What Ah actually means on a power tool battery

Battery listings put two numbers in front of you: volts and amp hours. The first is mostly fixed by the platform you have chosen. The second is where people overspend.

Volts set the ceiling, Ah sets the runtime

Voltage determines how hard the tool can work — an 18V drill will drive a bigger screw than a 12V one, whatever battery you fit. Amp hours determine how long it works before the battery is flat. A 5.0Ah battery does not make a drill more powerful than a 2.0Ah one. It makes it run roughly two and a half times longer.

There is one caveat. High-drain tools — circular saws, grinders, anything that pulls hard and continuously — can draw more current than a small battery can deliver comfortably. On those, a larger battery does deliver noticeably better performance, because it is not being pushed to its limit. On a drill, it does not.

Weight is the trade you are making

A 5.0Ah battery is roughly twice the weight of a 2.0Ah one and hangs off the bottom of the tool. For overhead work, or a long afternoon of driving screws, that weight is felt. Plenty of trades carry small batteries for drills and large ones for saws for exactly this reason.

What to actually buy

Two 2.0Ah batteries beat one 4.0Ah battery for most people. You charge one while using the other, the tool stays light, and you are never stopped completely. The exception is if you own high-drain tools, in which case one large battery plus one small is the better pair.

Where the value is

Battery prices do not scale linearly with capacity — a 4.0Ah is rarely twice the price of a 2.0Ah, so on paper the big one looks like value. Judge it on what you will actually do with the tool. Runtime you never use is weight you always carry.

Twin-pack batteries in a retailer's sale are usually the cheapest route into a platform, and they appear far more often than bare-tool discounts. Watch the platform rather than the individual product.