Replacing your rear wheel hub, the PowerTap G3 power meter is easy to install and calibrate. ANT+ compatible*, it can be used with a wide variety of devices, like the Garmin 920XT, Joule GPS by PowerTap, or an existing bike computer. It boasts accuracy of its metrics within +/- 1.5 percent, which is about as good as you'll find in any power meter. It's also one of the cheaper models on the market—perfect if you're looking to fine-tune your training on a limited budget.
*ANT+ is the technology that lets wireless devices communicate with each other.