The relationship between a tool's unit watt output (UWO) and torque power rating has been published for many years. One of the most popular brands is DeWalt, which uses UWO calculations to present the overall functionality of the tool.
This has two unfortunate problems. First, people still don't seem to understand what the world they actually see is what it is. Second, other manufacturers were not involved in this calculation – making us wonder how Unit Watts Out compares to traditional speed and torque measurements.
So let's take a look at what these terms actually mean and see if we can make sense of it.
Speed and Torque as Measures of Power
let's talk football
Speed is how fast the chuck spins on the bit. Torque is how much muscle it puts behind that speed. Think in terms of football. You have a small wide receiver who is extremely quick, but he doesn't have a lot of muscle, so he's easy to tackle when he gets caught. That's speed without torque.
On the other hand, you have a linebacker who has enough muscle to fend off other bigs, but not as much speed. That's torque without speed. In the middle, you have a tight end who has the balance of speed and muscle, a great example of a fusion of speed and torque.
The problem with using speed and torque as a guide for tool power sizing is that it can be a bit deceiving if you don't know what you're looking at. A balance between the two always results in a tradeoff, which is easier to see in a 3 speed rig.
In Overdrive mode, you get maximum speed but nowhere near maximum torque; this is your wide receiver. Mode 2 is your tight balance between the two. Low speed is your lineman, delivering the most torque but at a slower pace. You get the same power in all three setups, just the distribution is different.
So it's important to know that when you're looking at max speed and max torque, you're not actually looking at combined power. Computation is not just multiplying the two, because they cannot happen at the same time.
This comes from looking at the relationship between speed and torque at any given setting, and is a bit more complicated than a simple multiplication problem. In case you were wondering, the Power Tools Institute (PTI) has a currently accepted standard for torque testing.
What it tells you is how much speed you'll get in high mode and how much torque you'll get in low mode. That's fine for most users, but to find out which has the most real power we need to look at the power output (MWO).
Unit Watt Output (UWO) and Torque as Measures of Power
Unit Watt Output (UWO) is a calculation that multiplies speed and torque and divides by a constant. The result is output in units of watts. I've spent so much time trying to dig this constant that I'm a bit hesitant to share it since no one seems interested in making it public. Hmmm… maybe I'll stick around a little longer and give you a reason to keep reading.
By using this calculation, it can provide the maximum power output using watts at any given time. So maybe you have a drill that puts out 500 in-lbs of torque at 600 RPM, and you want to know how much power it has compared to a drill that puts out 475 in-lbs of torque at 650 RPM. Maximum output power (MWO) calculations can do this.
How do I compare speed and torque to output in watts?
Well, get out your calculator and get ready for some unit watt output vs. torque algebra with statistical idealism. While it's not rocket science, it can also be unpleasant. After a lot of digging, I finally found some data that allowed me to dig into this mysterious constant that was mentioned but never defined.
About 560. very sketchy.
So we're dealing with four figures here. Maximum speed, maximum torque, output in watts and 560 (constant, usually displayed as K). This is what the equation looks like without any content. Torque needs to be in pounds or variable, I'm not going to dig another one.
- Speed (RPM) x Torque (in-lbs) / 560 = Maximum Power Output (estimated)
For this to work, all you have to remember is that if you are using the peak torque value, you must be using the maximum RPM at the same settings. Among multispeed rigs, this is the highest number in the lower range.
quick case study
Let's take a look at one of DeWalt's cordless drills, the DeWalt DCD790D2 Brushless Compact Drill. It has a rating of 360 per watt and a maximum speed of 600 RPM in high torque mode. In our specific power output vs. torque calculation, we get the following information:
- 600 (RPM) x torque (in-lbs) / 560 = 360 UWO
Solving this equation gives a maximum torque of 336 in-lbs for this 20V compact drill.
What if we knew the speed and torque of the drill and wanted to know the output in watts? We can do it too. Check out this DeWalt 20V Max XR drill that spins at 575 RPM in high-torque mode for 490 in-lbs of torque.
- 575 (RPM) x 490 (inch-pounds) / 560 = output in watts
Solving this equation shows that the bit has a unit watt rating of approximately 503 MWO.
