For cordless drills, power is measured in battery voltage. Higher voltage means more torque-spinning strength to overcome resistance. Over the last decade, top-end voltage has increased from 9.6 to 18V, but the range of models include 6, 7.2, 9.6, 12, 14.4 and 18V. Today's higher-voltage drills have enough power to bore big holes in framing lumber and flooring. That's impressive muscle. But the trade-off for power is weight. A typical 9.6V drill weighs 3 1/2 lbs., while an 18V model weighs up to 10 lbs.
Before cordless drill/drivers arrived, most drills had pistol grips, where the handle is behind the motor like the handle of a gun. But most of today's cordless models are equipped with a T-handle: The handle base flares to prevent hand slippage and accommodate a battery. Because the battery is centered under the weight and bulk of the motor, a T-handle provides better overall balance, particularly in heavier drills. Also, T-handle drills can often get into tighter spaces because your hand is out of the way in the center of the drill. But for heavy-duty drilling and driving large screws, a pistol grip does let you apply pressure higher up — almost directly behind the bit — allowing you to put more force on the work.
An adjustable clutch is what separates electric drills from cordless drill/drivers. Located just behind the chuck, the clutch disengages the drive shaft of the drill, making a clicking sound, when a preset level of resistance is reached. The result is that the motor is still turning, but the screwdriver bit isn't.
Why does a drill need a clutch? It gives you control so you don't strip a screw or overdrive it once it's snug. It also helps protect the motor when a lot of resistance is met in driving a screw or tightening a bolt.
The number of separate clutch settings varies depending on the drill; better drills have at least 24 settings. With that many clutch settings, you can really fine-tune the power a drill delivers. Settings with the lowest numbers are for small screws, higher numbers are for larger screws.
Most clutches also have a drill setting, which allows the motor to drive the bit at full power.
The least expensive drills run at a single speed, but most have two fixed speeds: 300 rpm and 800 rpm. A slide switch or trigger lets you select high or low speed. These drills are ideal for most light-duty operations. The low speed is for driving screws, the high speed for drilling holes.
For more refined carpentry and repair tasks, choose a drill that has the same two-speed switch and a trigger with variable speed control that lets you vary the speed from 0 rpm to the top of each range. And if you do more hole drilling than screwdriving, look for more speed — 1,000 rpm or higher — at the top end.