The Unit Watts Out (UWO) vs torque power ratings on tools have been out for years. DeWalt is one of the most popular brands that use the UWO calculation to present the overall power of a tool.
There are two unfortunate issues with this. First, people still donโt seem to understand what in the world theyโre actually looking at. Second, the other manufacturers arenโt on board with this calculationโleaving us to wonder how Unit Watts Out compares to the traditional speed and torque measurements.
So letโs take a look at what these terms actually mean and see if we can make sense of it.
Speed and Torque as a Measure of Power
Letโs Talk Football for a Second
Speed is simply how fast the chuck is spinning on a drill. Torque is how much muscle itโs putting behind that speed. Think in terms of football. Youโve got a little wide receiver whoโs blazing fast, but he doesnโt have much muscle so heโs tackled easily when caught. Thatโs speed without torque.
On the other hand, youโve got a linemanย whoโs got plenty of muscle to fend off the other big guys, but not nearly as much speed. Thatโs torque without speed. In the middle, you have a tight end who has a balance of quickness and muscle and is a great example of the blend of speed and torque.
The problem with using speed and torque as guidelines of how much power a tool has is that it can be a little deceptive if you donโt know what youโre looking at. Thereโs a balance between the two that always results in a trade-off thatโs easier to see in a 3-speed drill.
Translation
On high-speed mode, you get the maximum speed, but nowhere near the maximum torque; itโs your wide receiver. Mode 2 is your tight end balance between the two. Low-speed is your lineman that delivers maximum torque, but slow speed. In all three settings, you get the same amount of power, but itโs just distributed differently.
So itโs important to know that when you look at the max speed and the max torque, youโre not actually seeing the combined power. The calculation doesnโt simply come from multiplying the two together, because they canโt occur at the same time.
That comes from looking at the relationship between speed and torque at any given setting and is a little more complicated than a simple multiplication problem.ย Just in case youโre wondering, the Power Tool Institute (PTI) has the currently accepted standards for torque testing.
What it does tell you is how much speed youโll get in high-speed mode and how much torque youโll get at low-speed mode. This is great for most users, but when it comes to finding out which has the most actualized power, we need to look at Unit Watts Out (MWO).
Unit Watts Out (UWO) vs Torque as a Measure of Power
Unit Watts Out (UWO) is a calculation that takes speed and torque, multiplies them together, then divides by a constant. The result is Unit Watts Out. I spent so much time trying to dig up that constant that Iโm kind of hesitant to share it since no one seems to be interested in making it public knowledge. Hmmmโฆ maybe Iโll hang on to that a little longer and give you a reason to read on.
By using this calculation, it offers the maximum power output at any given time using watts. So maybe youโve got a drill putting out 500 in-lbs of torque at 600 RPM and youโd like to know how much power that is compared to a drill putting out 475 in-lbs of torque at 650 RPM. The Max Watts Out (MWO) calculation does that.
How Can I Compare Speed and Torque vs Unit Watts Out?
All right, get out your calculators and get ready for some Unit Watts Out vs torque Algebra with a splash of statistical idealism. While this isnโt rocket science, it may not be pleasant, either. After way too much digging, I finally found some data that allowed me to hone in on this mysterious constant that people referred to, but never defined.
Itโs roughly 560. Very roughly.
So weโre dealing with four pieces of data here. Max Speed, Max Torque, Unit Watts Out, and 560 (the Constant, often shown as K). Hereโs what the equation looks like with nothing in it. Torque needs to be in in-lbs or the Constant changes, and Iโm not digging up another one.
- Speed (RPM) x Torque (inch-pounds) / 560 ย = Max Watts Out ย (estimated)
What you have to remember to make this work is that if you are using the peak torque figure, you must use the max RPMย in the same setting.ย In a multiple-speed drill, this is the highest number on the lower range.
Quick Case Study
Letโs look at one of DeWaltโs drills, the DeWaltย DCD790D2 Brushless Compact Drill. It has a Unit Watts Out rating of 360 and a max speed of 600 RPM in high torque mode. In our Unit Watts Out vs torque calculation we get the following:
- 600 (RPM) x Torque (inch pounds) / 560 = 360 UWO
Solving this equation leaves this 20V compact drill with a max torque of 336 in-lbs.
What if we know the speed and torque of a drill and would like to know the Unit Watts Out? We can do that, too. Letโs consider a DeWalt 20V Max XR drill that spins at 575 RPM with 490 in-lbs of torque (in high mode/speed).
- 575 (RPM) x 490 (inch-pounds) / 560 = Unit Watts Out
Solving this equation shows this drill has a Unit Watts Out rating of about 503 MWO.ย
Major Issues with the UWO Equation
There are several inherent problems with this Unit Watts Out vs torque equation, though. It will cause it to be relegated to use as a topic of conversation only. First of all, maximum power output actually occurs before top speed is achieved or max torque is applied in any given mode. Itโs somewhere in the middle.
