Its not just a case of knowing the power your outputting, you also need to know what that power is in relation to your current fitness, e.g. what % of FTP should you be working at?
If you’re new to training and/or getting fitter, rather than maintaining fitness, then your FTP will change, (hopefully increase), quite a lot. This therefore requires you to get tested regularly, at a cost, or guess your FTP, in which case why worry about getting accurate power readings….
It would be better, (read cheaper), to perform some of the tests documented by Joe Friel, which don’t need a power meter, as these are easily repeatable.