number that actually predicted how long the station would run. A portable power station built around a stack of high-capacity packs is really a box of watt-hours with an inverter attached.
Calculating watt-hours yourself
- Find the nominal voltage printed on the pack, usually 12V, 18V, 20V, 36V, or 54V.
- Find the amp-hour rating, usually 2.0, 4.0, 5.0, or 6.0.
- Multiply the two numbers to get watt-hours.
- Compare packs by watt-hours, then factor in weight and price.
Why brands still print amp-hours
Marketing inertia plays a part. Amp-hours were the standard for a decade, buyers recognize them, and a bigger amp-hour number sells even when the voltage differs. Print 5.0Ah on a pack and it sounds bigger than 4.0Ah; print 90Wh and shoppers reach for a calculator. Some brands now list watt-hours in the fine print, and regulators in other markets push the number harder than North American buyers do.
Voltage Tells Half the Story
Voltage determines what a tool can do. Higher-voltage systems deliver more power to high-draw tools like circular saws, grinders, and rotary hammers. Today’s lineups run from 12V for compact drivers and lights, through 18V and 20V Max for general work, up to 36V, 54V, and 60V for heavy equipment, with 40V platforms common for outdoor power tools. When you compare two packs at the same voltage, amp-hours are a fair proxy for runtime. When the voltages differ, watt-hours are the only honest comparison.
The 20V Max versus 18V naming causes constant confusion. Both describe the same five-cell platform; one brand quotes the peak charge voltage and the other quotes the nominal voltage, so a 20V Max tool and an 18V tool from the same brand family take the same packs. Knowing how voltage ratings and battery ecosystems develop helps you predict whether your packs will survive the next tool generation, because brands protect their platforms even as they push voltage higher.
What each voltage band is for
- 12V: compact drills, drivers, lights, and small saws for light duty and overhead work.
- 18V and 20V Max: the workhorse band for drilling, driving, and most job-site tasks.
- 36V and above: high-output saws, grinders, demolition tools, and outdoor power equipment.
Cell Technology Changes How Packs Are Built
A few years ago, a 4Ah pack from any major brand was built the same way from the same class of cells. That is no longer true. A 4Ah pack can be assembled from ten 18650 cells rated at 2Ah each, or from five larger 21700 cells rated at 4Ah each. Fewer cells mean less wiring, lower internal resistance, and usually a lighter pack with the same energy. The 21700 format, named for its 21mm by 70mm dimensions, now shows up in most high-capacity packs.
The battery management system inside the pack matters as much as the cells. It balances cell voltages, shuts the pack down on over-current and over-temperature, and in some designs talks to the tool and charger. This electronics layer is why unbranded aftermarket packs are a gamble: the cells may be fine, but the BMS may not match what the tool expects. As battery management systems grow more capable, packs gain fuel gauges, tool data logging, and thermal derating.
18650 versus 21700 at a glance
| Cell format | Typical capacity per cell | Common uses |
|---|---|---|
| 18650 | 2.0-3.5Ah | Compact and standard packs |
| 21700 | 3.0-5.0Ah | High-capacity and high-output packs |
What the BMS actually does
Balancing keeps all cells at similar voltages so one weak cell does not drag the whole pack down. Temperature monitoring stops charging and discharging at dangerous temperatures. Current limiting protects the tool and the pack from stall conditions, and communication lets the charger tailor its charge profile to the pack’s state.
Comparing Batteries Before You Buy
The fast way to compare packs is to run the watt-hour math, then check weight, price, and charger class. A cheap 6.0Ah pack that charges slowly can be a worse deal than a 4.0Ah pack with a fast charger, because recharge time is downtime on the job. Battery systems on modern construction jobs earn their keep when packs, chargers, and tools work as one unit.
- Confirm the pack voltage matches your tool platform.
- Convert amp-hours to watt-hours using the nominal voltage.
- Compare weight; heavy packs add fatigue on overhead work.
- Check the charger; fast charging shortens the gap between packs.
- Price the watt-hour by dividing pack price by watt-hours to compare value across brands.
Runtime math for a real job
A 72Wh pack on a tool that draws about 400W under load runs roughly 10 minutes of continuous use. Real duty cycles include trigger releases and idle time, so the same pack can stretch across a full morning of driving screws. Brushless motors convert more stored energy into work, which is why a brushless drill often outruns a brushed model on the same battery.
Spec Sheets That Work for You
Battery spec sheets are becoming more informative, and watt-hours are showing up alongside amp-hours. A few quick checks turn any spec sheet into a buying decision. Look at the nominal voltage first, then the amp-hours, then run the watt-hour multiplication. If two packs from the same brand hold the same watt-hours but weigh differently, the lighter one usually uses newer cells. If the prices differ sharply, the expensive pack often carries a better BMS and charge curve rather than more energy.
Three quick checks before checkout
- Same voltage: the bigger amp-hour rating wins on runtime.
- Different voltage: compute watt-hours before comparing anything.
- Same watt-hours: the lighter pack with a fast charger is usually the better buy.
The battery aisle changes faster than the tool aisle. Cell formats, BMS features, and voltage tiers keep shifting, and cordless power tool battery evolution shows no sign of slowing. Learn to read the two numbers on the label, convert them into watt-hours, and the marketing noise drops away. The pack that fits your tools, your workload, and your charger is the one worth buying.