Can pedal-based power meters be used as flat pedals?

On another forum (see responses by Anna Ronkainen), it was pointed out that if your shoes touch the power meter pods, that may cause erroneous power readings. Basically, you are introducing a side load to the strain gauge assembly in the pods. I am not an engineer, but my (extremely limited) understanding is that the strain gauge assembly is designed to isolate and measure the torque in one direction only, whereas when you pedal, you may be loading the pedals in two dimensions (vertically and horizontally, with respect to the pedals). If you put a side load on the strain gauge assembly by bumping it with a sneaker, I am not sure if that will introduce an error, but Ronkainen seemed to think it might.

My initial thought, based in comments on Argenti’s answer, was that the strain gauge basically measures how much you are bending the spindle. With sneakers, you are still bending the spindle, only you are losing a lot of power because of the poor shoe-pedal interface.

In principle, one could Google for a Look to flat pedal adapter (all current road power meter pedals are Look-based, although some use aftermarket cleats that are very similar but not 100% identical to Look cleats). Do note Rchung’s comment on Argenti’s answer in that case. I would still not recommend using street shoes on clipless power meter pedals, even if I am correct that the power meter would still measure correctly, because you are buying a very expensive item and using it in a pretty compromised use case; you’d be better off spending the money elsewhere.

As an aside, when the question was originally posed, we did not have a global pandemic going on. Right now, on Zwift, there are a number of less experienced riders asking how they can get exercise bikes or trainers to work with Zwift. Virtual cycling environments require either a measurement of your actual power, or they need a trainer with a known resistance curve (I.e. for a given wheel speed, what power is required to turn the trainer). Hence, this question might arise more frequently.

There have been power meters built into rear hubs, cranks, bottom brackets, chain tension readers, and even air-based meters, so you can get a power reading while riding for real, without having to use power-based pedals.

I have an old cycle computer headunit that estimates power based on speed and rate-of-change of elevation using a barometric sensor (ie doesn't account for wind) and that gives an instantaneous figure in 20 watt increments.

You could use Strava and their "estimated power" graph, though IMO its wildly inaccurate in detail though fair in the broad average.

The Power Pod is the main air-based one and is reviewed at https://www.dcrainmaker.com/2016/03/powerpod-depth-review.html Do note you'll need a proper cycling headunit to capture the data, your phone won't do.

Another tool is a fancy trainer that has a power meter built in. Downside is these are hard to ride down the road for real.