WATT, KILOWATT, VA, and POWER FACTOR

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Energy in general is defined as the capacity for doing work. Power by definition is the rate of work or energy flow (which are numerically the same):

It can be shown that in electrical circuits the instantaneous power is

The value of the main interest in electrical industry is an average value of p(t) over a complete AC cycle. This quantity is called

It represents that actual work done by an electric current or an actual energy consumed by a load to create for example heat, light or motion.

Electrical systems normally have inductors and capacitors, which are referred to as reactive components. Ideal reactive components do not dissipate any energy, but they draw currents and create voltage drops, which makes the impression that they actually do. This "imaginary power" is called

In this formula V and I are root-mean-square (RMS) values of voltage and current. For example, if V=120 volt (nominal value for U.S.), I=1 amp, then VA=120 volt-amp.

PF by definition is the ratio of real to apparent power:

People are often looking for a calculator to convert volt-amps (VA) to watts. Well, obviously you need to know the value of PF to do the calculation: W=VA×PF, where PF is in decimal. Likewise, you can convert watt to VA by using this formula: VA=W/PF.

Unfortunately, PF value is practically never stated in appliance's spec sheet. For old computers it used to be 0.6-0.65. Modern computers normally have an SMPS PSU with PFC that assures near unity PF. For the motor-driven appliances (such as refrigerators and air conditioners) this value is typically 0.6-0.9. If you don't know the PF of your device, assume the worse case of 0.6.

Enter any two known values and press "Calculate" to find the remaining value.

Reset before each new calculation.

A "Reset before each new calculation.

PF value measures how effectively electricity is being utilized. Here is a simple mechanical analogy.

In some US regions the utilities already installed residential digital electric meters, which compute W, VAR, and PF. They may surcharge you for VAR. However, so far most residential meters in U.S. are still rotating-disc devices that measure only real watts, so PF of your appliances does not affect the cost of your electricity. Therefore, using power factor correcting (PFC) devices will not reduce your electric bills as some claim. Nevertheless, PF of the appliances should be taken into account when choosing the size of a backup system, such as a generator or UPS. Also, lower PF will cause larger current in utility lines and additional voltage drop in the wiring. In an extreme case, this can cause overheating and premature failure of a motor and other equipment. Unlike most residential users, for commercial and industrial customers, an electric utility company may assess a surcharge when power factor drops below 0.95 or so.

Note that single-phase generators are usually rated for loads with PF=1, so their W and VA ratings are the same. Since typical appliances have PF=0.6-0.8, their VA consumption is 25-60% greater than their wattage. That's why generator output rating should be much greater than the net wattage of such motor-driven devices. For example, for 700 W load with PF=0.7 you need at least a 700/0.7=1000 W generator. Fortunately, nowadays an appliance's nameplate usually states its maximum current rather than wattage, so you don't need to know its PF: you just multiply the value of the current by nominal AC voltage (120V in US) to get the VA. For example, if your single-phase appliance is rated for 10 A maximum, it may consume up to 120×10=1200 VA. This is the number you should use when you do genset sizing.

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