Human power output is not a new concept; we’ve been monitoring caloric expenditure (the amount of heat released when burning a quantity of food) in the fitness industry for decades. But measuring human power through watts on the bike was unheard of until 1989, when California-based Team Strawberry, began using a system called the Power Pacer. They were looking for a more accurate measure for training output than heart rate. Specifically, for a way to resolve variables that heart rate training didn’t take into consideration, such as rider weight, bike weight, wind resistance, and variations in terrain. They wanted to know what level of power they were producing, which could be very high on say, a windy day, despite a low speed. Or very low, despite a high heart rate, if their legs were overly fatigued due to having raced the prior day. Their initial experimentation became the basis for the watt-measuring machine known as the Power Tap.

Before we get into the details of the Power Tap, a word about watts. A watt is simply a term used to define an amount of power (energy expended) over a period of time. Science-minded folks know that energy is typically represented as a joule, using a time period of one second. So one watt equals one joule of energy measured over one second; thus, 100 watts would equate to 100 joules of energy per second.

Now back to the Power Tap. Pure power on a bicycle is about how hard you push the pedals and how fast you are pedaling. It’s not how hard the wind is blowing, or how high your heart rate is, or how flat or steep the terrain is. To measure power in watts, a Power Tap hub containing a torque sensor attaches to your rear wheel. That reading combined with your wheel speed, is fed to a computer mounted on your handlebars. Voila – watts are calculated and displayed. Unfortunately, the power-measuring components inside the hub are proprietary information and not divulged – bike shops can’t even service them – only the manufacturer – but there are details regarding the hub.

Getting the hub integrated into your wheel is not so simple. The hub is constructed to be the axle of the wheel, and is available in either road (130-mm) or mountain (135-mm) bike axle lengths, in either alloy or steel. The only way to “install” it is to have a professional build you a custom wheel. Wheel specs will vary by the Power Tap manufacturer, but will have specific spoke hole diameter requirements, flange diameter requirements, and spoke measurements. The good news is, most manufacturers now make hubs compatible with any shifting set-up – be it Shimano or Campagnolo, and either an 8, 9, or 10 speed system. So that’s not a concern. To make the wheel installation simpler, some manufacturers offer the option to purchase the hub already built into a perfectly spec’ed wheel (for a much higher price, of course). And if you’re really picky, you can get a front hub to match the same color and sheen as the Power Tap back hub.

To read data from the hub, the first Power Taps used wires that ran up to the handlebar computer, from a sensor attached to the rear seat stay. But the latest and greatest models use wireless technology, which means less installation work, and less weight. A wireless system can weigh as little as 400-grams, compared to older technology weighing in at 600-grams. If you go wireless, be prepared to pay a 50- to 100-percent premium over what is already nearly a $1,000 undertaking.

But despite the complexities of watts, and the steep price of the Power Tap, bike racers swear that this is the superior way to train. Measuring watts, along with heart rate, speed, and cadence gives you everything you need to know to create and monitor a training program (by the way, Lance Armstrong hits 1,000-watts when racing in the Tour de France). Rest assured that manufacturers are scrambling to increase Power Tap technology while decreasing the price, ultimately making the Power Tap available to the masses.