Capacitor

By knowing the critical parameters of what the capacitor will be connected to and applying the appropriate differential equation.

_{Give me a sparse question and I'll give you a sparse answer.}

For motors requiring a capacitor (be it start or run) the sizing data is generally supplied with the product (especially if the product does not come with a capacitor).

Additionally the data is available off the motor manufacturer's website.

If nameplate do not give the information,contact an experienced appliance repair/motor rewinding shop.

That you will know the power fator data in your grid, to get a pf meter first. then you can know the reactive power, and can do the right compensation.