Well, you don't.
You really need to know the max rating of your motor, and max temp rating. That being said, the ratings may be a good guide, but lots of riders pump more amps through the motor than they are technically rated for. The danger is in overheating any one of the motor components or wires depending on the load, and time under load. If I had to guess, you could go to a 12-15 amp controller, but that would be my guess, and I would risk it on MY motor. Get a controller that will allow you to adjust the amperage output, such as a 21 amp KT Controller. You will need a matching KT display.
First I would read this:
"There's no such thing as a rated watt!" Read here for why we don't advertise a simple power rating for the motors we sell. Also, have a look at our Youtube video where Justin talks about the power to propel bicycles in <a href="https://www.youtube.com/watch?v=ALde6zhLPs0">"School of Watts"</a>.
ebikes.ca
And this:
https://ebikes.ca/tools/simulator.html
Paying attention to this:
- Overheat In: This is a prediction on how long it would take the motor to overheat (reach 150oC) from the starting temperature of the simulation, based on a comprehensive thermal motor model taking into account both the motor RPM and passing air velocity. If the motor has not had been thermally characterized in our wind tunnel, then it will say "Not Modeled" instead.
- Final Temp:This is a prediction of the motor core temperature after running for 2 hours at the current conditions for phase current, rpm, and velocity. The text will appear red if the final temp is over 150 oC. It is very common to run hub motors for short times at loads that cannot be sustained continuously, most steep hills are over in a matter of minutes rather than hours.