If you’ve walked the aisles of any large home improvement retailer lately, you’ve probably noticed that major brands’ cordless power tools typically fall into distinct power systems. These systems are almost always categorized by their voltage output, such as 4V, 12V, 20V, or 40V.
But what does that actually mean? Why is it always volts, and not watts, which is often how we see light bulbs categorized? The answer is rooted in physics and can get pretty technical, but simply put, voltage measures electrical pressure and describes the potential force that can power something like a motor. More voltage generally means more torque and, thus, better performance.
Many cordless tools don’t run at max volts all the time, as they often have variable-speed triggers that let you adjust how much power the tool outputs. In this case, 18V is the max output, not what you get every time you press the trigger. Watts, on the other hand, measure the total power consumed while the tool is in use. That’s why light bulbs are often measured in watts: Unlike power tools, they generally output the same amount of energy consistently, and a 40W light bulb will expend 40 watts nearly all of the time.
Do higher watts mean more power?
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Unlike voltage, watts measure power. So yes, higher wattage means more power. An 18V impact wrench used on its low setting is still an 18-volt tool. However, the wattage changes in real time depending on the speed setting. Because of this, the more wattage a cordless tool uses at any given moment, the faster the battery drains. That battery is a finite source of energy, after all, and higher wattage means you’re using more of it at once. The same goes for a corded tool. Your power supply may be unlimited, but higher wattage means a higher utility bill.
That’s why your bill and energy usage are measured in watt-hours (or kilowatt-hours). The difference between watts and watt-hours is that the latter equals one watt of power sustained continuously for one hour. A power tool expelling 1,000 watts for an hour has used 1,000 watt-hours of energy, or one kilowatt-hour (kWh). Watt-hours are also important when buying portable power stations, for example, as the kWh rating indicates how much power it can provide before running out of juice.
Higher wattage doesn’t necessarily mean better performance, though. It only tells you that a tool is using more power, not how it applies that power. Older power tools with brushed motors, for example, generate a lot of heat and friction, which wastes power. Newer, brushless tools will generally offer better performance for the same watts.
Watts, starting watts, and amps
Complicating things further is the difference between running watts and starting watts. Running watts indicate how much a tool uses when running normally, but some may require more power to get the motor up and running; this is its starting wattage, which could be twice or three times higher than running wattage.
You need to know what a tool’s starting wattage is because it could trip a breaker or trigger an automated shutdown if it exceeds what your battery, generator, or grid can provide. That’s why voltage is important, as it indicates a tool’s potential flow of electricity — an 18V tool running on an 18V battery will never transmit more than 18 volts of electricity.
But what about amps? Where do they come in? Amperage measures electrical current and completes a physics trifecta with volts and watts, since wattage is calculated by multiplying voltage and amperage. That means either volts or amps must increase to get higher power (wattage) at any given moment. A 12V tool, then, would need more amperage to output the same power as an 18V tool. Amps also indicate battery capacity via ampere-hours (Ah), which measures current over time. Thus, a 12 Ah battery in a 40V tool system has more capacity than a 6 Ah battery because it has twice the ampere-hours.