For heavy-duty cutting, a 4.0 Ah pack is a sensible battery size for cordless saw work. Ryobi rates its 18V ONE+ 5-1/2 in. circular saw kit with one 4.0 Ah battery at over 215 fast, clean cuts per charge [4]. That means you can finish a sizable batch of cuts before you stop to recharge.
How long any pack lasts depends on both its size and the job [5]. Ripping thick lumber drains a battery faster than trimming thin boards, so heavier work calls for a bigger pack or a spare.
If you cut all day, a kit with two batteries lets you keep one on the charger while the other runs [5].
Why does a heavy cut need a bigger battery?
Amp-hours, written as Ah, tell you how much charge a battery holds. A higher Ah number means the saw can run longer before the battery needs a recharge. Think of Ah as the size of a fuel tank: a bigger tank keeps you cutting through more boards before you refill it.
Heavy cutting asks much more of that tank. When a saw bites into dense lumber, such as thick hardwood or a stack of thick boards, it draws a lot of current. A small pack drains quickly under that load, even though it may handle light trim work without trouble.
Voltage and Ah are separate ratings, and both have to suit your saw. The battery’s voltage must match the saw’s voltage, or it will not fit or run. Within that voltage, choose the highest Ah your budget and weight tolerance allow, because extra capacity is what buys run time on a long cutting day.
Once you have the right pack, try it on a real cut. Make one full-depth cut through your usual material and see how far the charge drops. If the battery is nearly empty after a few cuts, step up in Ah rather than buying a second small pack.
Which battery sizes hold up for heavy cuts?
A 4Ah pack is the smallest size one forum poster would use on a heavy-draw tool like a saw. “I wouldn’t use anything less than a 4ah or larger battery on any heavy draw tool, i.e. saws.” [1] That is one person’s rule of thumb, not a test, but it gives you a clear floor for demanding work.
Another forum reply sets the bar a little lower and focuses on the saw itself. A modern lithium-ion saw running 3.0 or larger batteries is a usable and very effective tool. [2] A pack in that range is a reasonable middle ground for a current saw, and it suits you if you do not cut all day.
At the light end, a 1.5Ah pack draws doubt. One thread asks, “Worth trying to run a cordless circular saw with 1.5 amp hour battery?” [2] That question treats 1.5Ah as a light-duty choice, so save it for trim and occasional cuts.
Picture a day of ripping 2×6 framing. A pack at the 4Ah floor gives you a better chance of finishing the stack than a 1.5Ah pack, which you would be swapping and charging far more often. If you rip plywood or thick hardwood regularly, start at 4Ah and go up from there.
When do two batteries make sense for a cordless saw?
Two batteries make sense when you cut framing or thick hardwood most days and a single small pack would run down partway through the job. With one pack on the saw and a second on the charger, the cutting rarely has to stop.
What the top-rated saw shows
In Family Handyman’s guide to the best cordless circular saws of 2025, one saw is built to take two 18V batteries [3]. The reviewers said it delivers exceptional run-time and performed most like a corded saw [3]. If you plan heavy use, that is the kind of tool to look at first.
When two batteries are worth it
Think about a framing day. A single small pack may last through a few walls, then need a recharge in the middle of the job. With a second pack ready, you swap and keep cutting.
That matters most when you cut a lot of framing or thick hardwood. For occasional trim work, one pack is usually enough, and a second battery is an extra cost you may not use.
Which battery should you buy for your saw?
For heavy cutting, pick a battery of 4Ah or larger that matches your saw’s voltage. A forum poster says he would not go below that size on a heavy draw tool like a saw [1]. Match the voltage to your saw first, then choose the Ah size.
If your saw takes two batteries, a pair of larger packs gives the longest run time for big jobs. One top-rated cordless circular saw takes two 18V batteries and delivers exceptional run time [3]. Swapping in a charged pack keeps a long day of framing or thick hardwood cuts moving. For the next step, see How Long Do Cordless Saw Batteries Last Under Load?
Use a 1.5Ah pack only for light, occasional cuts, such as a few trim pieces around the house. For anything heavy, it runs down too fast.
The one thing to remember: for heavy work, buy 4Ah or larger in your saw’s voltage. Next, find out whether your saw takes one battery or two, then buy the matching pack.