Guides
How many batteries do you actually need?
Two packs of 240Wh is not the same as one pack of 480Wh, and the difference decides whether you finish.
Affiliate disclosure: we earn a commission if you buy through our links, at no extra cost to you. It never changes which product we rank first — here is how we're funded.
We have not tested these units. Every spec on this page comes from the manufacturer's own documentation and is linked to it. Every computed figure is arithmetic you can redo. Prices are checked daily — last verified January 1, 1970. Here is exactly how we pick.
Battery count is the question every cordless buyer asks after the purchase, when the mower stops two thirds of the way down the lawn. It is answerable in advance, with arithmetic, from numbers the manufacturers already publish.
Step one: lawn size to watt-hours
| Lawn | Mowing time | Energy per minute | Watt-hours needed |
|---|---|---|---|
| 1/4 acre (10,890 sq ft) | ~25-35 min | ~4Wh (20 in push) | ~140Wh |
| 1/3 acre | ~35-45 min | ~4-7Wh | ~270Wh |
| 1/2 acre (21,780 sq ft) | ~50-70 min | ~7.5Wh (21 in self-propelled) | ~450-600Wh |
| 1 acre (43,560 sq ft) | ~70-110 min | ~7.5Wh | ~600-1,000Wh |
| 2 acres | Seated | ~1,120Wh per acre (42 in rider) | ~2,240Wh |
Our arithmetic, using the per-minute rates derived from manufacturers' own published runtime claims. The riding figure comes from EGO's published two-acre coverage on 2,240Wh of installed packs.
Then compare with what is actually in the box. EGO ships 420Wh in one 7.5Ah pack. Greenworks 80V ships 320Wh. Toro publishes 324Wh. Kobalt installs 640Wh across two bays. Ryobi ships 480Wh in two packs — with a catch we come to next.
The sequential trap
This is the distinction that costs people an afternoon. Two 240Wh packs in a mower that draws from one at a time is not a 480Wh mower. It is a 240Wh mower with a spare you do not have to walk back to the garage for.
Does it matter? For total mowing time, no — 480Wh is 480Wh either way. For anything that needs sustained draw, and for the convenience of not stopping, yes. And it matters enormously for how you read a spec sheet: the Ryobi kit's 80-minute claim is a two-pack claim, not a one-pack one.
Step two: check the charge time before you rely on it
If one pack does not finish your lawn, your options are a second pack or a wait. How long a wait depends entirely on the charger, and the spread is enormous: Greenworks publishes 40 minutes for a 320Wh pack, Toro publishes 150 minutes for 324Wh.
At a 45% duty cycle — the Greenworks 80V's published 33 minutes of mowing against 40 of charging — a mow-wait-mow afternoon is workable. At Toro's rate it is not an afternoon, it is a project. The whole comparison is on the charging times page.
#ad Check the Greenworks LM2108S price on Amazon — how we're funded.
The answer, by lawn size
| Your lawn | Packs to buy | Why |
|---|---|---|
| Up to 1/3 acre | One, and no spare | A single 160-320Wh pack finishes the job with margin |
| 1/3 to 1/2 acre | One large, or two medium | You are close to the limit of one pack; a spare removes the risk |
| 1/2 to 1 acre | Two, minimum | 600-1,000Wh needed; no single walk-behind pack carries that |
| 1 to 2 acres | Four (rider) or three (walk-behind) | ~1,120Wh per acre on a rider by EGO's published coverage |
| Over 2 acres | Six, plus a recharge | EGO publishes six bays and up to 3 acres at full capacity |
Our arithmetic on published runtime claims, energy per minute and coverage figures. Add margin for thick or wet growth, which raises draw on any mower.
The Kobalt 80V dual-bay is the shortcut for the half- to-one acre band: 640Wh installed and a published one-acre rating, in one purchase rather than a mower plus a spare pack. The EGO LM2135SP is the shortcut for anyone who wants the biggest single pack rather than the biggest pair.
Why the spare pack is the better buy
- It removes the wait rather than shortening it. A faster charger is still a pause; a spare is a swap.
- It halves the cycles on each pack. Nobody publishes a cycle-life figure, but rotating two packs means each does half the work. What is actually published about pack life is thinner than you would hope.
- It runs your other tools. A spare mower pack is also the pack in your blower on a long autumn afternoon.
- It covers the bad day. Wet, thick grass raises draw. Margin is the whole point of a spare.
Three worked examples
A 4,000 sq ft front and back, 20-inch push mower
This is the case where buying a second battery is money spent on nothing. The published 40-minute runtime claim on a 160Wh pack is already double what the job needs, and the mower will be back on the charger before the next mow anyway.
A half acre, 21-inch self-propelled
Half an acre is where the decision actually bites, because the biggest single pack in most kits lands within a few tens of watt-hours of the requirement. Add 20 to 30% for thick or wet growth and one pack stops being enough on exactly the weeks you least want to stop.
