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
Unit of length in vacuum during a time interval of seconds." It is approximately equal to . Other units are derived from the metre by adding prefixes from the table below: For example, a kilometre is . In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or of a metre. Other non-SI units are derived from decimal multiples of the metre. The basic unit of length in the Imperial and U.S. customary systems is the yard, defined as exactly by international treaty in 1959. Common Imperial units and U.S. customary units of length include: In addition, the following