Metrication in the United States There are no U.S. customary units for electric current, potential difference, or charge since these concepts were developed after the international adoption of metric in science. The metric units ampere, volt, ohm and coulomb are the only units used. The SI term hertz has replaced the term "cycles per second" as a unit of frequency. Energy is often measured in watt hours, BTUs, therms or calories rather than the SI joule. Residential and commercial electrical energy use is metered and billed in kilowatt hours, kW·h, as in most metric countries. Force measurements regularly use the pound instead of the SI
2019 redefinition of SI base units countries adhering to the Metre Convention". The recommendations based on this mandate were presented to the 11th CGPM (1960) where they were formally accepted and given the name """ and its abbreviation "SI". Changing the underlying principles behind the definition of the SI base units is not without precedent. The 11th CGPM (1960) defined the SI metre in terms of the wavelength of krypton-86 radiation, replacing the pre-SI metre bar. The 13th CGPM (1967) replaced the original definition of the second (which was based on Earth's average rotation during 1750–1892) with a definition based on the frequency of the radiation