Blog post: swansont: Everything is Awesome, When You're a Part of the Measurement Team

Planck’s constant comes in because of an historical idiosyncrasy in the way power units have been defined. Since 1990, almost all electrical measurements have been calibrated using a system of units in which Planck’s constant, along with various other constants, are defined rather than measured.

 

By contrast, mechanical power relies on ordinary SI units, which rely on a measured value of Planck’s constant. “By comparing electrical power in conventional units to mechanical power in SI units, h can be determined,†say Chao and co.