Power Measurement Terminology – VA, PF and Watts

Watt is the difference? Why should I care? Where have I seen this before? Am I being misled by marketing enthusiasts who don’t know what a Watt is?

The Watt is a measure of power named after James Watt (1736-1819). In the mks system a watt is the rate of doing work equal to one joule per second. It is fundamentally a mechanical unit of measure, but is often applied to the measurement of electricity.

The VA is a simple equation for describing power in terms of Volts (V) and Amps (A). V times A equals Power in Watts. But it is interesting to note that there are two values that can be given to describe the same power source, both named Watts, but different values. (Can they both be correct?) One of these values includes the Power Factor (PF) of the load. Power Factor quantifies the relationship between the voltage waveform and the current waveform. Without getting too technical (you can get the scoop on Wikipedia), a device may measure 100 Watts of VA power while measuring only 50 Watts when a power factor of 0.50 is incorporated in the calculation.

The biggest offenders of misuse of these power measurement differences (to their avantage of course) are the makers of Compact Fluorescent Lighting and Light Emitting Diode lighting. I have measured CFLs with a Kill-A-Watt and found that the package states that is uses 18 Watts while it actually draws 28 watts from the power line. Since no-one has challenged them so far, they continue to mislead you into thinking that you are saving more energy than you really are. LED lights (not all though) are just as guilty of publishing this misleading information.

Here are some numbers that I have measured with my Kill-A-Watt. We have an array of CFLs in a display area that draws 2.4 Amps from the panel with a power factor of 0.65. The Kill-A-Watt measures 287 VA and it also measures 188 Watts at a PF of 0.65. The math is simple. 287 VA * 0.65 is 188 Watts.

So what does it cost? We are charged for the current drawn at 120 VAC, so 2.4 Amps X 120 Volts is 287 Watts. The packages will tell you that the power used is only 188 watts, but the power company will charge you for 287 VA. I call that the intentional misleading of an uninformed customer base. Sellers are supposed to educate the consumer, not take advantage of them.

Yes, the 18W CFL does put out as many lumens as a 40W light bulb, but the package should read “28VA” or “28 Watt CFL” or “18 Watt CFL at PF=0.65”. But to misuse the lower reading is outright false advertising.

So, we will use VA in our calculations on this web site a majority of the time and we will point out the use of Watts when the Power Factor is not 1.00 where Watts is equal to VA.

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