What is needed to work with Garmin Watch?
 

I see that Garmin has a speed & cadence sensor. Are there other brands that are compatible? Is a power meter the same thing?

Someone said cadence sensor is all that would be needed since gps would track speed, but if you did indoor training the speed sensor would be needed also.

Assuming a power meter measures something else since they are a lot more $.

I use this speed and cadence sensor on my mountain bike with my Vivoactive. I just got it, but it has worked fine on the couple rides I've taken since then.

GPS on the watch will track speed if you don't have a speed sensor, but I believe I've read that a speed sensor will be quicker to update on the watch, and the GPS generated speed will lag behind what you're actually doing or not update quite as often.

What watch are you looking at?

The fenix line, for instance, can be set to 1-second recording. GPS for speed is pretty responsive and almost automatic (when you stop at a red light for instance) and should be fine as long as you cycle in areas with good GPS signal.

A speed sensor would be better if you cycle where GPS signal is spotty (tall buildings, heavily wooded areas, tunnels ect). You will also need a speed sensor if you want to track and record the data on an indoor trainer.

Do you need a speed sensor? Depends on where you ride and if you want indoor trainer data. Do you need a cadence sensor? Again that depends on what you want. I do not use one as I don't care about cadence.

Any ANT+ sensor will work.

If you install a power meter, it will probably send cadence data (mine does. It is a Stages). The advantage of using Garmin's cadence sensor is that it takes longer to disconnect. My power meter will disconnect from my computer after about 2 minutes of inactivity. A long descent or waiting for others is enough to require a manual restart. The inconvenient of Garmin's is that it is easy to lose. It is attached to your crank by a gasket-like contraption that wears with time.

GPS-based real-time speed estimates are much less accurate than sensor-based. Doesn't matter for casual riding but could be annoying for serious training.

Is the old Garmin GSC-10 speed/cadence sensor compatible with the VAHR? Or does it use older protocols like the Edge 305 and is incompatible with the VAHR?

My gps app on my phone gives inaccurate speed readings and average speed but once it uploads, the web site corrects most of the errors. It's OK with me but may not be for you.