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Electric energy is often confused with electric power, but they are two different things;:

  • Power measures capacity to deliver electricity
  • Energy measures total electricity delivered

Electric energy is measured in Watt-hours (Wh), but most people are more familiar with the measurement on their electric bills, kilowatt-hours (1 kWh = 1,000-watt-hours). Electric utilities work at a larger scale and will commonly use megawatt-hours (1 MWh = 1,000 kWh).


There are devices that can convert current from one format to another, or from a higher voltage current to a lower voltage current and vice versa are universally referred to as “transformers.” Any time voltage or current type is transformed, there will always be some sort of energy loss, even if very small.

  • A transformer that converts a higher voltage current to a lower voltage current is called a “step down” transformer, and can work works by either converting high voltage low current loads to low voltage high current loads, or by adding resistance between two circuits to limit the voltage output, resulting in lower power being received on the output side.
  • A transformer that converts to a higher voltage is called a “step up” transformer, and works by converting low voltage but high currents into high voltage but low currents. A step up transformer does not add additional electrical power to the circuit, it only increases overall voltage.
  • A transformer that converts a current from DC to AC is called an inverter, and physically induces an alternating current on the output side. Inverters typically consume electrical power for the conversion process, and thus are less energy efficient than other forms of transformers.
  • A transformer that converts a current from an AC to DC can be called a "battery charger" (for charging batteries) or a "power supply" (for direct powering of a radio, etc.), depending on how the conversion process works.

Direct Current (DC)

The main characteristics characteristic of a Direct Current – or DC – is that the electrons within the current always flow in the same direction, from the side with a deficit to the side with a surplus. This is the kind of current supplied via the chemical effect by batteries, or via the photovoltaic effect by solar panels. The terminals are marked + and – to show the polarity of the circuit or generator. The voltage and current are constant in time.



  • Advantages: batteries Batteries can supply DC directly and it is possible to add the sources in parallel or series.
  • Disadvantages: In reality, the use of the batteries limits the voltage to a few volts (up to 24 volts in some vehicles). Those low voltages prevent the transportation of this type of current.


The frequency is defined as the number of sinusoidal oscillations per second:

  • 50 oscillations per second in Europe (50Hz).
  • 60 oscillations per second in the US (60Hz).

AC is the type of current supplied by electric utility companies because AC voltage can be increased and decreased with a transformer. This allows the power to be transported through power lines efficiently at high voltage and transformed to a lower, safer, voltage for use into in businesses and residences. Therefore, it is the form of electrical energy that consumers typically use when they plug an appliance into a wall socket.

  • Advantages: Can be transported over long distances without too much loss using high tension lines. It is easy to produce.
  • Disadvantages: AC current cannot be stored; it must be created. AC current can also pose a greater health hazard for living organisms that come into contact with it.



A single-phase current is the most common type of current, and thus is usually the configuration delivered by public networks, but also by a single-phase generator. A single-phase AC current is supplied via two lines (phase and neutral), usually with a 220 V voltage difference between them. Plugs can be inserted in both ways.

Because the voltage of a single-phase system reaches a peak value twice in each cycle, the instantaneous power is not constant and is mainly use for lighting and heating but cannot work with industrial motors.

A single-phase load may be powered from a three-phase distribution transformer allowing stand-along single-phase lighting circuit to be connected phase-to-neutral and a three-phase motor, an allowing a  three-phase motors motor to be connected to all three phases. This eliminates the need of a separate single-phase transformer.


Once Power needs are increased, in the use of large electrical motor for example, constancy and If there is an increased need for power, thin consistency and balance pay a key role. Three-phase is circuit is the common current configuration for electricity companies, and can also be produced with a three-phase generator. A three-phase current is the combination of three single phase currents.

To carry a given power with 3 separate single-phase cables, 9 wires are needed. To carry the same power in a three-phase cable, only 5 wires are required (3 phase, 1 neutral, 1 ground), which it is why there can be significant savings when properly planning a three-phase current: . Cost savings include saving on wires, cables, and also in apparatus using or producing electricity: three phase motor or alternator will . Three-phase motors or alternators will also be smaller than the single phase equivalents of the same power produced by three single phase equivalent unitsproduction.

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