General.The watt (abbreviated W) is the International System of Units' (SI) standard unit of power (energy per unit time), the equivalent of one joule per second. The watt is used to specify the rate at which electrical energy is dissipated, or the rate at which electromagnetic energy is radiated, absorbed, or dissipated. In DC (direct-current) and low-frequency AC (alternating current) electrical circuits and systems, power is the product of the current and the voltage. Power is also proportional to the ratio of the square of the voltage to the resistance, and to the product of the resistance and the square of the current. Consider a circuit in which the current, voltage, and resistance are all constant. If the current in amperes is represented by I, the voltage (or potential difference) in volts is represented by E, and the resistance in ohms is represented by R, then the following equations hold for power in watts, represented by P: P = EI; P = E²/R; P = I²R. In radio-frequency (RF) circuits and systems, the calculation of power becomes more complex. This is because, at high frequencies, AC is affected not only by resistance, but by reactance. In these situations, no simple formulas exist for the calculation of power. However, dissipated or radiated RF power in can be determined by direct measurement using an instrument called an RF wattmeter. In situations involving very high or very low power, prefix multipliers are commonly used to obtain power units. As the rate of dissipated or radiated power increases, one kilowatt (kW) is equal to 1000 W; one megawatt (MW) is equal to 10⁶W; one gigawatt (GW) is equal to 10⁹ W. As power decreases, one milliwatt (mW) is equal to 0.001 W; one microwatt (µW) is equal to 10^-6 W; one nanowatt (nW) is equal to 10^-9 W