Originally Posted by Branko D

Within 10% of actual watts is good enough for determining how hard you were trying, really. I don't have a power meter on my latest bike, so when I'm training on it based on perceived effort it tells me if I've paced it near correct or not.

The problem with estimates is that you don't have one real-time, which makes it of very limited utility. If you could get a 3s or 5s average estimate which is within 10%, you could pretty much train and pace off that, wouldn't be ideal, but still more useful than, say, looking at heart rate. Seeing an estimate after the ride, eh, not terribly useful.