Amp-Hours vs. Voltage: How to Actually Compare Cordless Tool Battery Runtime
Two cordless trimmers can both say '40V' on the box and still run for completely different lengths of time. The reason is that voltage and amp-hours (Ah) measure two different things, and most product listings only make one of them obvious.
Once you know what each number actually tells you, comparing battery-powered mowers, trimmers, blowers, and chainsaws across brands gets a lot less confusing — and a lot less dependent on trusting the marketing headline.
What voltage actually tells you
Voltage is a rough proxy for power and torque — it's why 80V chainsaws and mowers generally cut through tougher material than 20V trimmers. It also defines the battery platform: an 18V battery from one tool works in other tools on that same 18V platform from the same brand, which is a real convenience if you own several cordless tools.
What voltage does not tell you is how long a tool will run. Two tools on the same voltage platform can have wildly different runtimes depending on which battery pack is included, which is where the second number comes in.
One marketing wrinkle worth knowing: some brands advertise 'peak' or 'max' voltage, measured immediately off a fresh charge, rather than the nominal voltage the battery holds during most of its use. If a spec sheet lists an unusually round, high voltage number compared to competitors, check whether it's a peak or nominal rating before comparing platforms directly.
What amp-hours (Ah) actually tells you
Amp-hours measure capacity — essentially, how much energy the battery holds. Within the same voltage platform, a higher Ah rating means longer runtime: a 5.0Ah battery lasts roughly twice as long as a 2.5Ah battery on the same tool, all else being equal.
This is why the battery included with a tool matters as much as the tool itself. Two mowers from the same brand and voltage platform can ship with different batteries, and the one with the smaller Ah rating will run out mid-lawn while the other finishes the job.
It's worth checking the fine print on 'kit' listings for this reason. A trimmer or blower bundled as 'tool only' has no battery included at all, while a kit version might come with anything from a 2.0Ah to a 6.0Ah pack — two listings for the same physical tool can have very different real-world runtime and price depending on which battery is boxed with it.
How to compare two tools on different platforms
Comparing runtime across different voltage platforms takes one extra step, because voltage and amp-hours only tell the full story when multiplied together. Watt-hours (Wh) — voltage times amp-hours — is the number that's actually comparable across brands and platforms.
For example, a 40V battery rated at 4.0Ah stores about 160Wh (40 x 4.0). An 80V battery rated at 2.0Ah stores the same 160Wh (80 x 2.0), and — assuming similar tool efficiency — should offer roughly comparable runtime, despite the voltage numbers looking very different on the shelf.
If a listing doesn't state watt-hours directly, it's usually printed on the battery itself or in the manual, and you can always calculate it yourself from the voltage and Ah figures on the box.
Runtime depends on more than the battery spec
Watt-hours give you a fair way to compare batteries, but real-world runtime also depends on the tool drawing power from that battery. A more efficient motor stretches the same battery further, which is one reason two brands with identical watt-hour batteries can still post different runtime claims.
How you use the tool matters just as much. Running a blower or mower on a high-power setting, cutting thick or wet material, or working on an incline all draw more current and shorten runtime compared to manufacturer figures, which are typically measured under light, consistent conditions.
Cold weather has a similar effect: lithium-ion batteries temporarily lose some capacity in low temperatures, so a battery that comfortably finishes a task in summer may need a swap or a recharge doing the same job on a cold fall morning.
Battery platform compatibility saves money over time
Most cordless brands sell mowers, trimmers, blowers, and chainsaws that share one battery platform, so a single charger and a couple of batteries can power an entire yard's worth of tools. This is worth checking before buying your first cordless tool, since starting on a platform with a wide tool lineup gives you more flexibility as you replace gas equipment over time.
It's also why buying an extra, higher-Ah battery for tools you use most (a mower or a blower, typically) is often a better upgrade than buying a whole new tool — the runtime gain applies to every tool on that platform, not just one.
Frequently Asked Questions
- Is a higher voltage always better than a higher amp-hour rating?
- Not necessarily — they measure different things. Voltage relates to power and torque, while amp-hours relate to runtime. A high-voltage tool with a low-Ah battery can still run out quickly; the two specs need to be considered together, not ranked against each other.
- Can I use a higher-Ah battery than the one that came with my tool?
- Within the same voltage platform, yes — a higher-Ah battery from the same brand typically works and simply extends runtime. It also weighs more and costs more, so it's worth matching the upgrade to tools you actually run for extended periods.
- Why do two 40V batteries from different brands have different runtimes?
- Usually because their amp-hour ratings differ, but tool and motor efficiency also play a role. Comparing watt-hours (voltage x amp-hours) is a fairer way to line up batteries across brands than comparing voltage alone.
- Does battery voltage affect how long a battery lasts before it needs replacing?
- Not directly — battery lifespan is driven more by charge cycles, storage habits, and heat than by voltage. See our guide to winterizing outdoor power equipment for tips on storing lithium-ion batteries correctly during the off-season.