Chainsaw battery voltage explained: 18V vs 40V vs 80V

Voltage is power, not runtime. Higher voltage cuts thicker wood faster; it doesn't run for longer. Amp-hours (Ah) are runtime, not power. A bigger Ah battery at the same voltage lasts longer per charge; it doesn't cut any harder. By UK tier: 18-20V suits pruning and branches, 36-40V covers most gardens and firewood-sized logs, 60-80V approaches light petrol performance for bigger jobs.

Volts vs amp-hours: what each number actually does

The two numbers printed on a chainsaw battery answer different questions. Mixing them up is probably the most common confusion for first-time cordless buyers. Voltage is the electrical pressure the battery pushes through the motor. It's the main driver of chain speed and torque: how hard and how fast the saw can bite into wood. Amp-hours measure the battery's storage capacity: how much charge it holds, and therefore how long it keeps the saw running before you need to recharge or swap packs.

Think of it like a hosepipe. Voltage is the water pressure: it decides how forcefully the water comes out. Amp-hours is the size of the tank behind it: it decides how long the water keeps flowing before the tank runs dry. Fit a bigger tank and the pressure stays the same; you just get more time before it runs out. Fit a higher-Ah battery to the same voltage saw and the same rule applies: longer runtime and a heavier pack, not a saw that cuts through thicker wood.

This shows up when comparing two saws with the same voltage but different Ah ratings, or the same battery family sold in different pack sizes. A saw on an 18V platform with a 5.0Ah battery cuts at the same speed as the identical saw with a 3.0Ah battery; it just keeps going for longer between charges. A bigger battery won't fix a saw that feels underpowered. Only a higher voltage tier, or occasionally a better-designed brushless motor within the same voltage, fixes that.

Voltage tier to cutting capacity

The table below maps the three broad voltage bands UK retailers sell to what they're realistically good for. Treat these as typical ranges, not fixed limits. Motor design, chain sharpness and the wood species you're cutting all move the real-world number. Different nominal voltages within a band, 56V, 60V, 80V and 82V packs, for example, land in roughly the same performance tier despite the different label on the box.

Amp-hours in practice: a 36V saw running a 4.0Ah battery typically manages somewhere around 40 to 60 cuts through seasoned logs before it needs recharging, depending on wood hardness, moisture and chain sharpness. Move up to a 6.0Ah pack on the same 36V saw and you get roughly half as many cuts again, more runtime, not more power. If you need to cut for longer without stopping, buy more Ah or a second battery. If cuts are slow or the chain bogs down, that's a voltage problem, not an Ah one.

Voltage isn't the whole story: battery platforms

Picking a voltage tier from the table above also means buying into a specific brand's battery platform. Batteries are generally platform-specific. An 18V pack from one brand will not fit a 56V or 80V tool from another, and it usually won't fit a different brand's tool even at a matching voltage. The number on the label is not a compatibility promise.

Platform does matter inside a single brand's own range. Most manufacturers design one battery to run across dozens of tools: drills, strimmers, hedge trimmers, blowers, on the same voltage platform. If you already own cordless tools from a brand, check whether their chainsaw shares that battery before you buy. Sharing a platform can save a genuinely useful amount of money by skipping a second charger and pack. Don't assume this crosses brands, and don't assume two different brands' tools are interchangeable just because the box states the same voltage. Voltage tells you how powerful a saw is. Platform tells you what else you can run off the same batteries. Check both before you buy.

Which voltage do you actually need?

If you're only pruning, clearing hedges or cutting branches under about 15cm, an 18-20V saw is genuinely enough. It's lighter and cheaper to run too. If you're processing firewood-sized rounds up to around 25cm, which covers most UK garden use, 36-40V is the sensible default. Only step up to 60-80V if you're regularly cutting bigger rounds, felling small trees, or you want cordless power that gets close to a petrol saw without the fuel and maintenance.

Not sure where you land? Our free Fit Check tool asks three quick questions and matches your task to a power type and bar length in seconds. If this is your first saw, voltage is only half the decision. Safety features and proper training matter just as much; see the guide below.