A full acre, walk-behind
When a manufacturer's acreage rating and our watt-hour arithmetic disagree, we publish both rather than picking the flattering one. Kobalt rates the machine for an acre; our figures say an acre needs more energy than it carries. The honest reading is that its rating assumes lighter, faster mowing than our per-minute rate does.
What we cannot tell you
- How Kobalt's two ports discharge. The listing does not say, so we do not claim either behaviour.
- Real energy per minute on your lawn. Our per-minute rates come from published runtime claims under manufacturer conditions.
- How many cycles a pack lasts. Not published by any of these brands.
- What packs cost. We publish no unverified price. Put your own figures into the cost calculator.
The products discussed
Specs are the manufacturer's own and linked to the page they came from. Prices come from a live daily check; where we have no verified price we say so rather than print a stale one.
Ryobi 40V HP Brushless 21" Self-Propelled
Ryobi · Ryobi 40V · Our score 8.6/10
#ad No verified live price right now, so we show none rather than a stale one. How we're funded.
Two 6Ah packs and the longest published runtime claim on this page, on the platform with the cheapest route to a fourth and fifth tool.
Published specs
| Model | RY40HPLM01B |
|---|---|
| Deck | 21 in |
| Motor | Brushless, 40V HP |
| Batteries included (kit) | 2 x 40V 6.0Ah |
| Drive | Variable-speed rear-wheel self-propel |
| Runtime claim | Up to 80 min on 2 x 6Ah |
Source: 40V HP Brushless 21" Self-Propelled Mower, ryobitools.com
40V x 6.0Ah = 240Wh per pack; the kit ships two, so 480Wh total -- but only 240Wh is available at any moment, because the mower draws one pack at a time.
What the numbers favor
- 480Wh across two packs, the most total energy in any kit here
- Two packs means you swap and keep mowing instead of waiting on a charger
- Ryobi's 40V range is the broadest consumer battery line of the three platforms
What they don't
- Only one 240Wh pack is live at a time -- the 480Wh is sequential, not simultaneous
- Ryobi does not publish a cut-height range or weight on the product page, so both are unknown to us
you own nothing on Ryobi yet and expect to buy heavy tools later. 40V is the lowest voltage of the three platforms, and the ceiling shows on chainsaws and riding gear.
How it scores
Scores are our editorial weighting of the published specs above, not a test result. They are deliberately kept out of this page's structured data.
Kobalt 80V Max 21" Self-Propelled (dual-port)
Kobalt · Kobalt 80V Max · Our score 8.4/10
#ad No verified live price right now, so we show none rather than a stale one. How we're funded.
Two 4Ah bays, 80 published minutes and an acre rating -- the most runtime per purchase of any 80V walk-behind here.
Published specs
| Deck | 21 in, steel |
|---|---|
| Motor | Brushless |
| Batteries included | 2 x 80V 4.0Ah (dual port) |
| Runtime claim | Up to 80 min |
| Rated for | Up to 1 acre |
| Drive | Self-propelled RWD, 7 speed settings |
| Recharge | Approx. 60-90 min |
Source: Kobalt 80-Volt 21-in Self-propelled Battery Mower (2 batteries), lowes.com
80V x 4.0Ah = 320Wh per pack; two bays = 640Wh installed. Kobalt's listing does not state whether the two ports discharge simultaneously or in sequence, so we do not claim either -- what is verifiable is that 640Wh is on the machine when you start.
What the numbers favor
- 640Wh installed across two bays, the most of any walk-behind on this page
- Rated to 1 acre, twice the Greenworks 80V flagship's half-acre rating
- Seven published speed settings on the self-propel drive
- All Kobalt 80V packs are cross-compatible across the 80V outdoor line
What they don't
- Sold through Lowe's only, so availability and price track one retailer
- The listing does not publish weight, cut-height range or a noise figure
- Kobalt's 80V outdoor range is narrower than EGO's or Ryobi's
you want a platform rather than a mower. Kobalt is a single-retailer house brand -- fine for the tool, thinner as a decade-long ecosystem bet.
How it scores
Scores are our editorial weighting of the published specs above, not a test result. They are deliberately kept out of this page's structured data.
Greenworks 80V 21" 4-in-1 LM2108S
Greenworks · Greenworks 80V · Our score 8.4/10
#ad No verified live price right now, so we show none rather than a stale one. How we're funded.
The only mower here that publishes motor wattage, blade RPM, weight and an acreage rating -- and the only one whose battery warranty matches its tool warranty.
Published specs
| Model / SKU | LM2108S / 2550402 |
|---|---|
| Deck | 21 in |
| Max blade speed | 3200 RPM |
| Battery included | 80V 4.0Ah |
| Cut height | 1.38-4.0 in, 7 positions |
| Runtime claim | Up to 33 min |
| Weight | 75 lb |
| Rated for | Up to 1/2 acre |
| Warranty | 4-yr tool / 4-yr battery |
Source: 80V 21" 4-in-1 Self-Propelled Lawn Mower (LM2108S), greenworkstools.com
80V x 4.0Ah = 320Wh, using Greenworks' advertised pack voltage.
