You have a couple of options for your RS520 crankset as outlined below.
Left Crank Arm Power Meter for Shimano RS520
While there isn’t a specific RS520 crank arm power meter, you can buy a R7000 power meter made by 4iiii or Stages Cycling as the R7000 uses the same exact same interface (Hollowtech II) as the RS520.
Please note, the color of the R7000 power meter crank arm might not be exactly the same as your RS520 crankset, but this it will be very close and this is a cosmetic issue only and one that most customers are ok with.
This this type of power meter, you just swap out your current left crank arm for the new power-equipped one and you are good to go.
Please note, these power meters attach a sensor to the inside of the left crank arm. On some bikes, there is not enough room between the left crank arm and the chainstay for the sensor to clear the frame. Therefore, you should make sure you have at least 10 mm of clearance for Stages and 4iiii PRECISION 2 power meters and 8 mm of clearance for 4iiii PRECISION 3 power meters. Also, please make sure that you don’t have a bottom bracket mounted rear brake (used on some aero and TT bikes). Please refer to the image below.
Pros: Affordable, easy to install, don’t have to change crankset
Cons: Measures left leg power only
Crankset Power Meter (Spider Based)
This is a new crankset (power meter spider, crank arms and spindle), excluding chainrings. power2max makes wonderful power meters. They measure both left and right leg power, are reasonably priced and are as good as it gets in terms of accuracy, reliability and dependability. If you select the 24MM spindle option, it will fit your current bottom bracket (no need to change bottom brackets). Also, if you select the 4-bolt version, you should be able to use your existing RS520 Shimano chainrings on it.
Pros: Moderately priced, VERY reliable, measures both legs
Cons: Have to change your crankset
Power Meter Pedals
Power meter pedals can be installed on any crankset. This is a very popular power meter type as they fit any bike, are easy to install and can be moved from bike to bike if necessary. We carry power meter pedals from Favero, Garmin, SRM/LOOK and Wahoo. This includes pedals for SPD-SL, LOOK Keo as well as Speedplay cleats.
Pros: Easy to install, works on any bike, can move from bike to bike, don’t have to change crankset