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
Per unit tax may lead to substitution to cheaper brands. Per unit tax A per unit tax, or specific tax, is a tax that is defined as a fixed amount for each unit of a good or service sold, such as cents per kilogram. It is thus proportional to the particular quantity of a product sold, regardless of its price. Excise taxes, for instance, fall into this tax category. By contrast, an ad valorem tax is a charge based on a fixed percentage of the product value. Per unit taxes have administrative advantages when it is easy to measure quantities of the product