# Watt-hour per kilogram

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The watt-hour per kilogram (SI abbreviation: W⋅h/kg) is a unit of specific energy commonly used to measure the density of energy in batteries and capacitors. One watt-hour per kilogram is equal to 3,600 joules per kilogram.

The International System of Units is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the second, metre, kilogram, ampere, kelvin, mole, candela, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.

Specific energy is energy per unit mass. It is used to quantify, for example, stored heat and other thermodynamic properties of substances such as specific internal energy, specific enthalpy, specific Gibbs free energy, and specific Helmholtz free energy. It may also be used for the kinetic energy or potential energy of a body. Specific energy is an intensive property, whereas energy and mass are extensive properties.

In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of 1 metre against a force of 1 newton.

## Typical values

The batteries that Tesla uses in their electric cars have about 254 W⋅h/kg [1] , compared to supercapacitors that are typically between 3-10 W⋅h/kg [2] , albeit with research ongoing into enabling much higher values.

Tesla, Inc. is an American automotive and energy company based in Palo Alto, California. The company specializes in electric car manufacturing and, through its SolarCity subsidiary, solar panel manufacturing. It operates multiple production and assembly plants, notably Gigafactory 1 near Reno, Nevada, and its main vehicle manufacturing facility at Tesla Factory in Fremont, California. As of March 2019, Tesla sells the Model S, Model X, and Model 3 automobiles. It is accepting reservations for the Model Y, Roadster (2020), and Semi vehicles. Tesla also sells Powerwall and Powerpack batteries, solar panels, solar roof tiles, and some related products.

Nuclear batteries based on betavoltaics can give up to 3300 W⋅h/kg [3]

## Related Research Articles

The joule is a derived unit of energy in the International System of Units. It is equal to the energy transferred to an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre. It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).

Energy storage is the capture of energy produced at one time for use at a later time. A device that stores energy is generally called an accumulator or battery. Energy comes in multiple forms including radiation, chemical, gravitational potential, electrical potential, electricity, elevated temperature, latent heat and kinetic. Energy storage involves converting energy from forms that are difficult to store to more conveniently or economically storable forms.

The kilowatt hour is a unit of energy equal to 3.6 megajoules. If energy is transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatt hours is equal to the power in kilowatts multiplied by the time in hours. The kilowatt hour is commonly used as a billing unit for energy delivered to consumers by electric utilities.

The RAV4 EV is an all-electric version of the popular RAV4 SUV produced by Toyota. Two generations of the EV model were sold in California, and to fleets elsewhere in the USA, with a gap of almost ten years between them.

Energy density is the amount of energy stored in a given system or region of space per unit volume. Colloquially it may also be used for energy per unit mass, though the accurate term for this is specific energy. Often only the useful or extractable energy is measured, which is to say that inaccessible energy is ignored. In cosmological and other general relativistic contexts, however, the energy densities considered are those that correspond to the elements of the stress–energy tensor and therefore do include mass energy as well as energy densities associated with the pressures described in the next paragraph.

Grid energy storage is a collection of methods used to store electrical energy on a large scale within an electrical power grid. Electrical energy is stored during times when production exceeds consumption, and returned to the grid when production falls below consumption.

Betavoltaic devices, also known as betavoltaic cells, are generators of electric current, in effect a form of battery, which use energy from a radioactive source emitting beta particles (electrons). A common source used is the hydrogen isotope tritium. Unlike most nuclear power sources, which use nuclear radiation to generate heat, which then is used to generate electricity, betavoltaics use a non-thermal conversion process; converting the electron-hole pairs produced by the ionization trail of beta particles traversing a semiconductor.

Plutonium-238 is a radioactive isotope of plutonium that has a half-life of 87.7 years.

Watt is a unit of power named after Scottish engineer James Watt.

The Energy content of biofuel is a description of the chemical energy contained in a given biofuel, measured per unit mass of that fuel, as specific energy, or per unit of volume of the fuel, as energy density. A biofuel is a fuel, produced from living organisms. Biofuels include bioethanol, an alcohol made by fermentation—often used as a gasoline additive, and biodiesel, which is usually used as a diesel additive. Specific energy is energy per unit mass, which is used to describe the energy content of a fuel, expressed in SI units as joule per kilogram (J/kg) or equivalent units. Energy density is the amount of energy stored in a fuel per unit volume, expressed in SI units as joule per litre (J/L) or equivalent units.

An electric-vehicle battery (EVB) or traction battery is a battery used to power the propulsion of battery electric vehicles (BEVs). Vehicle batteries are usually a secondary (rechargeable) battery. Traction batteries are used in forklifts, electric golf carts, riding floor scrubbers, electric motorcycles, electric cars, trucks, vans, and other electric vehicles.

An electric car is an automobile that is propelled by one or more electric motors, using energy stored in rechargeable batteries. The first practical electric cars were produced in the 1880s. Electric cars were popular in the late 19th century and early 20th century, until advances in internal combustion engines, electric starters in particular, and mass production of cheaper gasoline vehicles led to a decline in the use of electric drive vehicles. In 1897, electric cars found their first commercial use in the USA. New York City taxis were electric, and they were manufactured by the Philadelphian Electric Carriage and Wagon company. During the 20th century, the main manufacturers of electric vehicles in the US were Anthony Electric, Baker, Columbia, Anderson, Edison, Riker, Milburn and others. Unlike gasoline-powered vehicles, the electric ones were quieter and did not require gear changes.

The watt is a unit of power. In the International System of Units (SI) it is defined as a derived unit of 1 joule per second, and is used to quantify the rate of energy transfer. In dimensional analysis, power is described by .

A liquid nitrogen vehicle is powered by liquid nitrogen, which is stored in a tank. Traditional nitrogen engine designs work by heating the liquid nitrogen in a heat exchanger, extracting heat from the ambient air and using the resulting pressurized gas to operate a piston or rotary motor. Vehicles propelled by liquid nitrogen have been demonstrated, but are not used commercially. One such vehicle, Liquid Air was demonstrated in 1902.

A capacitor vehicle or capa vehicle is a traction vehicle that uses supercapacitors to store electricity.

A supercapacitor (SC) is a high-capacity capacitor with capacitance values much higher than other capacitors that bridge the gap between electrolytic capacitors and rechargeable batteries. They typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries.

A Tesla Supercharger is a 480-volt DC fast-charging station built by American vehicle manufacturer Tesla Inc. for their all-electric cars. The Tesla Supercharger network of fast-charging stations was introduced beginning in 2012. The electric vehicle network consists of 12,011 individual Supercharger stalls at 1,422 locations worldwide, as of January 2019. Tesla Model S was the first car to be able to use the network, followed by the Tesla Model X and Tesla Model 3.

Research in lithium-ion batteries has produced many proposed refinements of lithium-ion batteries. Areas on research interest have focused on improving energy density, safety, rate capability, cycle durability, flexibility, and cost.

Zunum Aero is an aircraft manufacturer startup based in Kirkland, Washington. Backed by Boeing HorizonX and JetBlue Technology Ventures, the company has been working since 2013 on a family of hybrid electric regional aircraft up to 50 seats.

## References

1. "Tesla’s batteries have reached their limit – here’s how they could go further", theconversation.com, 2017-11-16
2. Hao Y, Santhakumar K. "Achieving Both High Power and Energy Density in Electrochemical Supercapacitors with Nanoporous Graphene Materials" (PDF): 3. Retrieved 2017-11-16.