Voltage and Amp-Hours Explained
Two numbers are printed on every cordless tool battery, they measure completely different things, and the one given the larger typeface is the less informative of the two. This guide separates them and gives you the third number — the one nobody prints — that actually predicts how long you will be working.
Voltage is a compatibility fact, not a performance one
Voltage is electrical pressure. In a tool platform it does two jobs: it decides which tools a pack will physically and electrically fit, and it sets a ceiling on how much power the tool can draw. It does not tell you how long the tool runs, and within a class it barely tells you how hard the tool pulls.
Our measured distribution across 37 packs puts the median at 20 V, the 25th percentile at 12 V and the full range from 3.7 V to 56 V. Voltage is the best-covered specification anywhere on this site at 93%, for the obvious reason that a pack which does not state it cannot be sold.
Why 20V MAX and 18V are usually the same pack
A lithium-ion cell measures about 4.0 V when fully charged and settles to about 3.6 V nominal under load. Five cells in series therefore read 20 V off the charger and 18 V nominal — two accurate descriptions of one pack.
Some brands print the charged figure with “MAX” attached, others print the nominal figure. Neither is dishonest and the difference is roughly 10%, which is entirely a labelling convention. A “20V MAX” tool is not more powerful than an “18V” tool for that reason alone.
| Marked | Nominal | Cells in series | Typical use |
|---|---|---|---|
| 12V MAX | 10.8 V | 3 | Compact drivers, precision and cabinet work |
| 20V MAX | 18 V | 5 | The main line — drills, drivers, saws, small outdoor tools |
| 40V / 40V MAX | 36 V | 10 | Mowers and larger outdoor equipment |
| 60V MAX | 54–56 V | 15 | Heavy outdoor equipment |
Compare marked against marked or nominal against nominal, never one against the other.
Amp-hours describe the pack, not the tool
Amp-hours (Ah) measure charge capacity — how much current the pack can supply for how long. A 4.0 Ah pack holds twice the charge of a 2.0 Ah pack at the same voltage, and will run the same tool roughly twice as long.
Roughly, because the relationship is not perfectly linear. Higher-capacity packs usually contain more cells in parallel, which lowers internal resistance, which means slightly less energy lost as heat under heavy load. A larger pack therefore tends to do marginally better than its capacity ratio suggests on demanding work, and it is also heavier.
Capacity has no effect on power. A 5.0 Ah pack does not make a drill drive harder than a 2.0 Ah pack does — it makes it do so for longer.
The number nobody prints: watt-hours
Neither figure alone lets you compare packs across voltage classes. Multiply them and you get energy in watt-hours, which does:
Watt-hours = nominal voltage × amp-hours
| Pack | Nominal V | Ah | Watt-hours |
|---|---|---|---|
| 12V MAX 2.0 Ah | 10.8 | 2.0 | 21.6 |
| 20V MAX 2.0 Ah | 18 | 2.0 | 36 |
| 20V MAX 5.0 Ah | 18 | 5.0 | 90 |
| 40V MAX 4.0 Ah | 36 | 4.0 | 144 |
Read down that column and the comparison that voltage alone obscures becomes obvious: a 40 V 4.0 Ah pack holds four times the energy of a 20 V 2.0 Ah pack, not twice, because both figures doubled. This is also why a mower and a drill are not on the same platform in most brands’ line-ups — our outdoor equipment data puts mowers at a 40 V median and 4.0 Ah median capacity, against 20 V and 2.0 Ah for trimmers and blowers.
Why our battery pages carry no watt-hour figures. Computing watt-hours needs a capacity figure we can stand behind, and this corpus does not supply one. Of 22 listings carrying capacity, four stated milliamp-hours in a field recording amp-hours and one recorded a pack as 2000Ah — a figure that would make a drill battery about half a home battery wall. We publish no energy density figure for that reason, and say so on the battery platform comparison rather than substituting a proxy. Get the capacity number from the manufacturer’s own page and check its unit before you multiply.
Which pack sizes to actually buy
Capacity is the specification where over-buying is easiest and least rewarded. Larger packs are heavier, and the mass hangs off the bottom of a tool you may be holding overhead.
| Work | Sensible capacity | Reasoning |
|---|---|---|
| Drills and impact drivers | 2.0 Ah | The lightest draw of any cordless category. A larger pack mostly adds weight to a tool used one-handed |
| Circular saws, sustained cutting | 4.0–5.0 Ah | High continuous current; a small pack sags under load and runs down quickly |
| String trimmers, blowers | 2.0–4.0 Ah | Our measured median for both categories is 2.0 Ah; step up if you clear a large area in one session |
| Mowers | 4.0 Ah and up | Our measured median is 4.0 Ah with a P75 of 5.0. Runtime here is measured in area covered, not minutes |
Two smaller packs usually beat one large one: you keep working while the other charges, and you carry less mass on the tool at any moment.
What runtime claims are worth
Very little, unless the pack capacity and the load condition are both stated alongside. “Up to 45 minutes” is a measurement of some specific tool doing some specific unstated work with some specific pack fitted, and none of those three variables is usually disclosed. We exclude runtime claims from this site entirely for that reason.
Amp-hours are stated on well under half these listings
Voltage appears on almost every tool in this corpus and amp-hours on considerably fewer, which is the wrong way round for a buyer: voltage is largely decided by the platform already owned, while amp-hours decide how long the tool runs before it stops. Where a battery is sold with a tool and the capacity is not stated, the honest assumption is that it is the smallest the manufacturer offers, because a large one is a selling point that gets printed.
Common questions
Does a higher-Ah pack make my tool more powerful?
No. It makes it run longer. Power is set by voltage and the tool’s motor and electronics; capacity is a fuel-tank measurement, not an engine one.
Will a 5.0 Ah pack fit a tool sold with a 2.0 Ah pack?
Within the same brand and voltage class, almost always yes — it will be physically larger and heavier. Check the tool’s own documentation rather than assuming, as some compact tools have clearance limits.
Is 40 V twice as good as 20 V?
It is twice the electrical pressure, which is not the same as twice as good. It matters for machines that draw sustained heavy current, which is why mowers sit there. For a drill it would be bulk you do not need.
Why do brands not print watt-hours?
Airlines and shippers require it, so it is often on the pack itself in small print, but it does not appear in marketing because a larger voltage number is easier to sell than a smaller energy one. It is the figure worth calculating yourself.