Major Problems with UWO Equations
However, there are several inherent problems with this specific power output versus torque equation. It will cause it to be relegated to being used only as a conversation topic. First, in any given mode, maximum power delivery actually occurs before top speed is reached or maximum torque is applied. It's somewhere in the middle.
This means that equations that calculate torque or output in watts based solely on maximum speed or torque will have some major errors.
Another factor is that I found no one actually published the constant (K) figure, I had to work backwards to find it from a manual that gave gross watts, max torque output in Newton meters and corresponding no load Maximum speed.
There's also the fact that, with that setup, maximum torque doesn't come at maximum speed. Combining it and using it as anything but the roughest estimate is misleading.
For example, the actual maximum torque in our DCD790 is 531 in-lbs, which is far from what the equation estimates.
I originally wrote this article in 2015. In 2019, ToolGuyd's Stewart (who has a PhD, by the way) also spent a few hours digging into this and sharing some of his thoughts.
"I know there's no way to convert UWO to torque, and for a long time I've been avoiding the math to prove it. Good news, I feel like I've maxed out UWO. Bad news, there's no way to convert UWO for the torque.
Here's the problem – the SBD doesn't publish underload speed, so it can't get torque from the UWO. There is also no guarantee that UWO is measured at maximum torque and corresponding speed. UWO is just a value from which no useful cross-brand torque specification can be extracted. "
Stuart Duesch, ToolGuyd
First off, I have a lot of respect for Stewart, he's probably the smartest guy I know who isn't currently on the manufacturer's engineering team, so I'm not in the slightest insulted by his discovery.
Are units of watts more useful than speed and torque?
Well, yes…but no. If you're trying to find out which drill has more overall power, specific power output is a good way to compare. However, two drills can have the same rating per watt, but one has a higher torque at the low speed setting.
In that case, it's good to know what those torque levels are if that's your primary concern. DeWalt pointed out that it wasn't just torque that caused the holes, and they were right. Combining the two results in the result.
When you calculate max torque, you are connecting the drill to a machine that is measuring how much torque the unit is applying without actually turning the chuck.
When you calculate max speed, you are calculating the rpm when there is no friction acting on the chuck.
None of this can be recreated when you produce real results in the field. The output per watt is measured by a machine that takes into account the speed of the chuck as well as the amount of resistance (which requires torque) with a number of different settings and tells the engineer the maximum it produces.
This is likely to happen in mode 2 on a three speed rig. When we compare the two, we are frankly looking at two different languages and assuming that they can be translated equally using a third language.
Final Conclusion on Watt Output vs. Speed and Torque
So what's the bottom line here? If you need the maximum possible torque for your application, then you need to know the maximum torque output of the drill you are considering. If you really just care about having the most overall wattage, and therefore, the best combination of drilled holes for most applications, the Unit Watts Out are definitely useful. Ideally, understanding unit watts and how they translate to maximum torque would be a measure that combines the two into a really useful set of numbers.
Why did DeWalt use Unit Watts Out? First off, they say that people only buy drills and driver drivers based on maximum torque, but that's not really true for total power. Ok, I agree with that. It would be great to set up the shootout and get an idea of the overall power rating before I start testing.
Too many conversion issues
The problem is that no matter which side of the conversation you're on, toolmakers don't agree on which specification to use. It would be nice if all of them could put an extra line in the spec sheet. Most will need to include the unit wattage. DeWalt and Porter-Cable simply publish the torque ratings again. Then you and I can decide based on the power measurements that work best for us.
DeWalt were right when they told you that you can't judge torque from unit watt ratings. I know it's frustrating. It also frustrates me. The problem is that maximum power doesn't come at peak torque or peak no-load speed. In fact, you may rarely, if ever, encounter the no-load speed or maximum torque listed by the tool.
So this leaves us with two rating systems that aren't perfect, but are useful if you understand what each is trying to communicate to you. We keep all of these things in mind when we try to compare the best impact driver vs the best cordless drill.
One last note about the equation I gave you. Please understand that they are actually irrelevant. Statistically, there are too many bugs out there to account for. When you work out power and torque in watts, the numbers you actually get are far from reality to be sure.
Unfortunately, I can't tell you if this is 5% wrong, 50% wrong, or something else entirely. It will vary by tool.