Anyone have experience with trainers with power? the kurt kinetic trainers ahve an optional piece for $80 that gives you power readings. Anyone know if it's accurate enough to be used for power training purposes?

Nashbar also has the "Watt Master" trainer that includes power readings and they also have another model on their site that supposedly provides power outputs. I'm not looking for anything perfect, but just want to do some basic tests more so than anything (power at threshold, for example)

Need to buy a triner because my gf is having her knee scoped and she'll be riding it for therapy so I figure since i want to get more serious with training during the winter may as well invest a little more in the power features if they are useful.

Yes and no. They don't measure power at all, they measure speed and then use a formula to calculate the power a hypothetical average rider would use to sustain that speed on the road. The formula is pretty consistent over the relatively narrow range you use in threshold training. So, if it underestimates your FTP by 10% at FTP effort it will be very close to underestimating it by 10% at 80% of FTP effort. So, you just use the number it thinks is your FTP and train away.

That said, it's really a waste of money. The formula is available on Kurt Kinetics website. The watt display is instantaneous only, it won't record your work out like a cyclometer will. Just get a cyclometer (if you don't have one) and then use the formula on the Kurt Kinetic website to determine what speed to do your intervals at ahead of time.

I just made an excel spreadsheet using Kurt Kinetic's formula. Taped it to my wall next to my trainer setup. Using my cyclo-computer, I just glance at the chart based on the speed I'm doing to tell me my wattage.

You really don't even need to bother trying to convert speed to watts... If a hard 20 min interval is 20mph then just do 20mph intervals.

Most of the time the resistance from trainer to trainer will not be the same, or at least not close enough to get a real power reading.

For example Cyclops posted that their Fluid2 Trainers are "ride tuned" 400 watts @ 25mph.

On the one I have this is not true. ~22.5mph is 400 watts. Cyclops Fluid 2 Review