Power meters for bikes seems very expensive. I have a wahoo speed and cadence sensor can I use them for measure power on road bike on Fenix 5? Or else what would the cheapest solution be?
Unfortunately there are too many variables (eg. road surface, wind speed/direction, aeroness of the bike, etc) for power to be estimated on speed and gradient alone. It does require a power meter to estimate.
Direct force power meters have certainly come down in price in the last couple of years, but they aren't what you would call "cheap". Stages left only is probably one of the cheapest. IQ2 is even cheaper, but is running late on delivery due to technical issues.
In terms of non-DFPM's, Powercal (estimates power from changes in heart rate) is a cheap option.
Indoor trainer apps like Zwift and Trainerroad can estimate power from an indoor trainer and speed sensor, but this won't give you power on outdoor rides.
Power meters for bikes seems very expensive. I have a wahoo speed and cadence sensor can I use them for measure power on road bike on Fenix 5? Or else what would the cheapest solution be?
Of course you need a power meter to measure power. Your questions sounds as if I ask: "Do I need a thermometer to measure temperature or could I just go by the feeling?"
Simplest option would be the favero assioma UNO pedals, currently retailing at €380 in Europe (same price as a Fenix 5). There are other cheaper options in the 250-350 range but depends on crank compatibility
The cheapest yet accurately enough power meter which I use is https://velocomp.com I am using it on my road and MTB bike with very good results. Hunt for discount periods, I paid mine 299 USD the full version. Now I see they have upgraded versions. See DCRainmaker review for more info, but I highly recommend it for a budget power meter.
