Most Accurate Power Meter?

It might seem superficially obvious to some people that "the most accurate power meter" might be the best power meter. Or maybe the most repeatably accurate power meter is the best? But then maybe the most repeatably accurate power meter involves: a huge sum of money; an inconvenient placement on your bike; and a usage case totally different to the one you need. I covered that a while ago whilst looking at the fabled 'Best Power Meter' for differing types of rider uses. But even if you want/need an accurate power meter then how do you know how accurate it really is? I mean REALLY know? Will the manufacturer test the accuracy of every power meter they ship? Will they test it meaningfully? Might that accuracy change because of flaws in your installation of it? Or might accuracy just change over time despite your best calibration efforts? I was going to post a quick table of Power Meter Accuracy. I was just trying to be helpful to someone, somewhere. Just to give them a steer. But as you've already guessed I opened a can of worms ;-) This is the list of stated manufacturer accuracy levels. We start by seeing the alphabetic brilliance of 4iiii being named as they are: +/- 1% 4iiii Precision Favero ASSIOMA (V2 firmware update for IAV) Rotor SRM Verve InfoCrank Garmin Vector 3 +/- 1.5% Powertap Hub/Pedals/Crank +/- 1.5% Quarq Riken +/- 1.5% Watteam PowerBeat- +/- 1.5% Garmin Vector 1 & 2 +/- 2% Easton Cinch FSA Powerbox Pioneer Crank Polar/Look Stages Favero bePRO +/- 3% Powerpod I may have missed some but that will do for now and please let me know if the manufacturer claimed figures are incorrect. BUT I don't believe the figures...at least not all of them. Cycling Weekly noted that in a test of 54 power meters (23 models), scientists " found that individual power meters deviated a lot, even when units came from the same manufacturer. The scientists are concerned that six units deviated by more than five percent, including products from Stages, Quarq and power2max.". The same study found SRM and PowerTap to be the best also noting that “Power meters used by elite and recreational cyclists vary considerably in their trueness.” Let's say I compare 4 power meters simultaneously, how do I know which, if any, are right? I guess you could compare: dual pedals; dual cranks; and a spider of some sort. But they should all be naturally different to a fourth PowerTap G4 wheel because of, for example, drive train loss. If 3 show the same readings does it mean they are all right? Or does it mean that as a reviewer I was given new and specially calibrated units by the manufacturer that maybe would not be given the same level of attention as a normal production model for you? I suppose that it is possible. I have no idea either way. The study referenced by Cycling Weekly, above, does kind of hint at something like that potentially happening. Cycling Weekly again note "The scientists don’t know whether the worst meters are not well calibrated before they leave the factory or if their set up deteriorates during use." So maybe the best review would be of a genuinely retail bought unit, and that would have to have been bought 'blind' without the manufacturer's knowledge. Maybe that unit should then get 1000 miles of use before tests commence? Just a thought. The units I get my hands on (temporarily from friends as well as manufacturers) mostly seem either 'about right' or patently wrong. I don't seem to find a middle ground. Is it important? Let's say I'm doing 250watts for 'quite a while' I could certainly tell if I was doing 260w (+4%) but I'm not sure that I could spot 2% variation at 255w even after half an hour. I might explain it away as having a good/bad day. So: is the +/- 3% of the PowerPod so bad? On that basis, maybe not. Summary for Jo-Average Cyclist: Accuracy and trueness seem to be really, really, really important. But on a day-to-day basis, I'm not so sure if most of us really, really, really know if we are getting that accuracy. So is it important? ;-) I might have to start to like optical heart rate monitors now ;-)