Stand in front of the cordless mower aisle at any large retailer and the marketing tells you that voltage is everything. A 40V mower is twice as good as a 20V mower, an 80V mower is the dream, and as long as the number on the box is big, you cannot go wrong. The reality is more subtle. A 40V mower with a tiny battery may run for 18 minutes and leave you stranded with half the lawn cut. A 36V mower with a generous battery may finish a 400m² garden on a single charge. The trick is reading the spec sheet properly, in the right order, and ignoring the headline voltage until you have checked four other numbers. This article walks through what each battery specification actually does, how to compare models on a like-for-like basis using watt-hours, and the trade-offs that decide whether you finish the lawn in one charge or three.
Voltage tells you power, not endurance
Volts are a measure of electrical pressure. Think of it as the force with which electrons are pushed through the motor windings. Higher voltage allows the motor to generate more torque at the cutting blade, which is what lets a mower power through long damp grass without bogging down. The 2026 cordless landscape sits in three voltage bands:
- 18V to 24V: small lightweight mowers for tiny gardens (under 100m² or about 1,000 square feet). Cordless equivalents of hover mowers.
- 36V to 48V: mid-range residential, the sweet spot for most domestic lawns up to 500m² (about 5,400 square feet).
- 56V to 80V (occasionally up to 82V): heavy-duty residential and light professional, capable of half-acre to one-acre work, often self-propelled.
What voltage will not tell you is how long the mower runs. A high-voltage mower with a small battery has plenty of cutting power but burns through energy in 15 minutes. A modest-voltage mower with a big battery runs for an hour. The headline voltage on the box is half the story.
Amp hours measure capacity, and capacity sets runtime
Amp-hours (Ah) measure how much current the battery can deliver over time. A 5Ah battery can deliver 5 amps for one hour, or 2.5 amps for two hours, or 10 amps for half an hour, before being depleted. Roughly, doubling the Ah rating doubles the runtime at the same workload.
Typical Ah ratings in cordless mower batteries range from 2Ah at the budget end (think Ryobi 18V starter kits) to 12Ah on premium self-propelled models. The rule of thumb is:
- 2 to 2.5Ah: 15 to 25 minutes of mowing, enough for a courtyard or front strip.
- 4 to 5Ah: 35 to 50 minutes, enough for a typical 200 to 300m² lawn.
- 6 to 7.5Ah: 50 to 70 minutes, suitable for 400 to 600m² gardens.
- 10 to 12Ah: 75 to 100 minutes, large half-acre and self-propelled use.
Two warnings. Manufacturer runtime claims are usually measured under unrealistic test conditions: short dry grass at minimum cutting height with the bag detached. Real-world runtime in damp May grass is typically 60 to 70 percent of the advertised figure. And the same battery in a self-propelled mower drains roughly 30 percent faster than in a push mower because the drive motor also drains the pack.
Watt-hours: the only number that lets you compare across brands
Here is the number nobody puts on the front of the box. Watt-hours (Wh) are calculated by multiplying volts by amp-hours: Wh = V x Ah. This single figure tells you the total energy stored in the battery, and unlike voltage or amp-hours alone, it lets you compare two batteries on different platforms fairly.
Worked examples from current 2026 models:
- EGO Power+ 56V 7.5Ah: 56 x 7.5 = 420Wh
- Ryobi 40V HP 6Ah: 40 x 6 = 240Wh
- Greenworks 80V 4Ah: 80 x 4 = 320Wh
- DeWalt 18V x2 (36V) 5Ah twin pack: 36 x 5 = 180Wh
- Bosch 18V 4Ah: 18 x 4 = 72Wh
Now you can compare them properly. The EGO at 420Wh has nearly six times the energy of the Bosch 18V at 72Wh and almost twice the energy of the Ryobi 40V 6Ah at 240Wh, despite being on a different voltage platform. If you have a 500m² lawn and you need to finish on one charge, the EGO will do it comfortably; the Bosch will not get past 50m² before dropping out.
