Watt-hours, always — and the only reason listings print amp-hours is that the number looks bigger on a low-voltage platform.
The arithmetic
Amp-hours measure charge. Watt-hours measure energy, which is what actually does work. Energy is charge times voltage, so:
A 18 V pack at 4.0 Ah holds about 72 watt-hours. A 36 V pack at 2.0 Ah holds about 72 watt-hours. A 40 V pack at 2.5 Ah holds about 100. A 18 V pack at 2.0 Ah holds about 36 — half the first one, despite the shelf presenting them as neighbours.
Within one platform, where the voltage is fixed, amp-hours are a perfectly good comparison and this is why the habit persists. Across platforms they are meaningless, and cordless garden shelves mix platforms freely.
The complication nobody prints
The voltage on the label is not always the same kind of voltage. Some makers state the nominal voltage of the cell string; others state the peak voltage of a fully charged pack, which is higher. The same hardware is therefore sold as 18 V by one brand and 20 V by another, and as 36 V or 40 V at the next size up.
For the arithmetic above this matters only slightly — use the figure you are given, consistently — but for the comparison between two brands’ headline numbers it matters a lot, because the higher-sounding platform may be the same cells.
What the figure still does not give you
A runtime. Energy divided by load is a runtime, and the load is the grass. A capacity figure is half of a calculation whose other half nobody publishes.
A lifespan. Rated capacity is measured on a new pack under a gentle discharge. How many seasons it survives depends on how you store it and how hard the machine works it.
How this site handles it
Every cordless page here prints the capacity and the platform voltage in the same line, because one without the other cannot be compared. Where a family page ranks packs, it ranks them on the product of the two, and says so.
Wherever this page and the maker’s own documentation disagree, the documentation wins. Every cutting width, cutting height, stated area, pack capacity, flow rate, head, sound level and application rate on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And the division this page refuses to hide: a runtime is pack energy divided by a load, and the load is your grass. A published figure is a bench result obtained on short, dry grass at a middle setting, with no uncertainty attached and no obligation to state its conditions. This page prints capacity and platform voltage together, and reports any runtime as the manufacturer’s.
The questions that keep coming in
Is a 36 V 2.0 Ah pack worse than an 18 V 4.0 Ah one?
They hold roughly the same energy — 72 watt-hours against 72 — so on that measure they are equivalent. What differs is everything else: weight distribution, the machines they fit, and whether you already own one platform.
Why do brands label the same pack 18 V and 20 V?
One quotes the nominal voltage of the cells and the other the peak voltage of a freshly charged pack. It is a naming convention, not a difference in hardware, and it has its own page in the standards section here.
Last reviewed 26 September 2026