That means that working the equation to calculate the torque or the Unit Watts Out based solely on max speed or torque is subject to some significant error.
The other factor is that no one I found actually publishes the Constant (K) figure, which I had to work backward to find from a manual that offered me total watts, max torque output as Newton Meters, and the corresponding no-load max speeds.
Then thereโs also the fact that the max torque does not occur at the max speed in that setting. Combining this together, and using this as anything other than the roughest of estimates would be misleading.
For example, the actual max torque in our DCD790 is 531 in-lbsโand thatโs WAY off the equationโs estimate.
ToolGuyd Weighs In
I originally wrote this article back in 2015. In 2019, Stuart over at ToolGuyd (who holds a Ph.D., by the way) spent a few hours digging into this as well and shared some of his thoughts.
ย โI knew thereโs no way to convert UWO to torque, and for the longest time Iโve been avoiding doing the math to prove it.ย Good news, I feel I have maximized my understanding of UWO.ย Bad news, thereโs no way to convert from UWO to torque.
Hereโs the problem โ SBD doesnโt publish under-load speeds, and so thereโs no way to get torque from UWO. Thereโs also no guarantee that UWO is measured at max torque and the corresponding speed. UWO is simply a value for which no useful cross-brand torque specs can be extracted.โ
โ Stuart Duesch, ToolGuyd
I have a ton of respect for Stuart, and heโs probably the smartest guy I know thatโs not currently on a manufacturerโs engineering team. Weโre not in the slightest bit insulted by his findings.
Is Unit Watts Out More Useful than Speed and Torque?
Well, yesโฆ and no. If you are trying to find what drill has more overall power, Unit Watts Out is a great way to compare. However, two drills can have the same Unit Watts Out rating, but one has higher torque on the low-speed setting.
In that case, it would be nice to know what those torque levels are if thatโs what youโre the most concerned about. DeWalt makes the point that itโs not just torque that creates a hole and theyโre right. It is a combination of the two that gets results.
When you calculate the maximum torque, youโre attaching the drill to a machine that is measuring how much torque is applied by the unit without actually turning the chuck.
When you calculate the maximum speed, youโre counting RPMs when there is no friction force acting against the chuck.
Neither of these can be recreated when youโre producing real results in the field. Unit Watts Out is measured by a machine that takes into account how much speed the chuck has along with the amount of resistance (which requires torque) and has several different settings and tells the engineer the maximum that it produced.
This most likely occurs in mode 2 on a three-speed drill. When we compare the two, weโre quite frankly looking at two different languages and assuming that a third language can be used to translate them equally.
The Final Word on Unit Watts Out vs Speed and Torque
So whatโs the bottom line here? If youย need the maximum possible torque for an application, then you need to know the max torque outputs for the drills you are considering. If youโre really just concerned about having the most overall power, and therefore, the best combination for drilling in most applications, then Unit Watts Out is absolutely useful. Ideally, knowing the Unit Watts Out and how it is translated as the max torque would be a measure that combines both into a truly useful set of numbers.
Why does DeWalt use Unit Watts Out? For starters, they say that people were purchasing drills and drivers simply based on the highest torque, but that wasnโt a true picture of total power. Okay, Iโm on board with that. It would be great to set up a shootout and know the overall power ratings before I even started testing.
Too Many Conversion Issues
The problem is that no matter which side of the conversation youโre on, tool manufacturers donโt agree on which specifications to use. It would be great if all of them would put one more line to their spec sheet. Most would need to include that Unit Watts Out value. DeWalt and Porter-Cable would simply need to publish torque ratings again. Then, you and I could decide based on which measurement of power was most applicable to us.
When DeWalt tells you that you canโt tell the torque from the Unit Watts Out rating, theyโre right. I know that itโs frustrating. Itโs frustrating to me as well. The issue is that the maximum amount of power doesnโt occur at peak torque or peak no-load speed. In fact, youโre likely to rarely, if ever, encounter a toolโs listed no-load speed or maximum torque.
So that leaves us with two rating systems that arenโt perfect but can be useful if you understand what each is trying to communicate to you. As we attempt to compare the best impact drivers and the best cordless drills, weโll keep all of these things in mind.
One last note about the equations I gave you. Please understand that they are virtually irrelevant. Statistically, there is too much error there to account for. When you calculate the Unit Watts Out from Power and Torque, youโre actually getting a number that is too far away from reality to have any confidence.
Unfortunately, I canโt tell you if thatโs a 5% error, 50% error, or something else entirely. It will change from tool to tool.
