Today I rode my rollers because the weather is cold and wet. I also find it a good intermediate workout that has both cycling and running benefits. It also gives me a chance to gauge my fitness. And the news was good. I rode pretty easily at 220W and my heart rate stayed nicely within the 145-155 range, with no real increase over time.
All in all I rode 1:33 minutes and burned an estimated 1,300 calories in the process. My average power was well above 200W. I had to stop a few times to readjust the roller belt because it kept coming off, but other than that there were no glitches.
My Ergomo "normalized" power (NP) output for the ride was 217W, which -for me- means a medium intensity ride. My average was 215W. I like the NP readout on the Ergomo. Although I have no idea how they calculate it or how accurate or valid it is, I have found it correlates well with my perceived effort. One thing that is clear is that NP is nearly identical to average power if you ride steady (as happens on rollers).
(the manual says (sic) "NP is calculated using a special formula that both smooths and weights your power output to better reflect physiological (especially metabolic) "costs" of variable-intensity efforts")
When I ride hard, my NP goes close to or over 250W while my average maybe as low as 200. When I rode with the Specialized Team last weekend my NP was 247 for a 2 hour ride that burned 1,260 calories over 30 miles. The average power was just below 200W for that ride no doubt reflecting the long downhill from Skyline to Menlo Park. When I ride easy NP stays below 200W.
I find NP is a better indicator than average watts. However since it correlates so well with my perceived effort I can usually guess my NP for a ride to within a few watts. And that tells me that listening to your body is really the best you can do.