What the numbers favor
- 4-year battery warranty -- a year longer than EGO's 3-year, on the component that fails first
- Publishes blade RPM, weight and acreage rating; the other two brands publish none of the three
- 40-minute recharge on the included charger, the fastest published here
- 4-in-1 deck adds turbo leaf pickup on top of mulch, bag and side discharge
What they don't
- 320Wh included, the least of the three flagships -- and the 33-minute runtime claim reflects that
- 75 lb is the only published weight here, and it is not light
you want one battery to finish a half-acre without a swap. Greenworks rates the mower for 1/2 acre but claims only 33 minutes on the included pack; those two numbers only reconcile if you buy a second battery.
How it scores
Scores are our editorial weighting of the published specs above, not a test result. They are deliberately kept out of this page's structured data.
EGO Power+ 21" Select Cut LM2135SP
EGO · EGO 56V ARC Lithium · Our score 9.1/10
#ad No verified live price right now, so we show none rather than a stale one. How we're funded.
The most watt-hours in the box of any flagship walk-behind here, on the platform with the widest tool range to grow into.
Published specs
| Deck | 21 in |
|---|---|
| Cutting torque | Up to 7.0 ft-lbs |
| Battery included | 56V 7.5Ah |
| Cut height | 1-4 in, 8 positions |
| Self-propel speed | 0.9-3.1 MPH |
| Runtime claim | Up to 60 min |
| Warranty | 5-yr tool / 3-yr battery |
Source: EGO POWER+ 21" Select Cut System (LM2135SP), egopowerplus.com
56V x 7.5Ah = 420Wh, using EGO's advertised pack voltage.
What the numbers favor
- 420Wh in the box -- the largest included pack of any flagship walk-behind on this page
- Eight cut-height positions from 1 to 4 in, the widest published range here
- Three interchangeable lower blades (mulching, high-lift bagging, extended runtime)
- 5-year tool warranty, the longest published on this page
What they don't
- EGO publishes torque (7.0 ft-lbs), not motor wattage, so it cannot be compared directly against Greenworks' 1500W figure
- The 7.5Ah pack that earns the runtime claim is also the expensive one to replace
your yard is under a quarter acre. You would be paying for 420Wh of headroom you will never discharge, and a 20 in push mower cuts the same grass.
How it scores
Scores are our editorial weighting of the published specs above, not a test result. They are deliberately kept out of this page's structured data.
Where to buy the tools on this page
Every link below is an Amazon affiliate link. We show no price we have not verified today, so where there is no live figure the link says "check price" and takes you to the listing to read it there.
Best for a household already on Ryobi
Ryobi 40V HP Brushless 21" Self-Propelled
Ryobi · Ryobi 40V
Best two-bay runtime
Kobalt 80V Max 21" Self-Propelled (dual-port)
Kobalt · Kobalt 80V Max
Best published spec sheet
Greenworks 80V 21" 4-in-1 LM2108S
Greenworks · Greenworks 80V
Best overall
EGO Power+ 21" Select Cut LM2135SP
EGO · EGO 56V ARC Lithium
#ad As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. How we're funded.
Common questions
How many batteries do I need for my lawn mower?
Is two small batteries the same as one big one?
Should I buy a spare battery?
How do I convert my lawn size into watt-hours?
Does a bigger battery make the mower more powerful?
How long will I wait if I run out mid-lawn?
Sources
- RYOBI 40V HP Brushless 21" Self-Propelled Mower, ryobitools.com
- Kobalt 80-Volt 21-in Self-propelled Battery Mower, lowes.com
- Greenworks 80V 21" Self-Propelled Mower (LM2108S), greenworkstools.com
- EGO POWER+ 21" Select Cut System (LM2135SP), egopowerplus.com
- Toro 60V Max 21 in. Recycler Self-Propel Lawn Mower (21356T), toro.com
Read next
- Guides
The questions that come before the shopping. Answered in the first paragraph, then explained.
- Your first cordless yard kit, in the order to buy it
The buying order, why the mower sets the platform, and the watt-hours a full Saturday needs.
- What size mower do I need for my yard?
Deck width and watt-hours by lawn size, with the mowing-time arithmetic for each.
- How long do yard tool batteries take to charge?
Every published charge time converted to watt-hours per minute, and the mow-wait-mow arithmetic.
- How long do electric lawn mower batteries last?
Runtime per charge is arithmetic. Lifespan in years is mostly warranty terms and honest uncertainty.
- 40V vs 80V: what the voltage on the sticker actually buys you
Volts x amp-hours = watt-hours. The one calculation that makes every battery spec comparable.