Your First Cordless Drill: What the Volts and Amp-Hours Mean

Walk into any hardware store or scroll an online listing and the cordless drills all look much the same: a pistol grip, a trigger, a battery, and a spray of numbers that supposedly tell you how good it is. Volts, amp-hours, newton-meters of torque, brushless this and max that. Most of it is real, some of it is marketing, and very little of it is explained in a way that helps you choose. This guide cuts through it. The goal is simple: to help you buy one drill that handles flat-pack furniture, a few shelves and the occasional hole in a brick wall, without overpaying for power you will never use.

Drill driver, combi or hammer drill?

There are three common types, and the names overlap in confusing ways.

A drill driver is the basic tool. It drills holes in wood, plasterboard, plastic and metal, and it drives screws. It has a clutch (the numbered collar behind the chuck) that lets you stop driving before you strip the screw head or sink it too deep. For assembling furniture and putting up shelves on timber or plasterboard, a drill driver is all most people ever need.

A combi drill (sold as a "hammer drill/driver" in some markets) adds a hammer action: a rapid tapping motion that helps the bit bite into masonry. It still drills and drives like a normal drill driver, so you lose nothing by having it. The hammer mode lets you make holes in brick, concrete block and mortar for wall plugs, which is what you need to hang something heavy on a masonry wall.

A dedicated SDS hammer drill is a different animal, using a special bit system for serious concrete work. For a first drill in a typical home, it is overkill, and it drives screws poorly. Ignore it unless you have a lot of drilling into solid concrete ahead of you.

For most readers the honest answer is a combi drill. It costs a little more than a plain driver but covers the occasional masonry hole, and the rest of the time behaves exactly like a normal drill.

Volts: 12V vs 18V, and that "20V max" confusion

Voltage is a rough guide to how much grunt a drill has. Higher voltage generally means more torque for driving big screws and drilling wide holes, at the cost of size and weight.

First, clear up the "20V max" business. In the United States you will see drills labelled "20V max" that are the same class of tool as an "18V" drill sold in the UK and Australia. The difference is purely how the number is measured: "20V max" is the voltage of a freshly charged battery at no load, while "18V" is the nominal voltage under use. They describe the same battery platform. Do not pay extra thinking 20V is a step up from 18V; it is not.

• 12V drills: light, compact and easy to hold overhead or in a cupboard. Plenty for flat-pack furniture, shelves into timber and plasterboard, and light drilling. They struggle with large holes and tough masonry.
• 18V / 20V max drills: the mainstream choice. More torque, longer runtime, bigger range of matching tools. Heavier, but not uncomfortably so for occasional use.

If your DIY is genuinely light and you value a small, low-weight tool, a 12V combi is a sensible and often cheaper pick. If you expect to do heavier jobs, or you want one drill that handles almost anything a home throws at it, 18V is the safer bet.

Amp-hours, brushless motors and what they actually change

If volts is roughly the push, amp-hours (Ah) is roughly the size of the fuel tank. A 4.0Ah battery stores about twice the energy of a 2.0Ah battery of the same voltage, so it runs longer between charges. It does not make the drill more powerful; it makes it last longer and, as a side effect, weigh more.

For occasional home use, a 2.0Ah battery is usually enough for an afternoon of furniture and shelves, and it keeps the tool light. A 4.0Ah battery suits longer sessions or bigger jobs. Two smaller batteries can be more useful than one large one, because you can charge one while using the other.

A brushless motor is worth understanding because it is heavily marketed. Older "brushed" motors use carbon contacts that rub and wear. Brushless motors replace that with electronics, which commonly means better efficiency, more runtime from the same battery, less heat and a longer lifespan. The trade-off is a higher price. For light, occasional use a good brushed drill is perfectly serviceable. If you will use it often, brushless is a reasonable thing to pay for. Treat it as a nice-to-have, not a requirement.

Chuck size, the battery platform and running costs

The chuck is the adjustable jaw that grips the bit. Home drills come in two sizes: 10mm (3/8 inch) and 13mm (1/2 inch). The number is the widest bit shank it accepts. A 10mm chuck covers the vast majority of household drill bits and screwdriver bits. A 13mm chuck accepts larger bits and is standard on bigger 18V drills. For most readers either is fine; do not choose a drill on chuck size alone.

The quietly important decision is the battery platform. Each brand's batteries fit only that brand's tools. If you think you might later add a cordless driver, sander, jigsaw or garden trimmer, it pays to pick a brand with a wide range so one set of batteries powers everything. Switching brands later means buying new batteries and chargers all over again. You are not just buying a drill; you are picking a system you may stay in for years.

Running costs are tiny, which is one genuine pleasure of cordless tools. A typical 18V, 4.0Ah battery holds roughly 0.072 kWh (18V × 4.0Ah = 72 watt-hours). Charging it from flat, allowing for some charger loss, uses well under 0.1 kWh. At about 17 US cents per kWh that is under 2 cents per charge; at around 25p in the UK, still only a couple of pence; at roughly 30 Australian cents, a fraction of a cent to a cent or two. Electricity prices vary by country and tariff, but either way, you could charge the battery most days for a year and barely notice it on your bill.

One safety note on the batteries themselves: lithium-ion packs dislike extremes. Avoid leaving them in a car or an unheated garage in hard frost or baking heat, charge them on a hard surface rather than on a bed or carpet, and stop using any battery that is swollen, damaged or getting very hot. When a pack finally dies, it should go to battery recycling or a tool retailer's take-back scheme, not the household bin, in the US, the UK and Australia alike.

What a home DIYer actually needs: jobs vs drill type

Here is the practical crossover. Match the drill to the jobs you really do, not the ones you imagine.

Before drilling into any wall, use an inexpensive cable and pipe detector and keep clear of the areas directly above and below sockets and switches, where wiring tends to run. Mains voltage differs by country, around 120V in the US and Canada and 230V to 240V in the UK and Australia, but a drill bit through a live cable is dangerous everywhere. If you are unsure what is behind a wall, stop and get an electrician or qualified tradesperson to check.

The short version

• For most homes, an 18V (or "20V max", the same thing) combi drill covers furniture, shelves and the odd masonry hole with room to spare.
• Volts is roughly power; amp-hours is roughly runtime. A 2.0Ah battery suits light jobs; two batteries beat one big one for convenience.
• Brushless motors run longer and last longer but cost more; they are a nice-to-have for occasional use, not essential.
• Pick the battery platform carefully if you may buy more tools later, because batteries only fit their own brand.
• Charging costs pennies, but keep lithium batteries out of extreme heat and cold, and always check for hidden cables and pipes before drilling.