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Electric Power Primer

Electric power is defined as the rate at which electrical energy is transferred by an electric circuit. The SI unit of electric power is the watt. Electrical power is distributed via cables and electricity poles or pylons.

When electric current flows in a circuit, it can transfer energy to do mechanical or thermodynamic work. Devices convert electrical energy into many useful forms, such as heat (electric heaters), light (light bulbs), motion (electric motors), sound (loudspeaker) or chemical changes. Electric power can be produced mechanically by generation, or chemically, or by direct conversion from light in photovoltaic cells, also it can be stored chemically in batteries.

Electric power, like mechanical power, is represented by the letter P in electrical equations. The term wattage is used colloquially to mean "electric power in watts."

Direct current
In direct current resistive circuits, instantaneous electrical power is calculated using Joule's Law, which is named after the British physicist James Joule, who first showed that heat and mechanical energy were interchangeable.

Alternating current
In alternating current circuits, energy storage elements such as inductance and capacitance may result in periodic reversals of the direction of energy flow. The portion of electric power flow that, averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known as real electric power (also referred to as active power). That portion of electric power flow due to stored energy, that returns to the source in each cycle, is known as reactive power.

Power triangle
The components of AC power
The relationship between real power, reactive power and apparent power can be expressed by representing the quantities as vectors. Real power is represented as a horizontal vector and reactive power is represented as a vertical vector. The apparent power vector is the hypotenuse of a right triangle formed by connecting the real and reactive power vectors. This representation is often called the power triangle. Using the Pythagorean Theorem, the relationship among real, reactive and apparent power is:

(apparent power)2 = (real power)2 + (reactive power)2
Real and reactive powers can also be calculated directly from the apparent power, when the current and voltage are both sinusoids with a known phase angle between them:
(real power) = (apparent power) * cos(theta)
(reactive power) = (apparent power) * sin(theta)

The ratio of real power to apparent power is called power factor and is a number always between 0 and 1.

In space
Electric power flows wherever electric and magnetic fields exist together and fluctuate in the same place. The simplest example of this is in electrical circuits, as the preceding section showed. In the general case, however, the simple equation P = IV must be replaced by a more complex calculation, the integral of the vector cross-product of the electrical and magnetic fields over a specified area, thus:

The result is a scalar since it is the surface integral of the Poynting vector.

Electric Power

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