Power Factor and Compact Fluorescent Bulbs (CFL)
What is the power factor?
What is the power factor of compact fluorecent bulbs?
Do manufacturers of compact fluorecent bulbs cheat with their promised energy savings?
Continue reading for answers to these questions.
The power factor (PF) is the ratio of real power vs. apparent power of a system. Electric appliances use Real power (measured in Watt, W) to produce work, for instance in the form of heat and light produced by a light bulb. Apparent power (measured in Volt Ampere, VA) is the product of voltage and current and is a rather abstract thing for most people (except for electrical engineers).
Apparent power is stored by inductors and capacitors in an electric circuit during each cycle, for instance in the electromagnetic field of a vaccum cleaner motor, and returned to the source in the same cycle (the generator in the power plant). The AC frequency in the United States is 60 Hz, so this cycle happens 60 times per second. Apparent power is not ‘consumed’ by appliances, and luckily, because it swings in and out of your house in each cycle, the net result on your residential electric meter is zero. Utilities do not charge residential customers for apparent power. However, apparent power requires additional current flowing across the grid, and thus creates distribution losses in transformers and power lines in the form of heat. Therefore, utilities have great interest in minimizing apparent power in the grid. They require industrial plants and other large consumers of electric power to take measures for compensating their power factor, or if that is not possible, will charge a penalty.
While incandescent bulbs have a power factor close to 1, and can be considered an almost ideal load, the electric circuit in a compact fluorecent bulb (also called ballast) causes apparent power and lowers the power factor.
Ideally, voltage and current are perfect sine waves. When you observe voltage and current on a resistor, both sine waves will appear undistorted and in phase (i.e. both curves have their maximum at the same time).
When you observe voltage and current on a CFL (as in the graph above), you can see that the curve of the current is no longer a sine, and that both waves are out of phase. The “out-of-phase-ness” indicates that the load causes apparent power. The odd shape is caused by non-linear characteristics of the electronics in the ballast and contributes to appearent power as well.
For an 11W CFL purchased at Ikea (a 40W-equivalent bulb), I measured a real power of 9W, an apparent power of 14VA, and a power factor of 0.65.
So what does this mean? It means that almost 50% more current is drawn from the grid to light up this bulb than it can convert into light and heat. But you don’t pay for this extra power, because it is not consumed. It oscillates back and forth between the generator and the (inductive) load. The apparent power causes distribution losses, but because there are so many different loads on the electric grid at any given time, the effect gets lost in the grand scheme of things, and even for a single residence the overall power factor is likely to be close to 1, despite a few CFL bulbs. If every residential customer in the country replaced all incandescent bulbs with CFLs at once, this situation could change. Electric utilities, sensing an opportunity for a new revenue streams, may begin to charge residential customers for apparent power at some point in the future.
The fact that CFLs have a power factor that is substantially smaller than 1 does not take away from the fact that they are a better choice than incandescent bulbs. Despite the fact that many a cradle-to-crave environmental impact analysis for CFLs on the web is based on assumptions of dubious origin, there is no doubt that their overall energy balance is superior to incandenscent light.
See this very good summary on Wikipedia for an in-depth explanation of the power factor.