The rule of thumb based on field testing across multiple cordless review sites: budget roughly 1Wh per square metre of lawn for an average push mower in normal grass. For self-propelled, allow 1.3Wh per square metre. So a 300m² garden needs at least a 300Wh battery for a push mower, or 400Wh for self-propelled, with some margin for longer grass and edges.
Why two batteries is almost always the right answer
For all but the smallest gardens, a kit that includes two batteries (or supports two batteries in the deck simultaneously) is the durable answer. Three reasons.
First, you can mow continuously by swapping while one charges. Most modern fast chargers refill a 5Ah pack in 60 to 80 minutes, which is roughly the same time it takes to deplete the other. With one battery you mow, stop, charge for an hour, mow again.
Second, dual-bay mowers such as the EGO LM2135SP and DeWalt DCMWSP255 use both batteries in sequence or parallel, which gives you total energy in the range of 800 to 1,200Wh on a single mowing session. That is enough for half an acre in damp grass without stopping.
Third, lithium-ion batteries degrade with cycle count rather than calendar age. Splitting your usage across two batteries doubles the effective lifespan of each, so you may not need to replace a pack for 8 to 10 years instead of 4 to 5.
Replacement battery cost and the platform lock-in trap
Lithium-ion battery packs lose capacity gradually with each charge cycle. A typical premium pack delivers 500 to 800 full cycles before dropping to 80 percent of its original capacity, which works out to 6 to 10 years for a once-a-week mower. After that, you are choosing between replacing the battery or replacing the whole mower.
Replacement pack prices at retail in 2026:
- EGO 56V 5Ah: around £220/$270
- EGO 56V 7.5Ah: around £340/$420
- Ryobi 40V 5Ah: around £140/$170
- Greenworks 80V 4Ah: around £220/$270
- Bosch 18V 4Ah: around £55/$70
- DeWalt 18V 5Ah: around £90/$110
This is where platform lock-in becomes the most underrated factor in cordless mower buying. If you also own (or might one day buy) a cordless drill, hedge trimmer, leaf blower or strimmer, picking the same battery platform across all of them means one charger, one set of spare packs, and significant savings over a decade. A household running Ryobi 40V or DeWalt 20V Max ends up with three or four spare batteries it can swap between tools, which removes most runtime anxiety. A household with three different brands needs nine separate batteries to achieve the same flexibility.
Cold weather, storage and the 30 percent rule
Lithium-ion battery chemistry slows down below 10 degrees C (50 degrees F) and stops accepting charge below 0 degrees C (32 degrees F). On the first cool morning of spring, a battery stored in the shed will deliver about 30 percent less runtime until it warms up. Bring the pack indoors the night before and the runtime returns to normal.
For winter storage, charge to about 50 percent state of charge (not fully charged, not fully discharged) and store between 5 and 25 degrees C (41 and 77 degrees F). A fully charged pack left for six months at room temperature degrades around 12 to 15 percent of total capacity; the same pack at 50 percent loses only 2 to 3 percent. Set a calendar reminder for October to part-discharge the pack before putting the mower away.
The buying checklist
When you stand in front of the box, work through this in order:
- Multiply the volts by the amp-hours to get watt-hours. Ignore the bold-faced voltage figure on the front.
- Compare the watt-hour total to your lawn area. Allow 1Wh per square metre for push mowers, 1.3Wh per square metre for self-propelled.
- Check whether the kit includes one or two batteries, and whether the mower deck takes two batteries at once.
- Check the price of a replacement pack from the manufacturer’s official channel. If it is more than 40 percent of the price of the mower, the platform is expensive to maintain.
- Cross-reference the battery platform with any other cordless tools you own. Same platform means one charger and shared spares.
Do that and you walk out with a tool that runs through May to October on one charge per lawn, costs less to maintain than the alternative, and still works long after the warranty has expired. The cutting width on the deck has barely featured in the decision, and that is the right outcome. A 38cm deck and a 46cm deck cut the same grass; the battery decides whether you finish the job.
