The problem with running is that there’s no way to constantly adjust your effort for best results. You often go out too slow or too fast, then feel awful—or worse, get hurt and leave the sport for good. A long list of heart rate monitors and GPS trackers have attempted to quantify performance, but none of them telegraph one simple metric tailored to a runner’s own body.

In the crowded field of wearable plastic gizmos, a new device promises to give you an objective number to guide your runs. Stryd, from startup Athlete Architect, borrows the idea of “power” from competitive cyclists (or maybe Nietzschean scholars), and applies it to running. Power is the kinetic energy you release with each step. To use the device, you clip on a sensor and view your power either on your smartphone or on the heart rate screen of many sports watches. (I like the way the Stryd folks have hacked the signals showing power so they show up as heart rate on sports watches instead of the metric they were designed for.) If you are using the smartphone display, a voice comes on every minute telling you the average power in watts. At the end of the run, an online page displays a tidy pie chart showing how much time was spent in the easy running zone, the medium aerobic zone, or the cookie-tossing anaerobic zone.

Stryd is still in beta, as was apparent during the two weeks I tried it out. The hardware has a rough 3-D printed housing and a rubber clip recycled from a competitor, but it gave me enough of a sense of the technology to know that it works well and could represent a radical shift in how we coach ourselves. If the company is able to go into manufacturing in early 2015 as planned, the gadget could help first-timers pace themselves and adjust their running form for less impact.

The sensor goes on a shoe or clips to a pair of shorts. Bob Parks/WIRED

Stryd itself is an acorn-sized nugget about the size of a Fitbit Zip. You wear it on your running shoes or shorts. Once I clipped it on, the phone’s screen began to show the instantaneous energy of each step (in watts) along with its impact (in pounds). Inside, according to company engineers, the device has accelerometers to measure the microsecond rate of change in a runner’s velocity along with a barometer to adjust for the grade of the road. From the limited description the company could give me, the secret sauce is an algorithm that uses acceleration and grade along with an anatomical modeling of the runner’s body to derive power, impact, ground contact time, and cadence. Before this, accelerometer-based wearables from Nike and Suunto were able to derive speed, distance, and cadence.

My first Stryd-tracked trek was a warm up and fast 400-meter repetitions on a hilly country road. I tried to keep the wattage consistent and was able to feel pretty comfortable. When I got back to my computer, the online report from Stryd showed that I was in the red zone for much of the run (average watts, 174.5), which made sense since it was supposed to be a tough day. Cyclists who use power typically log between 200 and 400 average watts, so the numbers are pretty consistent between cycling and running.

Chasing watts has been a sort of revelation for me. Power is the only feedback I’ve used that’s dead simple and actionable mid-gallop. Sometimes, keeping to a slow, even pace is harder than going all out. It’s particularly tough to pace yourself on rolling hills in contrast to a nice, flat track. But it’s crucial; running is one of the only sports you do most of your practice at a lower intensity than competition. The next day, I ran a recovery run (average watts, 115.3) on rolling hills, and the app correctly showed my pace as all green.

Power seems to be a good alternative to the more typical effort-based feedback, heart rate. Physiologists have often noted the difficulty for beginners to use heart rate. Jack Daniels, who is a physiologist and the coach of marathoner Ryan Hall, once told me that the age-based formulas used everywhere to guess heart max and heart zones are often way off. Even with accurate heart rate zones, monitors are often skewed by temperature, blood volume, and the lag before your effort really registers.

The impact graph for the 5K race. Kicking it home at the end might have caused some soreness the next day. Bob Parks/WIRED

Before giving the device back, my last test with Stryd was a mountainous 5K trail race (average watts, 197.5!) in which I tried out the “impact” feature. It showed how hard I was hitting the ground, which was apparently pretty hard. I remember that early in the race I treaded lightly down the hills, but on the last steep decline, in the rush of chasing the runner ahead, I let out all the stops. Sure enough, the graph (above) showed that all my running in the last meters is fringed in the high-impact red. Not surprisingly, an old running injury flared up in my foot the next day. Using Stryd in this way seems like the perfect recipe for improvement—by increasing my power over longer periods of time while dialing back impact with attention to form, I could be running faster and with less wear-and-tear. In other words: More watts, less pounds.

Bob Parks (@bobparks), a Wired correspondent, ran the 2014 Boston Marathon in 2:54:53.